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C# Essentials by Peter Drayton, Ben Albahari, Brad Merrill

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Interop with Native DLLs

PInvoke, short for Platform Invocation Services, lets C# access functions, structs, and callbacks in unmanaged DLLs. For example, perhaps you wish to call the MessageBox function in the Windows user32.dll:

int MessageBox(HWND hWnd, LPCTSTR lpText, 
               LPCTSTR lpCation, UINT uType);

To call this function, you can write a static extern method decorated with the DllImport attribute:

using System.Runtime.InteropServices;
[DllImport("user32.dll")]
static extern int MessageBox(int hWnd, string text, 
                             string caption, int type);

PInvoke then finds and loads the required Win32 DLLs and resolves the entry point of the requested function. The CLR includes a marshaler that knows how to convert parameters and return values between .NET types and unmanaged types. In this example the int parameters translate directly to four-byte integers that the function expects, and the string parameters are converted to null-terminated arrays of characters using one-byte ANSI characters under Win9x or two-byte Unicode characters under WinNT/Win2K.

Marshaling Common Types

The CLR marshaler is a .NET facility that knows about the core types used by COM and the Windows API and provides default translations to CLR types for you. The bool type, for instance, can be translated into a two-byte Windows BOOL type or a four-byte Boolean type. You can override a default translation using the MarshalAs attribute:

using System.Runtime.InteropServices; static extern int Foo([MarshalAs(UnmanagedType.LPStr)] ...

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