Marshal struct to unmanaged array

Question

I have a C# struct to represent a cartesian vector, something like this:

public struct Vector { private double x; private double y; private double z; //Some properties/methods }

Now I have an unmanaged C dll that I need to call with P/Invoke. Some methods expect a double[3] parameter.

The unmanaged C signature is something like

void Cross(double a[3], double b[3], double c[3]);

Is there any way to set up a P/Invoke signature so I can pass instances of my Vector struct and marshal them transparently to unmanaged double[3]? I would also need bidirectional marshaling as the unmanaged function needs to write the output to the argument array, so I guess I would need to marshal as LpArray.

Hans Passant · Accepted Answer · 2010-03-31 12:11:44Z

You can lie in your P/Invoke declaration, the members will align properly on all current CPU architectures to be readable as an array in unmanaged code:

[DllImport("blah.dll")] private static extern void Cross(ref Vector a, ref Vector b, ref Vector c);

Preet Sangha · Accepted Answer · 2010-03-31 11:16:15Z

I don't have my compilers to hand, but I wonder if you can use something like

[MarshalAs(...)] [StructLayout(LayoutKind::Sequential, Pack=1)] public struct Vector { private double x; private double y; private double z; //Some properties/methods }

See here and here and here

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