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• LINQ query checks for null

  - by user300992

  I have a userList, some users don't have a name (null). If I run the first LINQ query, I got an error saying "object reference not set to an instance of an object" error. var temp = (from a in userList where ((a.name == "john") && (a.name != null)) select a).ToList(); However, if I switch the order by putting the checking for null in front, then it works without throwing any error: var temp = (from a in userList where ((a.name != null) && (a.name == "john")) select a).ToList(); Why is that? If that's pure C# code (not LINQ), I think both would be the same. I don't have SQL profiler, I am just curious what will be the difference when they are being translated on SQL level.

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• Linq insert statement inserts nothing, does not fail either

  - by pietjepoeier

  I am trying to insert a new account in my Acccounts table with linq. I tried using the EntityModel and Linq2Sql. I get no insert into my database nor an exception of any kind. public static Linq2SQLDataContext dataContext { get { return new Linq2SQLDataContext(); } } try { //EntityModel Accounts acc = Accounts.CreateAccounts(0, Voornaam, Straat, Huisnummer, Stad, Land, 15, EmailReg, Password1); Entities.AddToAccounts(acc); Entities.SaveChanges(); //Linq 2 SQL Account account = new Account { City = Stad, Country = Land, EmailAddress = EmailReg, Name = Voornaam, Password = Password1, Street = Straat, StreetNr = Huisnummer, StreetNrAdd = Toevoeging, Points = 25 }; dataContext.Accounts.InsertOnSubmit(account); var conf = dataContext.ChangeConflicts; // No changeConflicts ChangeSet set = dataContext.GetChangeSet(); // 0 inserts, 0 updates, 0 deletes try { dataContext.SubmitChanges(); } catch (Exception ex) { } } catch (EntityException ex) { }

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• How to dynamically choose two fields from a Linq query as a result

  - by Dr. Zim

  If you have a simple Linq query like: var result = from record in db.Customer select new { Text = record.Name, Value = record.ID.ToString() }; which is returning an object that can be mapped to a Drop Down List, is it possible to dynamically specify which fields map to Text and Value? Of course, you could do a big case (switch) statement, then code each Linq query separately but this isn't very elegant. What would be nice would be something like: (pseudo code) var myTextField = db.Customer["Name"]; // Could be an enumeration?? var myValueField = db.Customer["ID"]; // Idea: choose the field outside the query var result = from record in db.Customer select new { Text = myTextField, Value = myValueField };

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• Changing an extention method from linq-to-sql to entity framework

  - by Jova

  I'm changing my project from working with Linq-to-SQL to working with Entity Framework. I have some extention methods that extend the classes created by LINQ and I'm wondering how to change them to work with the entities instead Here's an example. public static int GetPublishedArticlesCount(this Table<Article> source) { return GetPublishedArticles(source.Context as DataContext, null).Count(); } This method gets the number of published articles. Instead of using this Table<Article>, what should I use instead?

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• Linq to Sql Projection Help

  - by Micah

  I've reached the end of my Linq rope. Need your help! Heres my table structure first(all linq to sql objects): InventoryItems -ID -AmtInStock IventoryKits -ID InventoryKits_to_InventoryItems -InventoryItemID -InventoryKitID So i need to do a projection like the following var q2=from k in GetAllKits()//returns IQueryable<InventoryKit> select new VMPublication()//ViewModel Object { ID = k.ID, Name = k.Name, WebAmountInStock = ,//need to get the Min() AmtInStock from InventoryItems here ItemCode = k.ItemCode, WebAmountOrdered = k.AmtOrdered.ToString(), WebReminderAmount = "", WebAmountWarning="", Type = "Kit" }; i have no idea how to get that Min() of InventoryItem's AmtInStock in that query. Please help! Very Appreciated!

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• Case insensitive string compare in LINQ-to-SQL

  - by BlueMonkMN

  I've read that it's unwise to use ToUpper and ToLower to perform case-insensitive string comparisons, but I see no alternative when it comes to LINQ-to-SQL. The ignoreCase and CompareOptions arguments of String.Compare are ignored by LINQ-to-SQL (if you're using a case-sensitive database, you get a case-sensitive comparison even if you ask for a case-insensitive comparison). Is ToLower or ToUpper the best option here? Is one better than the other? I thought I read somewhere that ToUpper was better, but I don't know if that applies here. (I'm doing a lot of code reviews and everyone is using ToLower.) Dim s = From row In context.Table Where String.Compare(row.Name, "test", StringComparison.InvariantCultureIgnoreCase) = 0 This translates to an SQL query that simply compares row.Name with "test" and will not return "Test" and "TEST" on a case-sensitive database.

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• Using LINQ-To-Entities to Generate Information

  - by parminder

  I am working on a website where a user can add tags to their posted books, much like is currently done for questions on Stack Overflow. Classes: Books { bookId, Title } Tags { Id Tag } BooksTags { Id BookId TagId } Here are few sample records. Books BookId Title 113421 A 113422 B Tags Id Tag 1 ASP 2 C# 3 CSS 4 VB 5 VB.NET 6 PHP 7 java 8 pascal BooksTags Id BookId TagId 1 113421 1 2 113421 2 3 113421 3 4 113421 4 5 113422 1 6 113422 4 7 113422 8 Questions I need to write something in LINQ to entity queries which gives me data according to the tags: Query: bookIds where tagid = 1 Returns: bookid: 113421, 113422 Query 2: tags 1 and 2 Returns: 113421 I need tags and their count to to show in related tags, so in first case my related tags class should have following result. RelatedTags Tag Count 2 1 3 1 4 2 8 1 Second Case: RelatedTags Tag Count 3 1 4 1 How do I do this in LINQ?

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• How to update an element with a List using LINQ and C#

  - by Addie

  I have a list of objects and I'd like to update a particular member variable within one of the objects. I understand LINQ is designed for query and not meant to update lists of immutable data. What would be the best way to accomplish this? I do not need to use LINQ for the solution if it is not most efficient. Would creating an Update extension method work? If so how would I go about doing that? EXAMPLE: (from trade in CrudeBalancedList where trade.Date.Month == monthIndex select trade).Update( trade => trade.Buy += optionQty);

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• Can't enumerate LinQ results with left join

  - by nvtthang

  var itemSet = from item in da.GetList<Models.account>() join file in objFileStorageList on item.account_id equals file.parent_id into objFile from fileItem in objFile.DefaultIfEmpty() where item.company != null && item.company.company_id == 123 orderby item.updatedDate descending select new { Id = item.account_id, RefNo = item.refNo, StartDate = item.StartDate , EndDate = item.EndDate , Comment = item.comment, FileStorageID = fileItem != null ? fileItem.fileStorage_id : -1, Identification = fileItem != null ? fileItem.identifier : null, fileName = fileItem != null ? fileItem.file_nm : null }; It raises error message when I try to enumerate through collection result from Linq query above. LINQ to Entities does not recognize the method 'System.Collections.Generic.IEnumerable1[SCEFramework.Models.fileStorage] DefaultIfEmpty[fileStorage](System.Collections.Generic.IEnumerable1[SCEFramework.Models.fileStorage])' method, and this method cannot be translated into a store expression foreach (var item in itemSet) { string itemRef= item.RefNo; } Please suggest me any solutions. Thanks in advance.

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• Use LINQ and C# to make a new List from an old List

  - by Addie

  This should be pretty simple, but I am new at LINQ. I have a List of FillList structs. I'd like to use LINQ to create a new List where instead of having the number of buys and sells, I would have one variable containing the sum. For example, if the FillStruct structure has buy = 4 and sell = 2 then the NewFillStruct structure will have numlong = 2. If the FillStruct structure has buy = 2 and sell = 4 then the NewFillStruct structure will have numlong = -2. Here are the structures. struct FillStruct { int buy; int sell; string date; } struct NewFillStruct { int numlong; string date; }

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• username and password check linq query in c#

  - by b0x0rz

  this linq query var users = from u in context.Users where u.UserEMailAdresses.Any(e1 => e1.EMailAddress == userEMail) && u.UserPasswords.Any(e2 => e2.PasswordSaltedHash == passwordSaltedHash) select u; return users.Count(); returns: 1 even when there is nothing in password table. how come? what i am trying to do is get the values of email and passwordHash from two separate tables (UserEMailAddresses and UserPasswords) linked via foreign keys to the third table (Users). it should be simple - checking if email and password mach from form to database. but it is not working for me. i get 1 (for count) even when there are NO entries in the UserPasswords table. is the linq query above completely wrong, or...?

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• Building 'flat' rather than 'tree' LINQ expressions

  - by Ian Gregory

  I'm using some code (available here on MSDN) to dynamically build LINQ expressions containing multiple OR 'clauses'. The relevant code is var equals = values.Select(value => (Expression)Expression.Equal(valueSelector.Body, Expression.Constant(value, typeof(TValue)))); var body = equals.Aggregate<Expression>((accumulate, equal) => Expression.Or(accumulate, equal)); This generates a LINQ expression that looks something like this: (((((ID = 5) OR (ID = 4)) OR (ID = 3)) OR (ID = 2)) OR (ID = 1)) I'm hitting the recursion limit (100) when using this expression, so I'd like to generate an expression that looks like this: (ID = 5) OR (ID = 4) OR (ID = 3) OR (ID = 2) OR (ID = 1) How would I modify the expression building code to do this?

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• How to order the items in a nested LINQ-provided collection

  - by Carson McComas

  I've got a (SQL Server) database table called Category. And another database table called SubCategory. SubCategory has a foreign key relationship to Category. Because of this, thanks to LINQ, each Cateogory has a property called SubCategories and LINQ is nice enough to return all the SubCategories associated with my Category when I grab it. If I want to sort the Categories alphabetically, I can just do: return db.Categories.OrderBy(c => c.Name); However, I have no idea how to order the SubCategories collection inside each Category. My goal is to return a collection of Categories, where all of the SubCategory collections inside of them are ordered alphabetically by Name.

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• Convert SQL to LINQ to SQL

  - by Adam

  Hi I have the SQL query with c as ( select categoryId,parentId, name,0 as [level] from task_Category b where b.parentId is null union all select b.categoryId,b.parentId,b.name,[level] + 1 from task_Category b join c on b.parentId = c.categoryId) select name,[level],categoryId,parentId as item from c and I want to convert it to LINQ to SQL, yet my LINQ skills are not there yet. Could someone please help me convert this. It's the with and union statements that are making this a bit more complex for me. Any help appreciated.

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• LINQ Equivalent query

  - by GilliVilla

  I have a List<string> List<string> students; students.Add("123Rob"); students.Add("234Schulz"); and a Dictionary<string,string> Dictionary<string, string> courses = new Dictionary<string, string>(); courses .Add("Rob", "Chemistry"); courses .Add("Bob", "Math"); courses .Add("Holly", "Physics"); courses .Add("Schulz", "Botany"); My objective now is to get a List with the values - {Chemistry,Botany} . In other words, I am trying to get the LINQ equivalent of select value from [courses] where [courses].key in ( select [courses].key from courses,students where [students].id LIKE '%courses.key%' ) How should the LINQ query be framed?

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• Using condionals 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; }

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• Help with Linq and Generics

  - by Jonathan

  Hi to all. I'm triying to make a function that add a 'where' clause to a query based in a property and a value. This is a very simplefied version of my function. Private Function simplified(ByVal query As IQueryable(Of T), ByVal PValue As Long, ByVal p As PropertyInfo) As ObjectQuery(Of T) query = query.Where(Function(c) DirectCast(p.GetValue(c, Nothing), Long) = PValue) Dim t = query.ToList 'this line is only for testing, and here is the error raise Return query End Function The error message is: LINQ to Entities does not recognize the method 'System.Object CompareObjectEqual(System.Object, System.Object, Boolean)' method, and this method cannot be translated into a store expression. Looks like a can't use GetValue inside a linq query. Can I achieve this in other way? Post your answer in C#/VB. Chose the one that make you feel more confortable. Thanks

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• Translating this query in LINQ ? (list of like)

  - by Erick

  I would like to translate this query in LINQ ... it is very easy to construct if we do it in pure SQL but in dynamically created LINQ query (building a search query based on user input) it's a whole new story. SELECT * FROM MyTable WHERE 1=1 AND Column2 IN (1,2,3) AND ( Column1 LIKE '%a%' OR Column1 LIKE '%b%' ) Now to try to construct this we tried it this way : if(myOjb.Column2Collection != null) query = query.where(f => f.Column2.Contains(myOjb.Column2Collection)); if(myObj.Column1Collection != null) { // tough part here ? //query = query.Where(); ... } So what would be the best aproach to this normally ? Note that I am aware of the SqlMethod.Like, tho I can't figure a way to implement it here ...

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• Linq Query - Average Time (DateTime data types)

  - by Jade

  I have a database that has the following records in a DateTime field: 2012-04-13 08:31:00.000 2012-04-12 07:53:00.000 2012-04-11 07:59:00.000 2012-04-10 08:16:00.000 2012-04-09 15:11:00.000 2012-04-08 08:28:00.000 2012-04-06 08:26:00.000 I want to run a linq to sql query to get the average time from the records above. I tried the following: (From o In MYDATA Select o.SleepTo).Average() Since "SleepTo" is a datetime field I get an error on Average(). If I was trying to get the average of say an integer, the above linq query works. What do I need to do to get it to work for datetimes?

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• Deferred vs Immediate execution in Linq

  - by Jalpesh P. Vadgama

  In this post, We are going to learn about Deferred vs Immediate execution in Linq.  There an interesting variations how Linq operators executes and in this post we are going to learn both Deferred execution and immediate execution. What is Deferred Execution? In the Deferred execution query will be executed and evaluated at the time of query variables usage. Let’s take an example to understand Deferred Execution better. Example: Following is a code for that. using System; using System.Collections.Generic; using System.Linq; namespace ConsoleApplication1 { class Program { static void Main(string[] args) { var customers = new List<Customer>( new[] { new Customer{FirstName = "Jalpesh",LastName = "Vadgama"}, new Customer{FirstName = "Vishal",LastName = "Vadgama"}, new Customer{FirstName = "Tushar",LastName = "Maru"} } ); var newCustomers = customers.Where(c => c.LastName == "Vadgama"); customers.Add(new Customer {FirstName = "Teerth", LastName = "Vadgama"}); foreach (var c in newCustomers) { Console.WriteLine(c.FirstName); } } public class Customer { public string FirstName { get; set; } public string LastName { get; set; } } } } More on my personal blog @www.dotnetjalps.com

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• LINQ: Enhancing Distinct With The PredicateEqualityComparer

  - by Paulo Morgado

  Today I was writing a LINQ query and I needed to select distinct values based on a comparison criteria. Fortunately, LINQ’s Distinct method allows an equality comparer to be supplied, but, unfortunately, sometimes, this means having to write custom equality comparer. Because I was going to need more than one equality comparer for this set of tools I was building, I decided to build a generic equality comparer that would just take a custom predicate. Something like this: public class PredicateEqualityComparer<T> : EqualityComparer<T> { private Func<T, T, bool> predicate; public PredicateEqualityComparer(Func<T, T, bool> predicate) : base() { this.predicate = predicate; } public override bool Equals(T x, T y) { if (x != null) { return ((y != null) && this.predicate(x, y)); } if (y != null) { return false; } return true; } public override int GetHashCode(T obj) { if (obj == null) { return 0; } return obj.GetHashCode(); } } Now I can write code like this: .Distinct(new PredicateEqualityComparer<Item>((x, y) => x.Field == y.Field)) But I felt that I’d lost all conciseness and expressiveness of LINQ and it doesn’t support anonymous types. So I came up with another Distinct extension method: public static IEnumerable<TSource> Distinct<TSource>(this IEnumerable<TSource> source, Func<TSource, TSource, bool> predicate) { return source.Distinct(new PredicateEqualityComparer<TSource>(predicate)); } And the query is now written like this: .Distinct((x, y) => x.Field == y.Field) Looks a lot better, doesn’t it?

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• Distinct operator in Linq

  - by Jalpesh P. Vadgama

  Linq operator provides great flexibility and easy way of coding. Let’s again take one more example of distinct operator. As name suggest it will find the distinct elements from IEnumerable. Let’s take an example of array in console application and then we will again print array to see is it working or not. Below is the code for that. In this application I have integer array which contains duplicate elements and then I will apply distinct operator to this and then I will print again result of distinct operators to actually see whether its working or not. using System; using System.Collections.Generic; using System.Linq; using System.Text; namespace Experiment { class Program { static void Main(string[] args) { int[] intArray = { 1, 2, 2, 3, 3, 3, 4, 4, 4, 4, 5, 5, 5, 5, 5 }; var uniqueIntegers = intArray.Distinct(); foreach (var uInteger in uniqueIntegers) { Console.WriteLine(uInteger); } Console.ReadKey(); } } } Below is output as expected.. That’s cool..Stay tuned for more.. Happy programming. Technorati Tags: Linq,Distinct

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• Is LINQ to objects a collection of combinators?

  - by Jimmy Hoffa

  I was just trying to explain the usefulness of combinators to a colleague and I told him LINQ to objects are like combinators as they exhibit the same value, the ability to combine small pieces to create a single large piece. Though I don't know that I can call LINQ to objects combinators. I've seen 2 levels of definition for combinator that I generalize as such: A combinator is a function which only uses things passed to it A combinator is a function which only uses things passed to it and other standard atomic functions but not state The first is very rigid and can be seen in the combinatory calculus systems and in haskell things like $ and . and various similar functions meet this rule. The second is less rigid and would allow something like sum as it uses the + function which was not passed in but is standard and not stateful. Though the LINQ extensions in C# use state in their iteration models, so I feel I can't say they're combinators. Can someone who understands the definition of a combinator more thoroughly and with more experience in these realms give a distinct ruling on this? Are my definitions of 'combinator' wrong to begin with?

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• Simplify Your Code with LINQ

  - by dwahlin

  I’m a big fan of LINQ and use it wherever I can to minimize code and make applications easier to maintain overall. I was going through a code file today refactoring it based on suggestions provided by Resharper and came across the following method: private List<string> FilterTokens(List<string> tokens) { var cleanedTokens = new List<string>(); for (int i = 0; i < tokens.Count; i++) { string token = tokens[i]; if (token != null) { cleanedTokens.Add(token); } } return cleanedTokens; }   In looking through the code I didn’t see anything wrong but Resharper was suggesting that I convert it to a LINQ expression: In thinking about it more the suggestion made complete sense because I simply wanted to add all non-null token values into a List<string> anyway. After following through with the Resharper suggestion the code changed to the following. Much, much cleaner and yet another example of why LINQ (and Resharper) rules: private List<string> FilterTokens(IEnumerable<string> tokens) { return tokens.Where(token => token != null).ToList(); }

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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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