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  • Will C++1x support __stdcall or extern "C" capture-nothing lambdas?

    - by Daniel Trebbien
    Yesterday I was thinking about whether it would be possible to use the convenience of C++1x lambda functions to write callbacks for Windows API functions. For example, what if I wanted to use a lambda as an EnumChildProc with EnumChildWindows? Something like: EnumChildWindows(hTrayWnd, CALLBACK [](HWND hWnd, LPARAM lParam) { // ... return static_cast<BOOL>(TRUE); // continue enumerating }, reinterpret_cast<LPARAM>(&myData)); Another use would be to write extern "C" callbacks for C routines. E.g.: my_class *pRes = static_cast<my_class*>(bsearch(&key, myClassObjectsArr, myClassObjectsArr_size, sizeof(my_class), extern "C" [](const void *pV1, const void *pV2) { const my_class& o1 = *static_cast<const my_class*>(pV1); const my_class& o2 = *static_cast<const my_class*>(pV2); int res; // ... return res; })); Is this possible?

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  • Foreach/For loop alternative lambda function?

    - by mamu
    We use for or foreach to loop through collections and process each entries. Is there any alternative in all those new lambda functions for collections in C#? Traditional way of doing foreach(var v in vs) { Console.write(v); } Is there anything like? vs.foreach(v => console.write(v))

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  • Rewrite the foreach using lambda + C#3.0

    - by Newbie
    I am tryingv the following foreach (DataRow dr in dt.Rows) { if (dr["TABLE_NAME"].ToString().Contains(sheetName)) { tableName = dr["TABLE_NAME"].ToString(); } } by using lambda like string tableName = ""; DataTableExtensions.AsEnumerable(dt).ToList().ForEach(i => { tableName = i["TABLE_NAME"].ToString().Contains(sheetName); } ); but getting compile time error "cannot implicitly bool to string". So how to achieve the same.? Thanks(C#3.0)

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  • Lambda Expressions for a 5th Grader

    - by Randy Minder
    If you had to explain Lambda expressions to a 5th grader, how would you do it? And what examples might you give, or resources might you point them to? I may be finding myself in the position of having to teach this to 5th grade level developers and could use some assistance. Thanks very much.

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  • Entity Framework many-to-many using VB.Net Lambda

    - by bgs264
    Hello, I'm a newbie to StackOverflow so please be kind ;) I'm using Entity Framework in Visual Studio 2010 Beta 2 (.NET framework 4.0 Beta 2). I have created an entity framework .edmx model from my database and I have a handful of many-to-many relationships. A trivial example of my database schema is Roles (ID, Name, Active) Members (ID, DateOfBirth, DateCreated) RoleMembership(RoleID, MemberID) I am now writing the custom role provider (Inheriting System.Configuration.Provider.RoleProvider) and have come to write the implementation of IsUserInRole(username, roleName). The LINQ-to-Entity queries which I wrote, when SQL-Profiled, all produced CROSS JOIN statements when what I want is for them to INNER JOIN. Dim query = From m In dc.Members From r In dc.Roles Where m.ID = 100 And r.Name = "Member" Select m My problem is almost exactly described here: http://stackoverflow.com/questions/553918/entity-framework-and-many-to-many-queries-unusable I'm sure that the solution presented there works well, but whilst I studied Java at uni and I can mostly understand C# I cannot understand this Lambda syntax provided and I need to get a similar example in VB. I've looked around the web for the best part of half a day but I'm not closer to my answer. So please can somebody advise how, in VB, I can construct a LINQ statement which would do this equivalent in SQL: SELECT rm.RoleID FROM RoleMembership rm INNER JOIN Roles r ON r.ID = rm.RoleID INNER JOIN Members m ON m.ID = rm.MemberID WHERE r.Name = 'Member' AND m.ID = 101 I would use this query to see if Member 101 is in Role 3. (I appreciate I probably don't need the join to the Members table in SQL but I imagine in LINQ I'd need to bring in the Member object?) UPDATE: I'm a bit closer by using multiple methods: Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load Dim count As Integer Using dc As New CBLModel.CBLEntities Dim persons = dc.Members.Where(AddressOf myTest) count = persons.Count End Using System.Diagnostics.Debugger.Break() End Sub Function myTest(ByVal m As Member) As Boolean Return m.ID = "100" AndAlso m.Roles.Select(AddressOf myRoleTest).Count > 0 End Function Function myRoleTest(ByVal r As Role) As Boolean Return r.Name = "Member" End Function SQL Profiler shows this: SQL:BatchStarting SELECT [Extent1].[ID] AS [ID], ... (all columns from Members snipped for brevity) ... FROM [dbo].[Members] AS [Extent1] RPC:Completed exec sp_executesql N'SELECT [Extent2].[ID] AS [ID], [Extent2].[Name] AS [Name], [Extent2].[Active] AS [Active] FROM [dbo].[RoleMembership] AS [Extent1] INNER JOIN [dbo].[Roles] AS [Extent2] ON [Extent1].[RoleID] = [Extent2].[ID] WHERE [Extent1].[MemberID] = @EntityKeyValue1',N'@EntityKeyValue1 int',@EntityKeyValue1=100 SQL:BatchCompleted SELECT [Extent1].[ID] AS [ID], ... (all columns from Members snipped for brevity) ... FROM [dbo].[Members] AS [Extent1] I'm not certain why it is using sp_execsql for the inner join statement and why it's still running a select to select ALL members though. Thanks. UPDATE 2 I've written it by turning the above "multiple methods" into lambda expressions then all into one query, like this: Dim allIDs As String = String.Empty Using dc As New CBLModel.CBLEntities For Each retM In dc.Members.Where(Function(m As Member) m.ID = 100 AndAlso m.Roles.Select(Function(r As Role) r.Name = "Doctor").Count > 0) allIDs &= retM.ID.ToString & ";" Next End Using But it doesn't seem to work: "Doctor" is not a role that exists, I just put it in there for testing purposes, yet "allIDs" still gets set to "100;" The SQL in SQL Profiler this time looks like this: SELECT [Project1].* FROM ( SELECT [Extent1].*, (SELECT COUNT(1) AS [A1] FROM [dbo].[RoleMembership] AS [Extent2] WHERE [Extent1].[ID] = [Extent2].[MemberID]) AS [C1] FROM [dbo].[Members] AS [Extent1] ) AS [Project1] WHERE (100 = [Project1].[ID]) AND ([Project1].[C1] > 0) For brevity I turned the list of all the columns from the Members table into * As you can see it's just ignoring the "Role" query... :/

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  • Weak event handler model for use with lambdas

    - by Benjol
    OK, so this is more of an answer than a question, but after asking this question, and pulling together the various bits from Dustin Campbell, Egor, and also one last tip from the 'IObservable/Rx/Reactive framework', I think I've worked out a workable solution for this particular problem. It may be completely superseded by IObservable/Rx/Reactive framework, but only experience will show that. I've deliberately created a new question, to give me space to explain how I got to this solution, as it may not be immediately obvious. There are many related questions, most telling you you can't use inline lambdas if you want to be able to detach them later: Weak events in .Net? Unhooking events with lambdas in C# Can using lambdas as event handlers cause a memory leak? How to unsubscribe from an event which uses a lambda expression? Unsubscribe anonymous method in C# And it is true that if YOU want to be able to detach them later, you need to keep a reference to your lambda. However, if you just want the event handler to detach itself when your subscriber falls out of scope, this answer is for you.

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  • GridView ObjectDataSource LINQ Paging and Sorting using multiple table query.

    - by user367426
    I am trying to create a pageing and sorting object data source that before execution returns all results, then sorts on these results before filtering and then using the take and skip methods with the aim of retrieving just a subset of results from the database (saving on database traffic). this is based on the following article: http://www.singingeels.com/Blogs/Nullable/2008/03/26/Dynamic_LINQ_OrderBy_using_String_Names.aspx Now I have managed to get this working even creating lambda expressions to reflect the sort expression returned from the grid even finding out the data type to sort for DateTime and Decimal. public static string GetReturnType<TInput>(string value) { var param = Expression.Parameter(typeof(TInput), "o"); Expression a = Expression.Property(param, "DisplayPriceType"); Expression b = Expression.Property(a, "Name"); Expression converted = Expression.Convert(Expression.Property(param, value), typeof(object)); Expression<Func<TInput, object>> mySortExpression = Expression.Lambda<Func<TInput, object>>(converted, param); UnaryExpression member = (UnaryExpression)mySortExpression.Body; return member.Operand.Type.FullName; } Now the problem I have is that many of the Queries return joined tables and I would like to sort on fields from the other tables. So when executing a query you can create a function that will assign the properties from other tables to properties created in the partial class. public static Account InitAccount(Account account) { account.CurrencyName = account.Currency.Name; account.PriceTypeName = account.DisplayPriceType.Name; return account; } So my question is, is there a way to assign the value from the joined table to the property of the current table partial class? i have tried using. from a in dc.Accounts where a.CompanyID == companyID && a.Archived == null select new { PriceTypeName = a.DisplayPriceType.Name}) but this seems to mess up my SortExpression. Any help on this would be much appreciated, I do understand that this is complex stuff.

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  • Will C++0x support __stdcall or extern "C" capture-nothing lambdas?

    - by Daniel Trebbien
    Yesterday I was thinking about whether it would be possible to use the convenience of C++0x lambda functions to write callbacks for Windows API functions. For example, what if I wanted to use a lambda as an EnumChildProc with EnumChildWindows? Something like: EnumChildWindows(hTrayWnd, CALLBACK [](HWND hWnd, LPARAM lParam) { // ... return static_cast<BOOL>(TRUE); // continue enumerating }, reinterpret_cast<LPARAM>(&myData)); Another use would be to write extern "C" callbacks for C routines. E.g.: my_class *pRes = static_cast<my_class*>(bsearch(&key, myClassObjectsArr, myClassObjectsArr_size, sizeof(my_class), extern "C" [](const void *pV1, const void *pV2) { const my_class& o1 = *static_cast<const my_class*>(pV1); const my_class& o2 = *static_cast<const my_class*>(pV2); int res; // ... return res; })); Is this possible? I can understand that lambdas that capture variables will never be compatible with C, but it at least seems possible to me that capture-nothing lambdas can be compatible.

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  • Common lisp error: "should be lambda expression"

    - by Zachary
    I just started learning Common Lisp a few days ago, and I'm trying to build a function that inserts a number into a tree. I'm getting an error, * - SYSTEM::%EXPAND-FORM: (CONS NIL LST) should be a lambda expression From googling around, it seems like this happens when you have too many sets of parenthesis, but after looking at this for an hour or so and changing things around, I can't figure out where I could be doing this. This is the code where it's happening: (defun insert (lst probe) (cond ((null lst) (cons probe lst)) ((equal (length lst) 1) (if (<= probe (first lst)) (cons probe lst) (append lst (list probe)))) ((equal (length lst) 2) ((cons nil lst) (append lst nil) (insertat nil lst 3) (cond ((<= probe (second lst)) (insert (first lst) probe)) ((> probe (fourth lst)) (insert (fifth lst) probe)) (t (insert (third lst) probe))))))) I'm pretty sure it's occurring after the ((equal (length lst) 2), where the idea is to insert an empty list into the existing list, then append an empty list onto the end, then insert an empty list into the middle.

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  • lambda expressions in C#?

    - by Matt
    Hey I'm rather new to these could someone explain the significance (of the following code) or prehaps give a link to some useful information on lambda expressions? I encounter the following code in a test and I am wondering why someone would do this: foo.MyEvent += (o, e) => { fCount++; Console.WriteLine(fCount); }; foo.MyEvent -= (o, e) => { fCount++; Console.WriteLine(fCount); }; My instinct tells me it is something simple and not a mistake, but I don't know enough about these expressions to understand why this is being done. Regards,

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  • Rewriting VB.NET Lambda experession as a C# statement

    - by ChadD
    I downgraded my app from version 4 of the framework to version 4 and now I want to implement this VB.NET lambda function statement (which works on 3.5) Dim colLambda As ColumnItemValueAccessor = Function(rowItem As Object) General_ItemValueAccessor(rowItem, colName) and rewrite it in C#. This was my attempt: ColumnItemValueAccessor colLambda = (object rowItem) => General_ItemValueAccessor(rowItem, colName); When I did this, I get the following error: Error 14 One or more types required to compile a dynamic expression cannot be found. Are you missing references to Microsoft.CSharp.dll and System.Core.dll? C:\Source\DotNet\SqlSmoke\SqlSmoke\UserControls\ScriptUserControl.cs 84 73 SqlSmoke However, when I downgraded the app from version 4.0 of the framework to 3.5 (because our users only hae 3.5 and don't have rights to install 4.0). when I did this, the reference to "Microsoft.CSharp" was broken. Can I rewrite the VB.NET command in C# using syntax that is valid in C# 3.5 as I was able to in VB.NET? What would the syntax be?

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  • How to render Max(Substgring) with Lambda Extensions

    - by caifa
    Hi everybody. I'm using NHibernate with Lambda Extensions. I'd like to know how to nest a Max function with a Substring. The following statement retrieves Max("invoice_id") var ret = session .CreateCriteria<Invoice>() .SetProjection(Projections.Max("invoice_id")) .UniqueResult(); but in my case the field invoice_id is made in this way: 123452010 where 12345 is the invoice number, and 2010 is the current year. So I need to calculate the Max function only over the first 5 digits. How can I do it?

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  • Given a Member Access lambda expression, convert it to a specific string representation with full ac

    - by Nathan
    Given an Expression<Func<T, object>> (e.g. x = x.Prop1.SubProp), I want to create a string "Prop1.SubProp" for as deep as necessary. In the case of a single access (e.g. x = x.Prop1), I can easily do this with: MemberExpression body = (expression.Body.NodeType == ExpressionType.Convert) ? (MemberExpression)((UnaryExpression)expression.Body).Operand : (MemberExpression)expression.Body; return body.Member.Name; However, if there is deeper nesting, e.g. x = x.Prop1.SubProp1, this only gets the most deeply nested name, e.g. "SubProp1" instead of "Prop1.SubProp1" Is there anyway to access the full property path of a lambda expression?

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  • How to render Max(Substring) with Lambda Extensions

    - by caifa
    Hi everybody. I'm using NHibernate with Lambda Extensions. I'd like to know how to nest a Max function with a Substring. The following statement retrieves Max("invoice_id") var ret = session .CreateCriteria<Invoice>() .SetProjection(Projections.Max("invoice_id")) .UniqueResult(); but in my case the field invoice_id is made in this way: 12345.10 where 12345 is the invoice number, and 10 refers to the current year (2010). So I need to calculate the Max function only over the first 5 digits. How can I do it?

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  • get value of a property o => o.Property1 , defined in lambda

    - by Omu
    I need to get the value of a property defined in a lambda public static MvcHtmlString MyHelper<T, TProperty>( this HtmlHelper<T> html, Expression<Func<T, TProperty>> prop) { var value = \\ get the value of Prop1 (not the name "Prop1") ... } the intended usage is something like: public class FooViewModel { public string Prop1 { get;set; } } <%@ Page ViewPage<FooViewModel> %> <%=Html.MyHelper(o => o.Prop1) %>

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  • Convert the code into lambda/LINQ(C#3.0)

    - by Newbie
    How to convert the below code into lambda if (ds != null && ds.Tables.Count > 0) { dtAsset = ds.Tables["AssetData"]; dtCharecteristics = ds.Tables["CharacteristicsData"]; for (int i = 0; i < dtAsset.Rows.Count; i++) { for (int j = 0; j < dtCharecteristics.Rows.Count; j++) { if (dtAsset.Rows[i]["AssetId"].Equals(dtCharecteristics.Rows[j]["AssetId"])) { objAttributesCollection.Add(new Attributes { AttributeCode = Convert.ToString(dtCharecteristics.Rows[j]["AttributeCode"]), TimeSeriesData = fn(Convert.ToDateTime(dtCharecteristics.Rows[j]["StartDate"]), Convert.ToString(dtCharecteristics.Rows[j]["Value"])) }); } } objAssetCollection.Add(new Asset { AssetId = Convert.ToInt32(dtAsset.Rows[i]["AssetId"]), AssetType = Convert.ToString(dtAsset.Rows[i]["AssetCode"]), AttributeCollection = objAttributesCollection }); objAttributesCollection = new List<Attributes>(); } } I am using C#3.0 There is nothing wrong in the code but for the sake of learning I want to do this. Thanks

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  • Writing lambda functions in Scala

    - by user2433237
    I'm aware that you can write anonymous functions in Scala but I'm having trouble trying to convert a piece of code from Scheme. Could anyone help me convert this to Scala? (define apply-env (lambda (env search-sym) (cases environment env (empty-env () (eopl:error 'apply-env "No binding for ~s" search-sym)) (extend-env (var val saved-env) (if (eqv? search-sym var) val (apply-env saved-env search-sym))) (extend-env-rec (p-name b-var p-body saved-env) (if (eqv? search-sym p-name) (proc-val (procedure b-var p-body env)) (apply-env saved-env search-sym)))))) Thanks in advance

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  • What is wrong with the below statement(C#3.0 / Lambda)

    - by Newbie
    what is wrong in the below Enumerable.Range(0, objEntityCode.Count - 1).Select(i => { options.Attributes[i] = new EntityCodeKey { EntityCode = objEntityCode[i].EntityCodes , OrganizationCode = Constants.ORGANIZATION_CODE }; }) .ToArray(); Throwing 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. But this works Enumerable.Range(0, objEntityCode.Count - 1).ToList().ForEach(i => { options.Attributes[i] = new EntityCodeKey { EntityCode = objEntityCode[i].EntityCodes , OrganizationCode = Constants.ORGANIZATION_CODE }; } ); Using C#3.0. Purpose: I am learning LINQ / LAMBDA and trying to do the same program in different way. Thanks.

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