p/invoke C function that returns pointer to a struct

Question

How do I declare in C# a C function that returns a pointer to a structure?

I believe following is one way to do that, followed by Marshal.PtrToStructure to get actual structure value.

// C-function
SimpleStruct * Function(void);

// C# import
[DllImport("MyDll.dll")]
public static extern IntPtr Function();

1. Am I correct about that?
2. Are there other ways to accomplish the same? (It would be OK to get struct back by value)

Answer 1

I am not an expert here at all, but I happened to be looking at a piece of code (that i don't understand completely mind you) that is doing this same thing.

Here is what they are doing

[DllImport("")]
private static extern short MethodName([In,Out] ref StructureName variable);

and then on the structure they have the following attribute

[StructLayout(LayoutKind.Sequential, Size = #)]
public struct StructureName {}

I think the part you are looking for is the [In,Out] part, and since it's being passed via ref, you should be getting the same data back.

marked as community wiki so people can fix this if wrong.

Answer 2

Since the function returns a pointer (hopefully not a locally allocated one?) your best bet is to manually marshal it (via Marshal.PtrToStructure).

If it were a parameter you could create a managed version of the structure using the PInvoke Interop Assistant then pass it via ref or out.

Answer 3

Caveat: this will only work if the pointer returned is to memory already managed by the CLR

I believe what you are looking for is

// C# import
[DllImport("MyDll.dll")]
[return : MarshalAs(UnmanagedType.LPStruct)]
public static extern StructureName Function();

[StructLayout(LayoutKind.Sequential)]
public class StructureName {}

This should eliminate the need for any manual Marshal.PtrToStructure calls. Depending on what your structure contains, you may need to tag some fields with MarshalAs attributes as appropriate. MSDN has a good example of this.