C# in a Nutshell, Second Edition

Chapter 17. Integrating with Native DLLs

While it would be convenient if all applications could be written entirely in managed code, and you never had to depend on the services of “legacy” code, this situation is not likely to occur in the immediate future. For the purposes of this discussion, legacy code is defined as any code written in a traditional Microsoft language, such as Visual C++ or VB, for an already released operating system, such as Windows 2000 or Windows XP, or a virtual machine such as the VB runtime. Practically speaking, it is essential that managed code be able to draw on the services of unmanaged code in an efficient and well-integrated manner. Thankfully, the .NET Framework is an over-achiever in this area, providing excellent support for integrating with traditional DLLs as well as COM components and COM+ applications.

This section introduces and demonstrates the facilities for integrating with existing unmanaged code packaged in traditional DLLs. Unless otherwise stated, the types mentioned in this section all exist in either the System or System.Runtime.InteropServices namespaces.

Calling Into 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 DLL 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 ...