passing NULL (C++) to a function

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

Code:

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

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This function has the following C++ declaration

Code:

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

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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.