Interop C# with parameter of type VT_USERDEFINED

I try to call from C# a COM method that require that a parameter of type VARIANT is defined as VT_USERDEFINED (it also accept other types, but it have a special handling of VT_USERDEFINED).

The interface is:

    HRESULT GetRecord(
        [in] VARIANT varKey,
        [out] VARIANT* pvarRecord);

The question is how to call this method from C# where varKey is an integer defined as VT_USERDEFINED.

The COM interop DLL is generated by tlbimp.exe from the idl file in C++.

Edit 1 The code on the C++ side is like this:

    if (varKey.vt == VT_USERDEFINED)
    {
        // Sequential record number given
        UINT uintRecordNumber = varKey.uintVal;
        ...
    }