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• Is post-sudden-power-loss filesystem corruption on an SSD drive's ext3 partition "expected behavior"?

  - by Jeremy Friesner

  My company makes an embedded Debian Linux device that boots from an ext3 partition on an internal SSD drive. Because the device is an embedded "black box", it is usually shut down the rude way, by simply cutting power to the device via an external switch. This is normally okay, as ext3's journalling keeps things in order, so other than the occasional loss of part of a log file, things keep chugging along fine. However, we've recently seen a number of units where after a number of hard-power-cycles the ext3 partition starts to develop structural issues -- in particular, we run e2fsck on the ext3 partition and it finds a number of issues like those shown in the output listing at the bottom of this Question. Running e2fsck until it stops reporting errors (or reformatting the partition) clears the issues. My question is... what are the implications of seeing problems like this on an ext3/SSD system that has been subjected to lots of sudden/unexpected shutdowns? My feeling is that this might be a sign of a software or hardware problem in our system, since my understanding is that (barring a bug or hardware problem) ext3's journalling feature is supposed to prevent these sorts of filesystem-integrity errors. (Note: I understand that user-data is not journalled and so munged/missing/truncated user-files can happen; I'm specifically talking here about filesystem-metadata errors like those shown below) My co-worker, on the other hand, says that this is known/expected behavior because SSD controllers sometimes re-order write commands and that can cause the ext3 journal to get confused. In particular, he believes that even given normally functioning hardware and bug-free software, the ext3 journal only makes filesystem corruption less likely, not impossible, so we should not be surprised to see problems like this from time to time. Which of us is right? Embedded-PC-failsafe:~# ls Embedded-PC-failsafe:~# umount /mnt/unionfs Embedded-PC-failsafe:~# e2fsck /dev/sda3 e2fsck 1.41.3 (12-Oct-2008) embeddedrootwrite contains a file system with errors, check forced. Pass 1: Checking inodes, blocks, and sizes Pass 2: Checking directory structure Invalid inode number for '.' in directory inode 46948. Fix<y>? yes Directory inode 46948, block 0, offset 12: directory corrupted Salvage<y>? yes Entry 'status_2012-11-26_14h13m41.csv' in /var/log/status_logs (46956) has deleted/unused inode 47075. Clear<y>? yes Entry 'status_2012-11-26_10h42m58.csv.gz' in /var/log/status_logs (46956) has deleted/unused inode 47076. Clear<y>? yes Entry 'status_2012-11-26_11h29m41.csv.gz' in /var/log/status_logs (46956) has deleted/unused inode 47080. Clear<y>? yes Entry 'status_2012-11-26_11h42m13.csv.gz' in /var/log/status_logs (46956) has deleted/unused inode 47081. Clear<y>? yes Entry 'status_2012-11-26_12h07m17.csv.gz' in /var/log/status_logs (46956) has deleted/unused inode 47083. Clear<y>? yes Entry 'status_2012-11-26_12h14m53.csv.gz' in /var/log/status_logs (46956) has deleted/unused inode 47085. Clear<y>? yes Entry 'status_2012-11-26_15h06m49.csv' in /var/log/status_logs (46956) has deleted/unused inode 47088. Clear<y>? yes Entry 'status_2012-11-20_14h50m09.csv' in /var/log/status_logs (46956) has deleted/unused inode 47073. Clear<y>? yes Entry 'status_2012-11-20_14h55m32.csv' in /var/log/status_logs (46956) has deleted/unused inode 47074. Clear<y>? yes Entry 'status_2012-11-26_11h04m36.csv.gz' in /var/log/status_logs (46956) has deleted/unused inode 47078. Clear<y>? yes Entry 'status_2012-11-26_11h54m45.csv.gz' in /var/log/status_logs (46956) has deleted/unused inode 47082. Clear<y>? yes Entry 'status_2012-11-26_12h12m20.csv.gz' in /var/log/status_logs (46956) has deleted/unused inode 47084. Clear<y>? yes Entry 'status_2012-11-26_12h33m52.csv.gz' in /var/log/status_logs (46956) has deleted/unused inode 47086. Clear<y>? yes Entry 'status_2012-11-26_10h51m59.csv.gz' in /var/log/status_logs (46956) has deleted/unused inode 47077. Clear<y>? yes Entry 'status_2012-11-26_11h17m09.csv.gz' in /var/log/status_logs (46956) has deleted/unused inode 47079. Clear<y>? yes Entry 'status_2012-11-26_12h54m11.csv.gz' in /var/log/status_logs (46956) has deleted/unused inode 47087. Clear<y>? yes Pass 3: Checking directory connectivity '..' in /etc/network/run (46948) is <The NULL inode> (0), should be /etc/network (46953). Fix<y>? yes Couldn't fix parent of inode 46948: Couldn't find parent directory entry Pass 4: Checking reference counts Unattached inode 46945 Connect to /lost+found<y>? yes Inode 46945 ref count is 2, should be 1. Fix<y>? yes Inode 46953 ref count is 5, should be 4. Fix<y>? yes Pass 5: Checking group summary information Block bitmap differences: -(208264--208266) -(210062--210068) -(211343--211491) -(213241--213250) -(213344--213393) -213397 -(213457--213463) -(213516--213521) -(213628--213655) -(213683--213688) -(213709--213728) -(215265--215300) -(215346--215365) -(221541--221551) -(221696--221704) -227517 Fix<y>? yes Free blocks count wrong for group #6 (17247, counted=17611). Fix<y>? yes Free blocks count wrong (161691, counted=162055). Fix<y>? yes Inode bitmap differences: +(47089--47090) +47093 +47095 +(47097--47099) +(47101--47104) -(47219--47220) -47222 -47224 -47228 -47231 -(47347--47348) -47350 -47352 -47356 -47359 -(47457--47488) -47985 -47996 -(47999--48000) -48017 -(48027--48028) -(48030--48032) -48049 -(48059--48060) -(48062--48064) -48081 -(48091--48092) -(48094--48096) Fix<y>? yes Free inodes count wrong for group #6 (7608, counted=7624). Fix<y>? yes Free inodes count wrong (61919, counted=61935). Fix<y>? yes embeddedrootwrite: ***** FILE SYSTEM WAS MODIFIED ***** embeddedrootwrite: ********** WARNING: Filesystem still has errors ********** embeddedrootwrite: 657/62592 files (24.4% non-contiguous), 87882/249937 blocks Embedded-PC-failsafe:~# Embedded-PC-failsafe:~# e2fsck /dev/sda3 e2fsck 1.41.3 (12-Oct-2008) embeddedrootwrite contains a file system with errors, check forced. Pass 1: Checking inodes, blocks, and sizes Pass 2: Checking directory structure Directory entry for '.' in ... (46948) is big. Split<y>? yes Missing '..' in directory inode 46948. Fix<y>? yes Setting filetype for entry '..' in ... (46948) to 2. Pass 3: Checking directory connectivity '..' in /etc/network/run (46948) is <The NULL inode> (0), should be /etc/network (46953). Fix<y>? yes Pass 4: Checking reference counts Inode 2 ref count is 12, should be 13. Fix<y>? yes Pass 5: Checking group summary information embeddedrootwrite: ***** FILE SYSTEM WAS MODIFIED ***** embeddedrootwrite: 657/62592 files (24.4% non-contiguous), 87882/249937 blocks Embedded-PC-failsafe:~# Embedded-PC-failsafe:~# e2fsck /dev/sda3 e2fsck 1.41.3 (12-Oct-2008) embeddedrootwrite: clean, 657/62592 files, 87882/249937 blocks

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• MSDTC attempts to enlist client machine in a distributed transaction

  - by Ken

  Hi there We're seeing the following intermittent warning logged by MSDTC: A caller has attempted to propagate a transaction to a remote system, but MSDTC network DTC access is currently disabled on machine 'X'. Please review the MS DTC configuration settings. However, MSDTC is disabled on machine X by design - it's a client machine, and has no business being enlisted in the transaction! Several windows service endpoints hosting WCF services over TCP Single SQL Server 2005 instance beneath Linq to Sql Remote client receives event callbacks over WCF/TCP The issue is tricky to reproduce - usually following restart of services. We suspect a callback to the client machine is occurring within the context of a transaction. Just wondering if anyone has seen similar issues?? Ken

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• C# convert an IOrderedEnumerable<KeyValuePair<string, int>> into a Dictionary<string, int>

  - by Kache4

  I was following the answer to another question, and I got: // itemCounter is a Dictionary<string, int>, and I only want to keep // key/value pairs with the top maxAllowed values if (itemCounter.Count > maxAllowed) { IEnumerable<KeyValuePair<string, int>> sortedDict = from entry in itemCounter orderby entry.Value descending select entry; sortedDict = sortedDict.Take(maxAllowed); itemCounter = sortedDict.ToDictionary<string, int>(/* what do I do here? */); } Visual Studio's asking for a parameter Func<string, int> keySelector. I tried following a few semi-relevant examples I've found online and put in k => k.Key, but that gives a compiler error: 'System.Collections.Generic.IEnumerable<System.Collections.Generic.KeyValuePair<string,int>>' does not contain a definition for 'ToDictionary' and the best extension method overload 'System.Linq.Enumerable.ToDictionary<TSource,TKey>(System.Collections.Generic.IEnumerable<TSource>, System.Func<TSource,TKey>)' has some invalid arguments

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• C# select elements from IList

  - by user313884

  I have a list of objects: IList<O> O has several properties but only two of them are relevant: Date and Duration I want to "split" the list into several lists that contain only the objects that have matching Date and Duration Properties. Example: InitialList: 0- Date==1, Duration==7 1- Date==1, Duration==7 2- Date==2, Duration==7 3- Date==2, Duration==7 4- Date==2, Duration==14 5- Date==2, Duration==14 Desired result: IList<IList<O>: 0- 0- Date==1, Duration==7 1- Date==1, Duration==7 1- 0- Date==2, Duration==7 1- Date==2, Duration==7 2- 0- Date==2, Duration==14 1- Date==2, Duration==14 i know this can be done with some linq selects but am not sure how. thanks for the help.

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• WCF "DataContext accessed after Dispose"

  - by David Ward

  I have an application with numerous WCF services that make use of LINQ-To-SQL as the data access model. I am having lots of problems with the "DataContext accessed after Dispose" exception. I understand what this exception is and that it is occurring because I have not "initialised" the data that is trying to be accessed. I've read many articles that suggest that I called ToList() on any arrays before the parent object is returned by the service. My issue is that I am getting this exception and I don't know where it is originating from and therefore I don't know what hasn't been initialised. Can anyone advise how best to identify the root cause? (I have used the MS Service Trace Viewer and this doesn't seem to give me any further information)

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• Explicit construction of entity type [MyClass] in query is not allowed.

  - by Code Sherpa

  Hi. Like the title says, I have the following exception: Description: Event code: 3005 Event message: An unhandled exception has occurred. Exception information: Exception type: NotSupportedException Exception message: Explicit construction of entity type 'Company.Project.Core.Domain.Friend' in query is not allowed. I am using LINQ and have the following code in my datacontext: var friends2 = (dc.Friends .Where(f => f.MyFriendsAccountId == accountId && f.AccountId != accountId) .Select(f => new { f.FriendId, AccountId = f.MyFriendsAccountId, MyFriendsAccountId = f.AccountId, f.CreateDate, f.Timestamp })).Distinct(); result.AddRange(friends2 .Select(o => new Friend { FriendId = o.FriendId, AccountId = o.AccountId, CreateDate = o.CreateDate, MyFriendsAccountId = o.MyFriendsAccountId, Timestamp = o.Timestamp })); the final code block is throwing the error and I am pretty sure it is this statement that is the culprit: .Select( o => **new Friend** How should I be reworking my code to avoid this error? Code illustration appreciated. Thanks.

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• Dynamic Data Associate Related Table Value?

  - by davemackey

  I have create a LINQ-to-SQL project in Visual Studio 2010 using Dynamic Data. In this project I have two tables. One is called phones_extension and the other phones_ten. The list of columns in phones_extension looks like this: id, extension, prefix, did_flag, len, ten_id, restriction_class_id, sfc_id, name_display, building_id, floor, room, phone_id, department_id In phones_ten it looks like this: id, name, pbxid Now, I'd like to be able to somehow make it so that there is an association (or inheritance?) that essentially results in me being able to make a query like phones_extension.ten and it gives me the result of phones_ten.name. Right now I have to get phones_extension.ten_id and then match that against phones_ten.id - I'm trying to get the DBML to handle this translation automatically. Is this possible?

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• linq2sql : get generic type of table

  - by benpage

  i think this is a simple question but I've searched around and can't seem to find an answer easily. if you have var list = List<int>(); ... fill list ... and you want to get the generic type in list, i realise you could just type: var t = list.FirstOrDefault().GetType(); Is there another way to do this via just the list, rather than referring to the enumeration? Reason is, i have a System.Data.Linq.Table<TABLE1> and what i want to do is get the type of TABLE1 from it. so: var table = new DataContext().TABLE1s; // this is Table<TABLE1> var tableType = table.GetType().SomeMethod(); // i want tableType to equal TABLE1.GetType()

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• Query only the first detail record for each master record

  - by Neal S.

  If I have the following master-detail relationship: owner_tbl auto_tbl --------- -------- owner --- owner auto year And I have the following table data: owner_tbl auto_tbl --------- -------- john john, corvette, 1968 john, prius, 2008 james james, f-150, 2004 james, cadillac, 2002 james, accord, 2009 jeff jeff, tesla, 2010 jeff, hyundai, 1996 Now, I want to perform a query that returns the following result: john, corvette, 1968 jeff, hyundai, 1996 james, cadillac, 2002 The query should join the two tables, and sort all the records on the "year" field, but only return the first detail record for each master record. I know how to join the tables and sort on the "year" field, but it's not clear how (or if) I might be able to only retrieve the first joined record for each owner. Three related questions: Can I perform this kind of query using LINQ-to-SQL? Can I perform the query using T-SQL? Would it be best to just create a stored procedure for the query given its likely complexity?

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• linq2sql - where to enlist transaction (repository or bll)?

  - by Caroline Showden

  My app uses a business layer which calls a repository which uses linq to sql. I have an Item class that has an enum type property and an ItemDetail property. I need to implement a delete method that: (1) always delete the Item (2) if the item.type is XYZ and the ItemDetail is not null, delete the ItemDetail as well. My question is where should this logic be housed? If I have it in my business logic which I would prefer, this involves two separate repository calls, each of which uses a separate datacontext. I would have to wrap both calls is a System.Transaction which (in sql 2005) get promoted to a distributed transaction which is not ideal. I can move it all to a single repository call and the transaction will be handled implicitly by the datacontext but feel that this is really business logic so does not belong in the repository. Thoughts? Carrie

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• Is writing eSQL database agnostic?

  - by Robert Koritnik

  Using EF we can use LINQ to read data which is rather simple (especialy using fluent calls), but we have less control unless we write eSQL on our own. Is writing eSQL database actually data store independant code? So if we decide to change data store, can the same statements still be used? Is writing eSQL strings in your code pose any serious security threats similar to writing TSQL statements in plain strings? So we moved to SPs. Could we still mode eSQL scripts outside of code as well and use some other technique to make them a bit more secure?

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• ASP.NET MVC 2 RC2 Model Binding with NVARCHAR NOT NULL column

  - by Gary McGill

  I have a database column defined as NVARCHAR(1000) NOT NULL DEFAULT(N'') - in other words, a non-nullable text column with a default value of blank. I have a model class generated by the Linq-to-SQL Classes designer, which correctly identifies the property as not nullable. I have a TextAreaFor in my view for that property. I'm using UpdateModel in my controller to fetch the value from the form and populate the model object. If I view the web page and leave the text area blank, UpdateModel insists on setting the property to NULL instead of empty string. (Even if I set the value to blank in code prior to calling UpdateModel, it still overwrites that with NULL). Which, of course, causes the subsequent database update to fail. I could check all such properties for NULL after calling UpdateModel, but that seems ridiculous - surely there must be a better way? Please don't tell me I need a custom model binder for such a simple scenario...!

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• Sync Framework, Local Database Cache, and my DAL

  - by Refracted Paladin

  I am creating a WPF app that needs to allow users to work in a temporary disconnected state and I plan to use a Local Database Cache. My question's are about my data access layer. Do you typically create the whole DAL to point at the Cache or both and create a switching mechanism? Is Entity's a good way to go for my DAL against the Cache? I am used to L2S but my understanding is that I can't use that against SQLCE, correct? Thanks! PS: Any good resources out there for using Sync, Linq, and WPF? Tutorials, videos, etc?

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• Anonymous Methods / Lambda's (Coding Standards)

  - by Mystagogue

  In Jeffrey Richter's "CLR via C#" (the .net 2.0 edtion page, 353) he says that as a self-discipline, he never makes anonymous functions longer than 3 lines of code in length. He cites mostly readability / understandability as his reasons. This suites me fine, because I already had a self-discipline of using no more than 5 lines for an anonymous method. But how does that "coding standard" advice stack against lambda's? At face value, I'd treat them the same - keeping a lambda equally as short. But how do others feel about this? In particular, when lambda's are being used where (arguably) they shine brightest - when used in LINQ statements - is there genuine cause to abandon that self-discipline / coding standard?

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• IEnumerable<> to IList<>

  - by nachid

  I am using Linq to query my database and returning a generic IList. Whatever I tried I couldn't convert an IQueryable to an IList. Here is my code. I cannot write simpler than this and I don't understand why it is not working. public IList<IRegion> GetRegionList(string countryCode) { var query = from c in Database.RegionDataSource where (c.CountryCode == countryCode) orderby c.Name select new {c.RegionCode, c.RegionName}; return query.Cast<IRegion>().ToList(); } This returns an list with the right number of items but they are all empty Please help, I am bloqued with this for a couple of days now

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• LLBLGen: Copy table from one database to another

  - by StreamT

  I have two databases (SQL Server 2005) with the same table schemes. I need to copy data from source table to destination with some modification of data along the way. And if destination table already contains some data, then rows from source table should not override, but be added to the destination table. In our project we use LLBLGen and LINQ to LLBLGen to as ORM solution. Example: Table 1: Table 2: Table 1: Key Value Key Value Key Value 1 One 1 T2_One Result=> 1 One 2 Two 2 T2_Two 2 Two 3 Three 3 Three 4 T2_One 5 T2_Two

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• Problem in populating a dictionary using Enumerable.Range()

  - by Newbie

  If I do for (int i = 0; i < appSettings.Count; i++) { string key = appSettings.Keys[i]; euFileDictionary.Add(key, appSettings[i]); } It is working fine. When I am trying the same thing using Enumerable.Range(0, appSettings.Count).Select(i => { string Key = appSettings.Keys[i]; string Value = appSettings[i]; euFileDictionary.Add(Key, Value); }).ToDictionary<string,string>(); I am getting a compile time error The type arguments for method 'System.Linq.Enumerable.Select(System.Collections.Generic.IEnumerable, System.Func)' cannot be inferred from the usage. Try specifying the type arguments explicitly. Any idea? Using C#3.0 Thanks

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• where to enlist transaction with parent child delete (repository or bll)?

  - by Caroline Showden

  My app uses a business layer which calls a repository which uses linq to sql. I have an Item class that has an enum type property and an ItemDetail property. I need to implement a delete method that: (1) always delete the Item (2) if the item.type is XYZ and the ItemDetail is not null, delete the ItemDetail as well. My question is where should this logic be housed? If I have it in my business logic which I would prefer, this involves two separate repository calls, each of which uses a separate datacontext. I would have to wrap both calls is a System.Transaction which (in sql 2005) get promoted to a distributed transaction which is not ideal. I can move it all to a single repository call and the transaction will be handled implicitly by the datacontext but feel that this is really business logic so does not belong in the repository. Thoughts? Carrie

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• How to get all rows but specifc columns from a DataTable?

  - by Oliver

  Currently i am having some problems with getting some data out of a DataTable by selecting all rows, but only some columns. To be a little more descriptive here is a little example: Sample Data | ID | FirstName | LastName | Age | +----+-----------+----------+-----+ | 1 | Alice | Wannabe | 22 | | 2 | Bob | Consumer | 27 | | 3 | Carol | Detector | 25 | What i have So what we got from our GUI is a IEnumerable<DataColumn> selectedColumns and there we'll find two elements (FirstName and LastName). Now i need some result which contains all rows, but only the above two columns (or any other list of selected columns). So far i already used LINQ on several one dimensional objects, but this two dimensional object gives me a little headache. // The hard-coded way Table.AsEnumerable().Select(row => new { FirstName = row[1], LastName = row[2] }); // The flexible way Table.AsEnumerable().Select(row => row ???) But how can i now say, which columns from row should be selected by using my selectedColumns?

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• Is writing eSQL database independent or not?

  - by Robert Koritnik

  Using EF we can use LINQ to read data which is rather simple (especially using fluent calls), but we have less control unless we write eSQL on our own. Is writing eSQL actually data store independent code? So if we decide to change data store, can the same statements still be used? Does writing eSQL strings in your code pose any serious security threats similar to writing TSQL statements as plain strings in C# code? That's why SPs are recommended. Could we still move eSQL scripts outside of code and use some other technique to make them a bit more secure?

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• SubSonic IQueryable To String Array

  - by Daniel Draper

  Hey All, I have decided to use SubSonic (v3.0) for the first time and thoroughly enjoy it so far however I seem to have stumbled and I am hoping there is a nice neat solution. I have a users, roles and joining table. SubSonic (ActiveRecord) generated an entity User for my users table. A property of User is UserRoles and is of the type IQueryable this is my joining table. I want to convert the IQueryable column name RoleName to a string array. I have only just started playing with Linq as well and know of ToArray() but seem to be missing something or this isn't the function I want. I could iterate over each item in the UserRoles property but seems a little excessive. I appreciate your help! Cheers.

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• How can I generate an Expression tree that queries an object with List<T> as a property?

  - by David Robbins

  Forgive my clumsy explanation, but I have a class that contains a List: public class Document { public int OwnerId { get; set; } public List<User> Users { get; set; } public Document() { } } public class User { public string UserName { get; set; } public string Department { get; set; } } Currently I use PredicateBuilder to perform dynmica queries on my objects. How can I turn the following LINQ statement into an Expression Tree: var predicate= PredicateBuilder.True<User>(); predicate= predicate.And<User>(user => user.Deparment == "HR"); var deptDocs = documents.AsQueryable() .Where(doc => doc.Users .AsQueryable().Count(predicate) > 0) .ToList(); In other words var deptDocs = documents.HasUserAttributes("Department", "HR").ToList();

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• parsing expression trees with booleans

  - by Schotime

  I am trying to parse an expression tree for a linq provider and running into a little snag with booleans. I can parse this no problems. var p = products.Where(x=>x.IsAvailable == true).ToList(); however when its written like this? var p = products.Where(x=>x.IsAvailable).ToList(); i only get a MemberAccess to look at and i can't see how i deduce that it is true or false (!x.IsAvailable). Any help would be great. Thanks.

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• Mutating the expression tree of a predicate to target another type

  - by Jon

  Intro In the application I 'm currently working on, there are two kinds of each business object: the "ActiveRecord" type, and the "DataContract" type. So for example, we have: namespace ActiveRecord { class Widget { public int Id { get; set; } } } namespace DataContracts { class Widget { public int Id { get; set; } } } The database access layer takes care of "translating" between hierarchies: you can tell it to update a DataContracts.Widget, and it will magically create an ActiveRecord.Widget with the same property values and save that. The problem I have surfaced when attempting to refactor this database access layer. The Problem I want to add methods like the following to the database access layer: // Widget is DataContract.Widget interface DbAccessLayer { IEnumerable<Widget> GetMany(Expression<Func<Widget, bool>> predicate); } The above is a simple general-use "get" method with custom predicate. The only point of interest is that I 'm not passing in an anonymous function but rather an expression tree. This is done because inside DbAccessLayer we have to query ActiveRecord.Widget efficiently (LINQ to SQL) and not have the database return all ActiveRecord.Widget instances and then filter the enumerable collection. We need to pass in an expression tree, so we ask for one as the parameter for GetMany. The snag: the parameter we have needs to be magically transformed from an Expression<Func<DataContract.Widget, bool>> to an Expression<Func<ActiveRecord.Widget, bool>>. This is where I haven't managed to pull it off... Attempted Solution What we 'd like to do inside GetMany is: IEnumerable<DataContract.Widget> GetMany( Expression<Func<DataContract.Widget, bool>> predicate) { var lambda = Expression.Lambda<Func<ActiveRecord.Widget, bool>>( predicate.Body, predicate.Parameters); // use lambda to query ActiveRecord.Widget and return some value } This won't work because in a typical scenario, for example if: predicate == w => w.Id == 0; ...the expression tree contains a MemberAccessExpression instance which has a MemberInfo property (named Member) that point to members of DataContract.Widget. There are also ParameterExpression instances both in the expression tree and in its parameter expression collection (predicate.Parameters); After searching a bit, I found System.Linq.Expressions.ExpressionVisitor (its source can be found here in the context of a how-to, very helpful) which is a convenient way to modify an expression tree. Armed with this, I implemented a visitor. This simple visitor only takes care of changing the types in member access and parameter expressions. It may not be complete, but it's fine for the expression w => w.Id == 0. internal class Visitor : ExpressionVisitor { private readonly Func<Type, Type> dataContractToActiveRecordTypeConverter; public Visitor(Func<Type, Type> dataContractToActiveRecordTypeConverter) { this.dataContractToActiveRecordTypeConverter = dataContractToActiveRecordTypeConverter; } protected override Expression VisitMember(MemberExpression node) { var dataContractType = node.Member.ReflectedType; var activeRecordType = this.dataContractToActiveRecordTypeConverter(dataContractType); var converted = Expression.MakeMemberAccess( base.Visit(node.Expression), activeRecordType.GetProperty(node.Member.Name)); return converted; } protected override Expression VisitParameter(ParameterExpression node) { var dataContractType = node.Type; var activeRecordType = this.dataContractToActiveRecordTypeConverter(dataContractType); return Expression.Parameter(activeRecordType, node.Name); } } With this visitor, GetMany becomes: IEnumerable<DataContract.Widget> GetMany( Expression<Func<DataContract.Widget, bool>> predicate) { var visitor = new Visitor(...); var lambda = Expression.Lambda<Func<ActiveRecord.Widget, bool>>( visitor.Visit(predicate.Body), predicate.Parameters.Select(p => visitor.Visit(p)); var widgets = ActiveRecord.Widget.Repository().Where(lambda); // This is just for reference, see below Expression<Func<ActiveRecord.Widget, bool>> referenceLambda = w => w.Id == 0; // Here we 'd convert the widgets to instances of DataContract.Widget and // return them -- this has nothing to do with the question though. } Results The good news is that lambda is constructed just fine. The bad news is that it isn't working; it's blowing up on me when I try to use it (the exception messages are really not helpful at all). I have examined the lambda my code produces and a hardcoded lambda with the same expression; they look exactly the same. I spent hours in the debugger trying to find some difference, but I can't. When predicate is w => w.Id == 0, lambda looks exactly like referenceLambda. But the latter works with e.g. IQueryable<T>.Where, while the former does not (I have tried this in the immediate window of the debugger). I should also mention that when predicate is w => true, it all works just fine. Therefore I am assuming that I 'm not doing enough work in Visitor, but I can't find any more leads to follow on. Can someone point me in the right direction? Thanks in advance for your help!

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• Looking for the most painless non-RDBMS storage method in C#

  - by NateD

  I'm writing a simple program that will run entirely client-side. (Desktop programming? do people still do that?) and I need a simple way to store trivial amounts of data in a structured form, but really don't see any need to use a database system. What's more, some of the data needs to be serialized and passed around to different users, like some kind of "file" or perhaps a "document". (has anyone ever done that before?) So, I've looked at using .Net DataSets, LINQ, direct XML manipulation, and they all seem like they would get the job done, but I would like to know before I dive into any of them if there's one method that is generally regarded as easier to code than others. As I said, the amount of data to be stored is trivial, even if one hundred people all used the same machine we're not talking about more than 10 MB, so performance is not as large a concern as is codeability/maintainability. Thank you all in advance!

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