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

c# null pinvoke parameters ref
Nick picture Nick · Apr 10, 2009 · Viewed 47.3k times · Source

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

Andrew Hare picture Andrew Hare · Apr 10, 2009

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.

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