How to pass a nullable type to a P/invoked function [duplicate]

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It’s not possible to pass a Nullable type into a PInvoke’d function without some … interesting byte manipulation in native code that is almost certainly not what you want.

If you need the ability to pass a struct value as NULL to native code declare an overload of your PInvoke declaration which takes an IntPtr in the place of the struct and pass IntPtr.Zero

[DllImport("setupapi.dll", CharSet = CharSet.Auto, SetLastError = true)]
    static extern IntPtr SetupDiGetClassDevs(ref Guid ClassGuid, ref int enumerator, IntPtr hwndParent, uint Flags);
[DllImport("setupapi.dll", CharSet = CharSet.Auto, SetLastError = true)]
    static extern IntPtr SetupDiGetClassDevs(ref Guid ClassGuid, IntPtr enumerator, IntPtr hwndParent, uint Flags);

Note: I added a ref class to the first signature. If the native signature can take NULL, it is likely a pointer type. Hence you must pass value types by reference.

Now you can make calls like the following

if (enumerator.HasValue) { 
  SetupDiGetClassDevs(someGuid, ref enumerator.Value, hwnd, flags);
} else {
  SetupDiGetClassDevs(someGuid, IntPtr.Zero, hwnd, flags);
}

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