C#: How to pass null to a function expecting a ref?

Mapping oMapping = null;

FILES_GetMemoryMapping(MapFile, out size, MapName, out PacketSize, ref oMapping, out PagePerSector);

The reason you cannot pass null is because a ref parameter is given special treatment by the C# compiler. Any ref parameter must be a reference that can be passed to the function you are calling. Since you want to pass null the compiler is refusing to allow this since you are not providing a reference that the function is expecting to have.

Your only real option would be to create a local variable, set it to null, and pass that in. The compiler will not allow you to do much more than that.

I'm assuming that Mapping is a structure? If so you can have two versions of the FILES_GetMemoryMapping() prototype with different signatures. For the second overload where you want to pass null, make the parameter an IntPtr and use IntPtr.Zero

public static extern uint FILES_GetMemoryMapping(
    [MarshalAs(UnmanagedType.LPStr)] string pPathFile,
    out ushort Size,
    [MarshalAs(UnmanagedType.LPStr)] string MapName,
    out ushort PacketSize,
    IntPtr oMapping,
    out byte PagesPerSector);

Call example:

FILES_GetMemoryMapping(MapFile, out size, MapName,
   out PacketSize, IntPtr.Zero, out PagePerSector);

If Mapping is actually a class instead of a structure, just set the value to null before passing it down.

One way is to create a dummy variable, assign it null, and pass that in.