Calling Unmanaged Code from a Managed Application
The DllImport attribute is very useful when reusing existing unmanaged code in a managed application. For instance, your managed application might need to make calls to the unmanaged WIN32 API.

This common scenario is demonstrated in the following code sample where the MessageBox (located in User32.lib) is called:

#using <mscorlib.dll>
using namespace System::Runtime::InteropServices; 
// for DllImportAttribute

namespace SysWin32
{
   [DllImport("user32.dll", EntryPoint = "MessageBox", CharSet = Unicode)]
   int MessageBox(void* hWnd, wchar_t* lpText, wchar_t* lpCaption, 
                  unsigned int uType);
}

int main( )
{
   SysWin32::MessageBox(  0 , L"Hello world!", L"Greetings",  0  );
}
The main point of interest is the line of code containing DllImport. Based on the parameter values, this line of code tells the compiler to declare a function residing in the User32.dll and to treat all strings appearing in the signature (such as parameters or the return value) like Unicode strings. If the EntryPoint argument is missing, the default value is the name of the function. In addition, because the CharSet argument specifies Unicode, the common language runtime will first look for a function called MessageBoxW (W because of the Unicode specification). If the runtime does not find this function, it will then look for MessageBox and the corresponding decorated name depending on the calling convention. The only supported conventions are __cdecl and __stdcall.

When calling a function contained in a user-defined DLL, it is necessary to precede the DLL function declaration with extern "C", like this:

// The function declaration in SampleDLL.h file
extern "C" SAMPLEDLL_API int fnSampleDLL(void);
For more information on other supported parameter values, see DllImport.