You should change the Invoke definition like this:

void Invoke(
            int dispId,
            ref Guid riid,
            int lcid,
            System.Runtime.InteropServices.ComTypes.INVOKEKIND wFlags,
            ref System.Runtime.InteropServices.ComTypes.DISPPARAMS pDispParams,
            [Out, MarshalAs(UnmanagedType.LPArray)] object[] result,
            IntPtr pExcepInfo,
            IntPtr puArgErr);

and use it like this:

public void Invoke(int dispId, ref Guid riid, int lcid, System.Runtime.InteropServices.ComTypes.INVOKEKIND wFlags, ref System.Runtime.InteropServices.ComTypes.DISPPARAMS pDispParams, out object result, IntPtr pExcepInfo, IntPtr puArgErr)
{
    string name;
    dispIdNameMap.TryGetValue(dispId, out name);
    if (result != null)
    {
        result[0] = name;
    }
}