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• Take,Skip and Reverse Operator in Linq

  - by Jalpesh P. Vadgama

  I have found three more new operators in Linq which is use full in day to day programming stuff. Take,Skip and Reverse. Here are explanation of operators how it works. Take Operator: Take operator will return first N number of element from entities. Skip Operator: Skip operator will skip N number of element from entities and then return remaining elements as a result. Reverse Operator: As name suggest it will reverse order of elements of entities. Here is the examples of operators where i have taken simple string array to demonstrate that. C#, using GeSHi 1.0.8.6 using System; using System.Collections.Generic; using System.Linq; using System.Text;     namespace ConsoleApplication1 {     class Program     {         static void Main(string[] args)         {             string[] a = { "a", "b", "c", "d" };                           Console.WriteLine("Take Example");             var TkResult = a.Take(2);             foreach (string s in TkResult)             {                 Console.WriteLine(s);             }               Console.WriteLine("Skip Example");             var SkResult = a.Skip(2);             foreach (string s in SkResult)             {                 Console.WriteLine(s);             }               Console.WriteLine("Reverse Example");             var RvResult = a.Reverse();             foreach (string s in RvResult)             {                 Console.WriteLine(s);             }                       }     } } Parsed in 0.020 seconds at 44.65 KB/s Here is the output as expected. hope this will help you.. Technorati Tags: Linq,Linq-To-Sql,ASP.NET,C#.NET

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• Dynamic Data: how to filter dropdown for foreign key on edit page

  - by Leonv

  I have Organisation with a foreign key to a Manager. Managers can be active, or inactive. On the Dynamic Data edit page for Organisation, I need to filter the dropdown for Manager to only show active records. I started out by making a custom version of DynamicData\FieldTemplates\ForeignKey_Edit.ascx and setting a UIHint to the new field template on Organisation.Manager. But, how to customize the linq or sql query that runs to load the Managers? Using Linq-to-SQL and DynamicDataFutures

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• LINQ dynamic query

  - by ile

  How to dynamically generate LINQ query: int[] IDArray = {55, 36}; public IQueryable<ContactListView> FindAllContacts(int loggedUserID, int[] IDArray) { var result = ( from contact in db.Contacts //Start of dynamic part... where contact.UserID == loggedUserID foreach (var id in IDArray) { where contact.UserID == id } // End of dynamic part orderby contact.ContactID descending select new ContactListView { ContactID = contact.ContactID, FirstName = contact.FirstName, LastName = contact.LastName }); return result; } Thanks, Ile

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• Retrieve data using Dynamic Query and Linq to SQL

  - by GigaPr

  Hi I have a really complicated dynamic query that i want to use to retrieve data from the database I am working in .net 3.5 sql server 2008 i created a stored procedure that accepts a varchar(max) as input parameter and does execute (@SqlQuery) it executes but does not return anything I really would like to use LINQ as all my project is implemented using linq Any Idea how to do it what is the problem?

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• .NET 4.0 Dynamic object used statically?

  - by Kevin Won

  I've gotten quite sick of XML configuration files in .NET and want to replace them with a format that is more sane. Therefore, I'm writing a config file parser for C# applications that will take a custom config file format, parse it, and create a Python source string that I can then execute in C# and use as a static object (yes that's right--I want a static (not the static type dyanamic) object in the end). Here's an example of what my config file looks like: // my custom config file format GlobalName: ExampleApp Properties { ExternalServiceTimeout: "120" } Python { // this allows for straight python code to be added to handle custom config def MyCustomPython: return "cool" } Using ANTLR I've created a Lexer/Parser that will convert this format to a Python script. So assume I have that all right and can take the .config above and run my Lexer/Parser on it to get a Python script out the back (this has the added benefit of giving me a validation tool for my config). By running the resultant script in C# // simplified example of getting the dynamic python object in C# // (not how I really do it) ScriptRuntime py = Python.CreateRuntime(); dynamic conf = py.UseFile("conftest.py"); dynamic t = conf.GetConfTest("test"); I can get a dynamic object that has my configuration settings. I can now get my config file settings in C# by invoking a dynamic method on that object: //C# calling a method on the dynamic python object var timeout = t.GetProperty("ExternalServiceTimeout"); //the config also allows for straight Python scripting (via the Python block) var special = t.MyCustonPython(); of course, I have no type safety here and no intellisense support. I have a dynamic representation of my config file, but I want a static one. I know what my Python object's type is--it is actually newing up in instance of a C# class. But since it's happening in python, it's type is not the C# type, but dynamic instead. What I want to do is then cast the object back to the C# type that I know the object is: // doesn't work--can't cast a dynamic to a static type (nulls out) IConfigSettings staticTypeConfig = t as IConfigSettings Is there any way to figure out how to cast the object to the static type? I'm rather doubtful that there is... so doubtful that I took another approach of which I'm not entirely sure about. I'm wondering if someone has a better way... So here's my current tactic: since I know the type of the python object, I am creating a C# wrapper class: public class ConfigSettings : IConfigSettings that takes in a dynamic object in the ctor: public ConfigSettings(dynamic settings) { this.DynamicProxy = settings; } public dynamic DynamicProxy { get; private set; } Now I have a reference to the Python dynamic object of which I know the type. So I can then just put wrappers around the Python methods that I know are there: // wrapper access to the underlying dynamic object // this makes my dynamic object appear 'static' public string GetSetting(string key) { return this.DynamicProxy.GetProperty(key).ToString(); } Now the dynamic object is accessed through this static proxy and thus can obviously be passed around in the static C# world via interface, etc: // dependency inject the dynamic object around IBusinessLogic logic = new BusinessLogic(IConfigSettings config); This solution has the benefits of all the static typing stuff we know and love while at the same time giving me the option of 'bailing out' to dynamic too: // the DynamicProxy property give direct access to the dynamic object var result = config.DynamicProxy.MyCustomPython(); but, man, this seems rather convoluted way of getting to an object that is a static type in the first place! Since the whole dynamic/static interaction world is new to me, I'm really questioning if my solution is optimal or if I'm missing something (i.e. some way of casting that dynamic object to a known static type) about how to bridge the chasm between these two universes.

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• dynamic urls and links on one web page

  - by John

  I am trying to figure out how to create dynamic links and urls on a static webpage. What I want to do is the following: I have a single webpage for example: MYWEBPAGEdotCOM/INDEX.HTML that will always look the same, except for one link on the page. the link would be on the page for example: LINK TO AFFILIATE: affiliatedotCOM/my-affiliate_code_here_DYNAMIC_REFERER the only thing would change is the "DYNAMIC_REFERER" with every dynamic url on this page: MYWEBPAGEdotCOM/INDEX.PHP_id=test1 MYWEBPAGEdotCOM/INDEX.PHP_id=test2 MYWEBPAGEdotCOM/INDEX.PHP_id=test3 MYWEBPAGEdotCOM/INDEX.PHP_id=test4 which would only hange the dynamic link on the page to: affiliatedotCOM/my-affiliate_code_here_test1 affiliatedotCOM/my-affiliate_code_here_test2 affiliatedotCOM/my-affiliate_code_here_test3 affiliatedotCOM/my-affiliate_code_here_test4 Can someone tell me how I could go about doing this? I just dont want to have to make 100's of pages, as this would prevent me from having to do so.

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• dynamic urls and links on one web page

  - by John

  I am trying to figure out how to create dynamic links and urls on a static webpage. What I want to do is the following: I have a single webpage for example: MYWEBPAGEdotCOM/INDEX.HTML that will always look the same, except for one link on the page. the link would be on the page for example: LINK TO AFFILIATE: affiliatedotCOM/my-affiliate_code_here_DYNAMIC_REFERER the only thing would change is the "DYNAMIC_REFERER" with every dynamic url on this page: MYWEBPAGEdotCOM/INDEX.PHP_id=test1 MYWEBPAGEdotCOM/INDEX.PHP_id=test2 MYWEBPAGEdotCOM/INDEX.PHP_id=test3 MYWEBPAGEdotCOM/INDEX.PHP_id=test4 which would only hange the dynamic link on the page to: affiliatedotCOM/my-affiliate_code_here_test1 affiliatedotCOM/my-affiliate_code_here_test2 affiliatedotCOM/my-affiliate_code_here_test3 affiliatedotCOM/my-affiliate_code_here_test4 Can someone tell me how I could go about doing this? I just dont want to have to make 100's of pages, as this would prevent me from having to do so.

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• Problem with Mapping Linq-to-Sql on different Types

  - by csharpnoob

  Hi, maybe someone can help. I want to have on mapped Linq-Class different Datatype. This is working: private System.Nullable<short> _deleted = 1; [Column(Storage = "_deleted", Name = "deleted", DbType = "SmallInt", CanBeNull = true)] public System.Nullable<short> deleted { get { return this._deleted; } set { this._deleted = value; } } Sure thing. But no when i want to place some logic for boolean, like this: private System.Nullable<short> _deleted = 1; [Column(Storage = "_deleted", Name = "deleted", DbType = "SmallInt", CanBeNull = true)] public bool deleted { get { if (this._deleted == 1) { return true; } return false; } set { if(value == true) { this._deleted = (short)1; }else { this._deleted = (short)0; } } } I get always runtime error: [TypeLoadException: GenericArguments[2], "System.Nullable`1[System.Int16]", on 'System.Data.Linq.Mapping.PropertyAccessor+Accessor`3[T,V,V2]' violates the constraint of type parameter 'V2'.] I can't change the database to bit.. I need to have casting in mapping class.

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• c# linq to xml to list

  - by WtFudgE

  I was wondering if there is a way to get a list of results into a list with linq to xml. If I would have the following xml for example: <?xml version="1.0"?> <Sports xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema"> <SportPages> <SportPage type="test"> <LinkPage> <IDList> <string>1</string> <string>2</string> </IDList> </LinkPage> </SportPage> </SportPages> </Sports> How could I get a list of strings from the IDList? I'm fairly new to linq to xml so I just tried some stuff out, I'm currently at this point: var IDs = from sportpage in xDoc.Descendants("SportPages").Descendants("SportPage") where sportpage.Attribute("type").Value == "Karate" select new { ID = sportpage.Element("LinkPage").Element("IDList").Elements("string") }; But the var is to chaotic to read decently. Isn't there a way I could just get a list of strings from this? Thanks

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• ASP MVC C#: LINQ Foreign Key Constraint conflicts

  - by wh0emPah

  I'm having a problem with LINQ. I have 2 tables (Parent-child relation) Table1: Events (EventID, Description) Table2: Groups (GroupID, EventID(FK), Description) Now i want to create an Event an and a child. Event e = new Event(); e.Description = "test"; Datacontext.Events.InsertOnSubmit(event) Group g = new Group(); g.Description = "test2"; g.EventID = e.EventID; Datacontext.Groups.InsertOnSubmit(g); Datacontext.SubmitChanges(); When i debug, i can see that after inserting the event. the EventID has gotten a new value (auto increment). But when Datacontext.SubmitChanges(); gets called. I get the following exception "The INSERT statement conflicted with the FOREIGN KEY constraint ... I know this can be solved by creating a relation in the LINQ diagram between Events and groups. And then setting the entity itself. But i don't want to load the events everytime i ask a list of groups. All i need is some way that when inserting the group fails, the event insert won't be comitted in the database. Sorry if this is a bit unclear, My english isn't really good. Thanks in advance!

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• LINQ self referencing query

  - by Chris

  I have the following SQL query: select p1.[id], p1.[useraccountid], p1.[subject], p1.[message], p1.[views], p1.[parentid], case when p2.[created] is null then p1.[created] else p2.[created] end as LastUpdate from forumposts p1 left join ( select parentid, max(created) as [created] from forumposts group by parentid ) p2 on p2.parentid = p1.id where p1.[parentid] is null order by LastUpdate desc Using the following class: public class ForumPost : PersistedObject { public int Views { get; set; } public string Message { get; set; } public string Subject { get; set; } public ForumPost Parent { get; set; } public UserAccount UserAccount { get; set; } public IList<ForumPost> Replies { get; set; } } How would I replicate such a query in LINQ? I've tried several variations, but I seem unable to get the correct join syntax. Is this simply a case of a query that is too complicated for LINQ? Can it be done using nested queries some how? The purpose of the query is to find the most recently updated posts i.e. replying to a post would bump it to the top of the list. Replies are defined by the ParentID column, which is self-referencing.

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• Tracking Down a Stack Overflow in My Linq Query

  - by Lazarus

  I've written the following Linq query: IQueryable<ISOCountry> entries = (from e in competitorRepository.Competitors join c in countries on e.countryID equals c.isoCountryCode where !e.Deleted orderby c.isoCountryCode select new ISOCountry() { isoCountryCode = e.countryID, Name = c.Name }).Distinct(); The objective is to retrieve a list of the countries represented by the competitors found in the system. 'countries' is an array of ISOCountry objects explicitly created and returned as an IQueryable (ISOCountry is an object of just two strings, isoCountryCode and Name). Competitors is an IQueryable which is bound to a database table through Linq2SQL though I created the objects from scratch and used the Linq data mapping decorators. For some reason this query causes a stack overflow when the system tries to execute it. I've no idea why, I've tried trimming the Distinct, returning an anonymous type of the two strings, using 'select c', all result in the overflow. The e.CountryID value is populated from a dropdown that was in itself populated from the IQueryable so I know the values are appropriate but even if not I wouldn't expect a stack overflow. Can anyone explain why the overflow is occurring or give good speculation as to why it might be happening? EDIT As requested, code for ISOCountry: public class ISOCountry { public string isoCountryCode { get; set; } public string Name { get; set; } }

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• LINQ to XML via C#

  - by user70192

  Hello, I'm new to LINQ. I understand it's purpose. But I can't quite figure it out. I have an XML set that looks like the following: <Results> <Result> <ID>1</ID> <Name>John Smith</Name> <EmailAddress>[email protected]</EmailAddress> </Result> <Result> <ID>2</ID> <Name>Bill Young</Name> <EmailAddress>[email protected]</EmailAddress> </Result> </Results> I have loaded this XML into an XDocument as such: string xmlText = GetXML(); XDocument xml = XDocument.Parse(xmlText); Now, I'm trying to get the results into POCO format. In an effort to do this, I'm currently using: var objects = from results in xml.Descendants("Results") select new Results // I'm stuck How do I get a collection of Result elements via LINQ? I'm particularly confused about navigating the XML structure at this point in my code. Thank you!

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• Getting values from DataGridView back to XDocument (using LINQ-to-XML)

  - by Pretzel

  Learning LINQ has been a lot of fun so far, but despite reading a couple books and a bunch of online resources on the topic, I still feel like a total n00b. Recently, I just learned that if my query returns an Anonymous type, the DataGridView I'm populating will be ReadOnly (because, apparently Anonymous types are ReadOnly.) Right now, I'm trying to figure out the easiest way to: Get a subset of data from an XML file into a DataGridView, Allow the user to edit said data, Stick the changed data back into the XML file. So far I have Steps 1 and 2 figured out: public class Container { public string Id { get; set; } public string Barcode { get; set; } public float Quantity { get; set; } } // For use with the Distinct() operator public class ContainerComparer : IEqualityComparer<Container> { public bool Equals(Container x, Container y) { return x.Id == y.Id; } public int GetHashCode(Container obj) { return obj.Id.GetHashCode(); } } var barcodes = (from src in xmldoc.Descendants("Container") where src.Descendants().Count() > 0 select new Container { Id = (string)src.Element("Id"), Barcode = (string)src.Element("Barcode"), Quantity = float.Parse((string)src.Element("Quantity").Attribute("value")) }).Distinct(new ContainerComparer()); dataGridView1.DataSource = barcodes.ToList(); This works great at getting the data I want from the XML into the DataGridView so that the user has a way to manipulate the values. Upon doing a Step-thru trace of my code, I'm finding that the changes to the values made in DataGridView are not bound to the XDocument object and as such, do not propagate back. How do we take care of Step 3? (getting the data back to the XML) Is it possible to Bind the XML directly to the DataGridView? Or do I have to write another LINQ statement to get the data from the DGV back to the XDocument? Suggstions?

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• Linq join with an inner collection

  - by bronze

  Hi, I am trying a LINQ to Object query on 2 collections Customer.Orders Branches.Pending.Orders (Collection within a collection) I want to output each branch which is yet to deliver any order of the customer. var match = from order in customer.Orders join branch in Branches on order equals branch.Pending.Orders select branch; This does not work, I get : The type of one of the expressions in the join clause is incorrect. Type inference failed in the call to 'GroupJoin'. From my search, I think this is because Order or collection of Orders does not implement equals. If this query worked, it will still be wrong, as it will return a branch if the customer's and pending orders match exactly. I want a result if any of the order matches. I am learning Linq, and looking for a approach to address such issues, rather than the solution itself. I would have done this in SQL like this; SELECT b.branch_name from Customers c, Branches b, Orders o WHERE c.customer_id = o.customer_id AND o.branch_id = b.branch_id AND c.customer_id = 'my customer' AND o.order_status = 'pending'

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• Linq is returning too many results when joined

  - by KallDrexx

  In my schema I have two database tables. relationships and relationship_memberships. I am attempting to retrieve all the entries from the relationship table that have a specific member in it, thus having to join it with the relationship_memberships table. I have the following method in my business object: public IList<DBMappings.relationships> GetRelationshipsByObjectId(int objId) { var results = from r in _context.Repository<DBMappings.relationships>() join m in _context.Repository<DBMappings.relationship_memberships>() on r.rel_id equals m.rel_id where m.obj_id == objId select r; return results.ToList<DBMappings.relationships>(); } _Context is my generic repository using code based on the code outlined here. The problem is I have 3 records in the relationships table, and 3 records in the memberships table, each membership tied to a different relationship. 2 membership records have an obj_id value of 2 and the other is 3. I am trying to retrieve a list of all relationships related to object #2. When this linq runs, _context.Repository<DBMappings.relationships>() returns the correct 3 records and _context.Repository<DBMappings.relationship_memberships>() returns 3 records. However, when the results.ToList() executes, the resulting list has 2 issues: 1) The resulting list contains 6 records, all of type DBMappings.relationships(). Upon further inspection there are 2 for each real relationship record, both are an exact copy of each other. 2) All relationships are returned, even if m.obj_id == 3, even though objId variable is correctly passed in as 2. Can anyone see what's going on because I've spent 2 days looking at this code and I am unable to understand what is wrong. I have joins in other linq queries that seem to be working great, and my unit tests show that they are still working, so I must be doing something wrong with this. It seems like I need an extra pair of eyes on this one :)

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• Using conditionals in Linq Programatically

  - by Mike B

  I was just reading a recent question on using conditionals in Linq and it reminded me of an issue I have not been able to resolve. When building Linq to SQL queries programatically how can this be done when the number of conditionals is not known until runtime? For instance in the code below the first clause creates an IQueryable that, if executed, would select all the tasks (called issues) in the database, the 2nd clause will refine that to just issues assigned to one department if one has been selected in a combobox (Which has it's selected item bound to the departmentToShow property). How could I do this using the selectedItems collection instead? IQueryable<Issue> issuesQuery; // Will select all tasks issuesQuery = from i in db.Issues orderby i.IssDueDate, i.IssUrgency select i; // Filters out all other Departments if one is selected if (departmentToShow != "All") { issuesQuery = from i in issuesQuery where i.IssDepartment == departmentToShow select i; } By the way, the above code is simplified, in the actual code there are about a dozen clauses that refine the query based on the users search and filter settings.

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• Delaying LINQ to SQL Select Query Execution

  - by Maxim Z.

  I'm building an ASP.NET MVC site that uses LINQ to SQL. In my search method that has some required and some optional parameters, I want to build a LINQ query while testing for the existence of those optional parameters. Here's what I'm currently thinking: using(var db = new DBDataContext()) { IQueryable<Listing> query = null; //Handle required parameter query = db.Listings.Where(l => l.Lat >= form.bounds.extent1.latitude && l.Lat <= form.bounds.extent2.latitude); //Handle optional parameter if (numStars != null) query = query.Where(l => l.Stars == (int)numStars); //Other parameters... //Execute query (does this happen here?) var result = query.ToList(); //Process query... Will this implementation "bundle" the where clauses and then execute the bundled query? If not, how should I implement this feature? Also, is there anything else that I can improve? Thanks in advance.

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• EF/LINQ: Where() against a property of a subtype

  - by ladenedge

  I have a set of POCOs, all of which implement the following simple interface: interface IIdObject { int Id { get; set; } } A subset of these POCOs implement this additional interface: interface IDeletableObject : IIdObject { bool IsDeleted { get; set; } } I have a repository hierarchy that looks something like this: IRepository<T <: BasicRepository<T <: ValidatingRepository<T (where T is IIdObject) I'm trying to add a FilteringRepository to the hierarchy such that all of the POCOs that implement IDeletableObject have a Where(p => p.IsDeleted == false) filter applied before any other queries take place. My goal is to avoid duplicating the hierarchy solely for IDeletableObjects. My first attempt looked like this: public override IQueryable<T> Query() { return base.Query().Where(t => ((IDeletableObject)t).IsDeleted == false); } This works well with LINQ to Objects, but when I switch to an EF backend I get: "LINQ to Entities only supports casting Entity Data Model primitive types." I went on to try some fancier parameterized solutions, but they ultimately failed because I couldn't make T covariant in the following case for some reason I don't quite understand: interface IQueryFilter<out T> // error { Expression<Func<T, bool>> GetFilter(); } I'd be happy to go into more detail on my more complicated solutions if it would help, but I think I'll stop here for now in hope that someone might have an idea for me to try. Thanks very much in advance!

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• LINQ to objects: Is there

  - by Charles

  I cannot seem to find a way to have LINQ return the value from a specified accessor. I know the name of the accessors for each object, but am unsure if it is possible to pass the requested accessor as a variable or otherwise achieve the desired refactoring. Consider the following code snippet: // "value" is some object with accessors like: format, channels, language row = new List<String> { String.Join(innerSeparator, (from item in myObject.Audio orderby item.Key ascending select item.Value.format).ToArray()), String.Join(innerSeparator, (from item in myObject.Audio orderby item.Key ascending select item.Value.channels).ToArray()), String.Join(innerSeparator, (from item in myObject.Audio orderby item.Key ascending select item.Value.language).ToArray()), // ... } I'd like to refactor this into a method that uses the specified accessor, or perhaps pass a delegate, though I don't see how that could work. string niceRefactor(myObj myObject, string /* or whatever type */ ____ACCESSOR) { return String.Join(innerSeparator, (from item in myObject.Audio orderby item.Key ascending select item.Value.____ACCESSOR).ToArray()); } I have written a decent amount of C#, but am still new to the magic of LINQ. Is this the right approach? How would you refactor this?

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• LINQ to SQL: Reusable expression for property?

  - by coenvdwel

  Pardon me for being unable to phrase the title more exact. Basically, I have three LINQ objects linked to tables. One is Product, the other is Company and the last is a mapping table Mapping to store what Company sells which products and by which ID this Company refers to this Product. I am now retrieving a list of products as follows: var options = new DataLoadOptions(); options.LoadWith<Product>(p => p.Mappings); context.LoadOptions = options; var products = ( from p in context.Products select new { ProductID = p.ProductID, //BackendProductID = p.BackendProductID, BackendProductID = (p.Mappings.Count == 0) ? "None" : (p.Mappings.Count > 1) ? "Multiple" : p.Mappings.First().BackendProductID, Description = p.Description } ).ToList(); This does a single query retrieving the information I want. But I want to be able to move the logic behind the BackendProductID into the LINQ object so I can use the commented line instead of the annoyingly nested ternary operator statements for neatness and re-usability. So I added the following property to the Product object: public string BackendProductID { get { if (Mappings.Count == 0) return "None"; if (Mappings.Count > 1) return "Multiple"; return Mappings.First().BackendProductID; } } The list is still the same, but it now does a query for every single Product to get it's BackendProductID. The code is neater and re-usable, but the performance now is terrible. What I need is some kind of Expression or Delegate but I couldn't get my head around writing one. It always ended up querying for every single product, still. Any help would be appreciated!

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• Linq generic Expression in query on "element" or on IQueryable (multiple use)

  - by Bogdan Maxim

  Hi, I have the following expression public static Expression<Func<T, bool>> JoinByDateCheck<T>(T entity, DateTime dateToCheck) where T : IDateInterval { return (entityToJoin) => entityToJoin.FromDate.Date <= dateToCheck.Date && (entityToJoin.ToDate == null || entityToJoin.ToDate.Value.Date >= dateToCheck.Date); } IDateInterval interface is defined like this: interface IDateInterval { DateTime FromDate {get;} DateTime? ToDate {get;} } and i need to apply it in a few ways: (1) Query on Linq2Sql Table: var q1 = from e in intervalTable where FunctionThatCallsJoinByDateCheck(e, constantDateTime) select e; or something like this: intervalTable.Where(FunctionThatCallsJoinByDateCheck(e, constantDateTime)) (2) I need to use it in some table joins (as linq2sql doesn't provide comparative join): var q2 = from e1 in t1 join e2 in t2 on e1.FK == e2.PK where OtherFunctionThatCallsJoinByDateCheck(e2, e1.FromDate) or var q2 = from e1 in t1 from e2 in t2 where e1.FK == e2.PK && OtherFunctionThatCallsJoinByDateCheck(e2, e1.FromDate) (3) I need to use it in some queries like this: var q3 = from e in intervalTable.FilterFunctionThatCallsJoinByDateCheck(constantDate); Dynamic linq is not something that I can use, so I have to stick to plain linq. Thank you Clarification: Initially I had just the last method (FilterFunctionThatCallsJoinByDateCheck(this IQueryable<IDateInterval> entities, DateTime dateConstant) ) that contained the code from the expression. The problem is that I get a SQL Translate exception if I write the code in a method and call it like that. All I want is to extend the use of this function to the where clause (see the second query in point 2)

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• LINQ to SQL Queries odd Materialization

  - by ptoinson

  I ran across an interesting Linq to SQL, uh, feature, the other day. Perhaps someone can give me a logical explanation for the reasoning behind the results. Take the code below as my example which utilizes the AdventureWorks database setup in a Linq to SQL DataContext. This is a clip from my unit test. The resulting customer returned from a call to both CustomerQuery_Test_01() and CustomerQuery_Test_02() is the same. However, the query executed on the SQLServer are different is a major way. The method CustomerQuery_Test_01 us causing the entire Customer table to be materialized, which the call to CustomerQuery_Test_02 is only causing the single customer to be materialized. The resulting SQL Queries are at the bottom of this post. Anyone have a good reason for this? To me, it was highly non-intuitive. protected virtual Customer GetByPrimaryKey(Func<Customer, bool> keySelection) { AdventureWorksDataContext context = new AdventureWorksDataContext(); return (from r in context.Customers select r).SingleOrDefault(keySelection); } [TestMethod] public void CustomerQuery_Test_01() { Customer customer = GetByPrimaryKey(c => c.CustomerID == 2); } [TestMethod] public void CustomerQuery_Test_02() { AdventureWorksDataContext context = new AdventureWorksDataContext(); Customer customer = (from r in context.Customers select r).SingleOrDefault(c => c.CustomerID == 2); } Query for CustomerQuery_Test_01 (notice the lack of a where clause) SELECT [t0].[CustomerID], [t0].[NameStyle], [t0].[Title], [t0].[FirstName], [t0].[MiddleName], [t0].[LastName], [t0].[Suffix], [t0].[CompanyName], [t0].[SalesPerson], [t0].[EmailAddress], [t0].[Phone], [t0].[PasswordHash], [t0].[PasswordSalt], [t0].[rowguid], [t0].[ModifiedDate] FROM [SalesLT].[Customer] AS [t0] Query for CustomerQuery_Test_02 (notice the where clause) SELECT [t0].[CustomerID], [t0].[NameStyle], [t0].[Title], [t0].[FirstName], [t0].[MiddleName], [t0].[LastName], [t0].[Suffix], [t0].[CompanyName], [t0].[SalesPerson], [t0].[EmailAddress], [t0].[Phone], [t0].[PasswordHash], [t0].[PasswordSalt], [t0].[rowguid], [t0].[ModifiedDate] FROM [SalesLT].[Customer] AS [t0] WHERE [t0].[CustomerID] = @p0

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• xen 4.1 host priodically dropping network packets of domU

  - by Dyutiman Chakraborty

  I have xen 4.1 Host running on a ubuntu 12.04 LTS Server with ip 153.x.x.54. I have setup 2 VMs on it, namely, "dev.mydomain.com" and "web.mydomain.com" with ips 195.X.X.2 and 195.x.x.3 respectively. For network the VMs connect through xendbr0 (xen-bridge), and can accces the network properly. I can also login to the VMs with ssh with no issue. However when I ping any of the VMs, there is a high amount of periodic packet drop. If I the ping the xen host (dom0) there is no packet drop. Following is a output of "tcpdump | grep ICMP" on dOM0 while I was pinging one of the domU tcpdump: verbose output suppressed, use -v or -vv for full protocol decode listening on eth0, link-type EN10MB (Ethernet), capture size 65535 bytes 05:19:55.682493 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 30, length 64 05:19:56.691144 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 31, length 64 05:19:57.698776 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 32, length 64 05:19:58.706784 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 33, length 64 05:19:59.714751 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 34, length 64 05:20:00.723144 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 35, length 64 05:20:01.730349 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 36, length 64 05:20:02.739017 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 37, length 64 05:20:03.746806 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 38, length 64 05:20:06.770326 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 41, length 64 05:20:07.778801 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 42, length 64 05:20:08.786481 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 43, length 64 05:20:09.794720 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 44, length 64 05:20:10.802395 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 45, length 64 05:20:11.810770 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 46, length 64 05:20:12.818511 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 47, length 64 05:20:13.826817 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 48, length 64 05:20:14.835125 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 49, length 64 05:20:15.842138 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3460, seq 50, length 64 05:20:18.274072 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 1, length 64 05:20:19.282347 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 2, length 64 05:20:20.290746 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 3, length 64 05:20:21.297910 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 4, length 64 05:20:22.305656 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 5, length 64 05:20:23.314369 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 6, length 64 05:20:24.322055 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 7, length 64 05:20:25.329782 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 8, length 64 05:20:26.338473 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 9, length 64 05:20:27.346411 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 10, length 64 05:20:28.354175 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 11, length 64 05:20:29.361640 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 12, length 64 05:20:30.370026 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 13, length 64 05:20:31.377696 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 14, length 64 05:20:32.386151 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 15, length 64 05:20:33.394118 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 16, length 64 05:20:34.402058 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 17, length 64 05:20:35.409002 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 18, length 64 05:20:36.417692 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > web.mydomain.com: ICMP echo request, id 3461, seq 19, length 64 05:20:36.496916 IP6 fe80::3285:a9ff:feec:fc69 > ip6-allnodes: HBH ICMP6, multicast listener querymax resp delay: 1000 addr: ::, length 24 05:20:36.499112 IP6 fe80::21c:c0ff:fe6c:c091 > ff02::1:ff6c:c091: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff6c:c091, length 24 05:20:36.507041 IP6 fe80::227:eff:fe11:fa3f > ff02::1:ff00:2: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff00:2, length 24 05:20:36.523919 IP6 fe80::21c:c0ff:fe77:6257 > ff02::1:ff77:6257: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff77:6257, length 24 05:20:36.544785 IP6 fe80::54:ff:fe12:ea9a > ff02::1:ff12:ea9a: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff12:ea9a, length 24 05:20:36.581740 IP6 fe80::5604:a6ff:fef1:6da7 > ff02::1:fff1:6da7: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:fff1:6da7, length 24 05:20:36.600103 IP6 fe80::8a8:8aa0:5e18:917a > ff02::1:ff18:917a: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff18:917a, length 24 05:20:36.601989 IP6 fe80::227:eff:fe11:fa3e > ff02::1:ff11:fa3e: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff11:fa3e, length 24 05:20:36.611090 IP6 fe80::dcad:56ff:fe57:3bbe > ff02::1:ff57:3bbe: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff57:3bbe, length 24 05:20:36.660521 IP6 fe80::54:ff:fe02:1d31 > ff02::1:ff00:6: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff00:6, length 24 05:20:36.698871 IP6 fe80::21e:8cff:feb4:9f89 > ff02::1:ffb4:9f89: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ffb4:9f89, length 24 05:20:36.776548 IP6 fe80::54:ff:fe12:ea9a > ff02::1:ff01:7: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff01:7, length 24 05:20:36.781910 IP6 fe80::54:ff:fe8f:6dd > ff02::1:ff00:3: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff00:3, length 24 05:20:36.865475 IP6 fe80::21c:c0ff:fe4a:ae9f > ff02::1:ff4a:ae9f: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff4a:ae9f, length 24 05:20:36.908333 IP6 fe80::dcad:45ff:fe90:84db > ff02::1:ff90:84db: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff90:84db, length 24 05:20:36.919653 IP6 fe80::54:ff:fe12:ea9a > ff02::1:ff00:7: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff00:7, length 24 05:20:36.924276 IP6 fe80::59a2:2a4a:2082:6dee > ff02::1:ff82:6dee: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff82:6dee, length 24 05:20:37.001905 IP6 fe80::54:ff:fe8f:6dd > ff02::1:ff8f:6dd: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff8f:6dd, length 24 05:20:37.042403 IP6 fe80::54:ff:fe95:54f2 > ff02::1:ff95:54f2: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff95:54f2, length 24 05:20:37.090992 IP6 fe80::21c:c0ff:fe77:62ac > ff02::1:ff77:62ac: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff77:62ac, length 24 05:20:37.098118 IP6 fe80::d63d:7eff:fe01:b67f > ff02::1:ff01:b67f: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff01:b67f, length 24 05:20:37.118784 IP6 fe80::54:ff:fe12:ea9a > ff02::202: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::202, length 24 05:20:37.168548 IP6 fe80::54:ff:fe02:1d31 > ff02::1:ff02:1d31: HBH ICMP6, multicast listener reportmax resp delay: 0 addr: ff02::1:ff02:1d31, length 24 05:20:41.743286 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 1, length 64 05:20:41.743542 IP dev.mydomain.com > ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in: ICMP echo reply, id 3463, seq 1, length 64 05:20:42.743859 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 2, length 64 05:20:42.743952 IP dev.mydomain.com > ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in: ICMP echo reply, id 3463, seq 2, length 64 05:20:43.745689 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 3, length 64 05:20:43.745777 IP dev.mydomain.com > ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in: ICMP echo reply, id 3463, seq 3, length 64 05:20:44.746706 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 4, length 64 05:20:44.746796 IP dev.mydomain.com > ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in: ICMP echo reply, id 3463, seq 4, length 64 05:20:45.747986 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 5, length 64 05:20:45.748082 IP dev.mydomain.com > ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in: ICMP echo reply, id 3463, seq 5, length 64 05:20:46.749834 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 6, length 64 05:20:46.749920 IP dev.mydomain.com > ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in: ICMP echo reply, id 3463, seq 6, length 64 05:20:47.750838 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 7, length 64 05:20:47.751182 IP dev.mydomain.com > ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in: ICMP echo reply, id 3463, seq 7, length 64 05:20:48.751909 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 8, length 64 05:20:48.751991 IP dev.mydomain.com > ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in: ICMP echo reply, id 3463, seq 8, length 64 05:20:49.752542 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 9, length 64 05:20:49.752620 IP dev.mydomain.com > ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in: ICMP echo reply, id 3463, seq 9, length 64 05:20:50.754246 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 10, length 64 05:20:51.753856 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 11, length 64 05:20:52.752868 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 12, length 64 05:20:53.754174 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 13, length 64 05:20:54.753972 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 14, length 64 05:20:55.753814 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 15, length 64 05:20:56.753391 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 16, length 64 05:20:57.753683 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 17, length 64 05:20:58.753487 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 18, length 64 05:20:59.754013 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 19, length 64 05:21:00.753169 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 20, length 64 05:21:01.753757 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 21, length 64 05:21:02.753307 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 22, length 64 05:21:03.753021 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 23, length 64 05:21:04.753628 IP ABTS-North-Dynamic-226.X.X.122.airtelbroadband.in > dev.mydomain.com: ICMP echo request, id 3463, seq 24, length 64 ^C479 packets captured 718 packets received by filter 238 packets dropped by kernel 3 packets dropped by interface You see the ping request is not responed to initially, then for a moment it is replied back and then again no reply. I have tried everything (to the best of my knowledge) to fix this, but can't find any answer Any help will be greatly appreciated Thanks.

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• Dynamic Class Inheritance For PHP

  - by VirtuosiMedia

  I have a situation where I think I might need dynamic class inheritance in PHP 5.3, but the idea doesn't sit well and I'm looking for a different design pattern to solve my problem if it's possible. Use Case I have a set of DB abstraction layer classes that dynamically compiles SQL queries, with one DAL class for each DB type (MySQL, MsSQL, Oracle, etc.). Each table in the database has its own class that extends the appropriate DAL class. The idea is that you interact with the table classes, but never directly use the DAL class. If you want to support a different DB type for your app, you don't need to rewrite any queries or even any code, you simply change a setting that swaps one DAL class out for another...and that's it. To give you a better idea of how this is used, you can take a look at the DAL class, the table classes, and how they are used on this StackExchange Code Review page. To really understand what I'm trying to do, please take a look at my implementation first before suggesting a solution. Issues The strategy that I had used previously was to have all of the DAL classes share the same class name. This eliminated autoloading, so I had to manually load the appropriate DAL class in a switch statement. However, this approach presents some problems for testing and documentation purposes, so I'd like to find a different way to solve the problem of loading the correct DAL class more elegantly. Update to clarify the issue The problem basically boils down to inconsistencies in the class name (pre-PHP 5.3) or class namespace (PHP 5.3) and its location in the directory structure. At this point, all of my DAL classes have the same name, DBObject, but reside in different folders, MySQL, Oracle, etc. My table classes all extend DBObject, but which DBObject they extend varies depending on which one has been loaded. Basically, I'm trying to have my cake and eat it too. The table classes act as a stable API and extend a dynamic backend, the DAL (DBObject) classes. It works great, but I outsmarted myself and because of the inconsistencies with the class names and their locations, I can't autoload the DBObject, which makes running unit tests and generating API docs impossible for the DBObject classes because the tests and docs rely on auto-loading. Just loading the appropriate DBObject into memory using a factory method won't work because there will be times when I need to load multiple DBObjects for testing. Because the classes currently share a name, this causes a class is already defined error. I can make exceptions for the DBObjects in my test code, obviously, but I'm looking for something a little less hacky as there may future instances where something similar would need to be done. Solutions? Worst case scenario, I can continue my current strategy, but I don't like it very much, especially as I'll soon be converting my code to PHP 5.3. I suspect that I can use some sort of dynamic inheritance via either namespaces (preferred) or a dynamic class extension, but I haven't been able to find good examples of this implemented in the wild. In your answers, please suggest either an alternate pattern that would work for this use case or an example of dynamic inheritance done right. Please assume PHP 5.3 with namespaced code. Any code examples are greatly encouraged. The preferred constraints for the solution are: DAL class can be autoloaded. DAL classes don't share the same exact same namespace, but share the same class name. As an example, I would prefer to use classes named DbObject that use namespaces like Vm\Db\MySql and Vm\Db\Oracle. Table classes don't have to be rewritten with a change in DB type. The appropriate DB type is determined via a single setting only. That setting is the only thing that should need to change to interchange DB types. Ideally, the setting check should occur only once per page load, but I'm flexible on that.

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