/Visual Studio 2008/CSPInvokeDll/ReadMe.txt

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            CONSOLE APPLICATION : CSPInvokeDll Project Overview

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Summary:



Platform Invocation Services (P/Invoke) in .NET allows managed code to call 

unmanaged functions that are implemented and exported in unmanaged DLLs. This 

VC# code sample demonstrates using P/Invoke to call the functions exported by 

the native DLLs: CppDynamicLinkLibrary.dll, user32.dll and msvcrt.dll.





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Sample Relation:



CSPInvokeDll -> CppDynamicLinkLibrary

CSPInvokeDll P/Ivnoke the functions exported by the native DLL 

CppDynamicLinkLibrary.





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Implementation:



A. P/Invoke functions exposed from a native C++ DLL module.



Step1. Declare the methods as having an implementation from a DLL export.

First, declare the method with the static and extern C# keywords. Next, 

attach the DllImport attribute to the method. The DllImport attribute allows 

us to specify the name of the DLL that contains the method. The general 

practice is to name the C# method the same as the exported method, but we can

also use a different name for the C# method. Specify custom marshaling 

information for the method's parameters and return value, which will override 

the .NET Framework default marshaling. 



For example, 



    [DllImport("CppDynamicLinkLibrary.dll", CharSet = CharSet.Auto,

        CallingConvention = CallingConvention.Cdecl)]

    internal static extern int GetStringLength1(string str);



These tools can help your write the correct P/Invoke declartions.



	Dumpbin: View the export table of a DLL



	PInvoke.NET: PInvoke.net is primarily a wiki, allowing developers to find, 

	edit and add PInvoke* signatures, user-defined types, and any other info 

	related to calling Win32 and other unmanaged APIs from managed code such 

	as C# or VB.NET.

	

	PInvoke Interop Assistant: It is a toolkit that helps developers to 

	efficiently convert from C to managed P/Invoke signatures or verse visa. 



Step2. Call the methods through the PInvoke signatures. For example:



	string str = "HelloWorld";

    int length;

    length = NativeMethod.GetStringLength1(str);



B. P/Invoke C++ classes exposed from a native C++ DLL module.



There is no easy way to call the classes in a native C++ DLL module through 

P/Invoke. Visual C++ Team Blog introduced a solution, but it is complicated: 

http://go.microsoft.com/?linkid=9729423.



The recommended way of calling native C++ class from .NET are:



  1) use a C++/CLI class library to wrap the native C++ module, and your .NET 

     code class the C++/CLI wrapper class to indirectly access the native C++ 

     class.



  2) convert the native C++ module to be a COM server and expose the native 

     C++ class through a COM interface. Then, the .NET code can access the 

     class through .NET-COM interop.



C. Unload the native DLL module.



You can unload the DLL by first calling GetModuleHandle to get the handle of 

the module and then calling FreeLibrary to unload it. 



    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]

    static extern IntPtr GetModuleHandle(string moduleName);



    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]

    [return: MarshalAs(UnmanagedType.Bool)]

    static extern bool FreeLibrary(IntPtr hModule);



    // Unload the DLL by calling GetModuleHandle and FreeLibrary. 

    if (!FreeLibrary(GetModuleHandle(moduleName)))

    {

        Console.WriteLine("FreeLibrary failed w/err {0}", 

        Marshal.GetLastWin32Error());

    }





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References:



MSDN: Platform Invoke Tutorial

http://msdn.microsoft.com/en-us/library/aa288468.aspx



MSDN: Using P/Invoke to Call Unmanaged APIs from Your Managed Classes

http://msdn.microsoft.com/en-us/library/aa719104.aspx



MSDN: Calling Win32 DLLs in C# with P/Invoke

http://msdn.microsoft.com/en-us/magazine/cc164123.aspx



PInvoke.NET

http://www.pinvoke.net/



PInvoke Interop Assistant 

http://www.codeplex.com/clrinterop





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