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

Question

I've got the following function:

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

Which I would like to call like this:

FILES_GetMemoryMapping(MapFile, out size, MapName,     out PacketSize, null, out PagePerSector); 

Unfortunately, I cannot pass null in a field that requires type ref Mapping and no cast I've tried fixes this.

Any suggestions?

Answer 1

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.

Answer 2

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.