Marshaling an array of C # structure and using it in COM - what are the correct marshaling attributes

I ran into the following problem for almost a week: I have an array of C # structure that 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 mated to SysInt or SysUInt).

The IDL file is expanded / displayed using the 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[]
);

Interface with#:

 [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);
}

MyStruct definition:

[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
    }

A snippet of code that calls the COM method call:

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);
//....
}