Unions in C# - incorrectly aligned or overlapped with a non-object field

Question

I am marshaling via PInvoke to a native C dll which expects the following call.

private static extern int externalMethod(IntPtr Data, [MarshalAs(UnmanagedTyp         


        

Answer 1

Just use the A/B/C/D structs directly and skip the union. In your extern calls, simply substitute the correct struct in the method declaration.

extern void UnionMethodExpectingA( A a );

If the unmanaged methods actually accept a union and behave differently based on the type passed, then you can declare different extern methods that all end up calling the same unmanaged entry point.

[DllImport( "unmanaged.dll", EntryPoint="ScaryMethod" )]
extern void ScaryMethodExpectingA( A a );

[DllImport( "unmanaged.dll", EntryPoint="ScaryMethod" )]
extern void ScaryMethodExpectingB( B b );

Updated for "length" parameter. The logic still applies. Just create a "wrapper" method and do the same thing.

void CallScaryMethodExpectingA( A a )
{
  ScaryMethodExpectingA( a, Marshal.SizeOf( a ) );
}