passing NULL (C++) to a function

[reptile]

Hi all.
I created a structure in C#, DEVMODE, which is placed in winuser.h in C++.
Then I imported a function: ChangeDisplaySettings in the following way

[ DllImport( "user32", EntryPoint = "ChangeDisplaySettingsW", CallingConvention = CallingConvention.StdCall ) ]
public static extern int ChangeDisplaySettings(
			ref DEVMODE lpDevMode,
			uint dwFlags );

This function has the following C++ declaration

WINUSERAPI
LONG
WINAPI
ChangeDisplaySettingsW(
    IN LPDEVMODEW  lpDevMode,
    IN DWORD       dwFlags);

I need to pass C++ NULL value for lpDevMode to this function in C#. How to do that?
When I pass C# null, compiler gives an error.

Thanx for attention.