Marshaling an array of C# structure and use it in COM-What are the right Marshaling Attributes

Question

I am facing the following problem for almost a week: I have an array of c# structure which needs to be sent to a COM application. But when I call the COM method I get the following

error: System.Runtime.InteropServices.MarshalDirectiveException' occurred

Additional information: Cannot marshal 'parameter #5': Invalid managed/unmanaged type combination (Int/UInt must be paired with SysInt or SysUInt).

The IDL file is extended/derived with a C# interface.

Here is the method defined as in the IDL:

[helpstring("Method MyCallbackMehtod")]
    HRESULT MyRequestFinished(
        [in] long    callId,
        [in] unsigned int nrElemArray1,
        [in, size_is(nrElemArray1)] MyStruct ElemArray1[],
        [in] unsigned int nrElemArray2,
        [in, size_is(nrElemArray2)] MyStruct ElemArray2[]
);

The c# interface:

 [ComImport, Guid("xxxxxxxx-xxxx-xxxx-xxx-xxxxxxxxxx")]
    [InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    public interface IMyInterface
    {

        void MyRequestFinished(
            [In] 
            long callId,

            [In] 
            uint nrElemArray1,

            [In,MarshalAs(UnmanagedType.LPArray,SizeParamIndex=1)] 
            IntPtr ElemArray1,

            [In]
            uint nrElemArray2,

            [In, MarshalAs(UnmanagedType.LPArray,SizeParamIndex = 3)]  
            IntPtr ElemArray2);
}

The definition of MyStruct is:

[StructLayout(LayoutKind.Sequential,Pack=1)]
    public struct MyStruct
    {
        public double setValue;
        public double actualValue;
        [MarshalAs(UnmanagedType.I4)]
        public MyEnum myResult;
    }

    [ComVisible(true)]
    public enum MyEnum 
    {
        Val1,

        Val2,

        Val3
    }

The code snippet where the call to the COM method is called:

IntPtr pMyElemArray1  = IntPtr.Zero;
IntPtr pMyElemArray2 = IntPtr.Zero;

MyStruct[] MyElemArray1= GetArray1();
MyStruct[] MyElemArray2= GetArray2();

int lengthElemArray1= MyElemArray1.Length;
int lengthElemArray2= MyElemArray2.Length;

pMyElemArray1 = Marshal.AllocCoTaskMem(Marshal.SizeOf(MyElemArray1[0]) * lengthElemArray1);
pMyElem2 = Marshal.AllocCoTaskMem(Marshal.SizeOf(MyElemArray2[0]) * lengthElemArray2);

int rundef = (int)pMyElemArray1 ;
for (int i = 0; i < lengthElemArray1; i++)
{
     Marshal.StructureToPtr(MyElemArray1 [i], (IntPtr)rundef, false);
     rundef += Marshal.SizeOf(MyElemArray1[i]);
}

rundef = (int)pMyElemArray2;
for (int i = 0; i < lengthElemArray2; i++)
{
    Marshal.StructureToPtr(MyElemArray2[i], (IntPtr)rundef, false);
    rundef += Marshal.SizeOf(MyElemArray2[i]);
}

// Notify COM component
//here i get the error
  myComObject.MyRequestFinished(callId,
                       ((uint)lengthElemArray1),
                       pMyElemArray1,
                       ((uint)lengthElemArray2),
                       pMyElemArray2);
//....
}

Answer 1

LPArray is not applicable to parameters of type IntPtr . Try either changing the parameter type to an array or removing the MarshalAs attribute.

void MyRequestFinished(
    [In]
    long callId,

    [In]
    uint nrElemArray1,

    [In, MarshalAs(UnmanagedType.LPArray, SizeParamIndex = 1)]
    MyStruct[] ElemArray1,

    [In]
    uint nrElemArray2,

    [In]  
    IntPtr ElemArray2);
}

Personally, I prefer to pass in an array rather than an IntPtr as it results in much simpler logic at the call site.