Marshalling arrays from C# to C++ and back: PInvokeStackImbalance

Question

Hi,

I have a C++ function that I'd like to access from C#. The problem is I keep getting PInvokeStackImbalance exceptions and I don't know why. Everything runs fine and as expected when checking for that exception is turned off.

The signature of my C++ function is:

extern "C" double solveQP(
    int32_t n, int32_t mE, int32_t mI,
    double *p_G, double *p_g0,
    double *p_CE, double *p_ce0,
    double *p_CI, double *p_ci0,
    double *p_x)

and what I've been using to access it is:

        [DllImport("libQuadProg.dll")]
        [return: MarshalAs(UnmanagedType.R8)]
        private static extern double solveQP(
            [In, MarshalAs(UnmanagedType.I4)] int n,
            [In, MarshalAs(UnmanagedType.I4)] int mE,
            [In, MarshalAs(UnmanagedType.I4)] int mI,
            [In, MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.R8)] double[] p_G,
            [In, MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.R8)] double[] p_g0,
            [In, MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.R8)] double[] p_CE,
            [In, MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.R8)] double[] p_ce0,
            [In, MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.R8)] double[] p_CI,
            [In, MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.R8)] double[] p_ci0,
            [In, MarshalAs(UnmanagedType.LPArray, ArraySubType = UnmanagedType.R8)] double[] p_x);

I've also tried it with just the UnmanagedType.LPArray option and nothing at all. I figure there is one detail about PInvoke that I just do not get and I'd appreciate it if someone pointed it out.

Answer 1

You need to use the DllImport's CallingConvention property. Cdecl is required here since you didn't declare the C function as __stdcall. You don't need [MarshalAs], the values you use are already the default. Thus:

    [DllImport("libQuadProg.dll", CallingConvention = CallingConvention.Cdecl)]
    private static extern double solveQP(
        int n, int mE, int mI,
        double[] p_G,
        // etc...
    }