reference architecture - Page 146

Cross-language Extension Method Calling

- by Tom Hines

Extension methods are a concise way of binding functions to particular types. In my last post, I showed how Extension methods can be created in the .NET 2.0 environment. In this post, I discuss calling the extensions from other languages. Most of the differences I find between the Dot Net languages are mainly syntax. The declaration of Extensions is no exception. There is, however, a distinct difference with the framework accepting excensions made with C++ that differs from C# and VB. When calling the C++ extension from C#, the compiler will SOMETIMES say there is no definition for DoCPP with the error: 'string' does not contain a definition for 'DoCPP' and no extension method 'DoCPP' accepting a first argument of type 'string' could be found (are you missing a using directive or an assembly reference?) If I recompile, the error goes away. The strangest problem with calling the C++ extension from C# is that I first must make SOME type of reference to the class BEFORE using the extension or it will not be recognized at all. So, if I first call the DoCPP() as a static method, the extension works fine later. If I make a dummy instantiation of the class, it works. If I have no forward reference of the class, I get the same error as before and recompiling does not fix it. It seems as if this none of this is supposed to work across the languages. I have made a few work-arounds to get the examples to compile and run. Note the following examples: Extension in C# using System; namespace Extension_CS { public static class CExtension_CS { //in C#, the "this" keyword is the key. public static void DoCS(this string str) { Console.WriteLine("CS\t{0:G}\tCS", str); } } } Extension in C++ /****************************************************************************\ * Here is the C++ implementation. It is the least elegant and most quirky, * but it works. \****************************************************************************/ #pragma once using namespace System; using namespace System::Runtime::CompilerServices; //<-Essential // Reference: System.Core.dll //<- Essential namespace Extension_CPP { public ref class CExtension_CPP { public: [Extension] // or [ExtensionAttribute] /* either works */ static void DoCPP(String^ str) { Console::WriteLine("C++\t{0:G}\tC++", str); } }; } Extension in VB ' Here is the VB implementation. This is not as elegant as the C#, but it's ' functional. Imports System.Runtime.CompilerServices ' Public Module modExtension_VB 'Extension methods can be defined only in modules. <Extension()> _ Public Sub DoVB(ByVal str As String) Console.WriteLine("VB" & Chr(9) & "{0:G}" & Chr(9) & "VB", str) End Sub End Module Calling program in C# /******************************************************************************\ * Main calling program * Intellisense and VS2008 complain about the CPP implementation, but with a * little duct-tape, it works just fine. \******************************************************************************/ using System; using Extension_CPP; using Extension_CS; using Extension_VB; // vitual namespace namespace TestExtensions { public static class CTestExtensions { /**********************************************************************\ * For some reason, this needs a direct reference into the C++ version * even though it does nothing than add a null reference. * The constructor provides the fake usage to please the compiler. \**********************************************************************/ private static CExtension_CPP x = null; // <-DUCT_TAPE! static CTestExtensions() { // Fake usage to stop compiler from complaining if (null != x) {} // <-DUCT_TAPE } static void Main(string[] args) { string strData = "from C#"; strData.DoCPP(); strData.DoCS(); strData.DoVB(); } } } Calling program in VB Imports Extension_CPP Imports Extension_CS Imports Extension_VB Imports System.Runtime.CompilerServices Module TestExtensions_VB <Extension()> _ Public Sub DoCPP(ByVal str As String) 'Framework does not treat this as an extension, so use the static CExtension_CPP.DoCPP(str) End Sub Sub Main() Dim strData As String = "from VB" strData.DoCS() strData.DoVB() strData.DoCPP() 'fake End Sub End Module Calling program in C++ // TestExtensions_CPP.cpp : main project file. #include "stdafx.h" using namespace System; using namespace Extension_CPP; using namespace Extension_CS; using namespace Extension_VB; void main(void) { /*******************************************************\ * Extension methods are called like static methods * when called from C++. There may be a difference in * syntax when calling the VB extension as VB Extensions * are embedded in Modules instead of classes \*******************************************************/ String^ strData = "from C++"; CExtension_CPP::DoCPP(strData); CExtension_CS::DoCS(strData); modExtension_VB::DoVB(strData); //since Extensions go in Modules }

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