C# Definition:

    [ComImport(),
    InterfaceType(ComInterfaceType.InterfaceIsIUnknown),
    GuidAttribute("000214e4-0000-0000-c000-000000000046")]
    public interface IContextMenu
    {
        [PreserveSig()]
        int QueryContextMenu(
            uint hMenu,
            uint indexMenu,
            int idCmdFirst,
            int idCmdLast,
            uint uFlags);

        [PreserveSig()]
        void InvokeCommand(IntPtr pici);

        [PreserveSig()]
        void GetCommandString(
            int idcmd,
            uint uflags,
            int reserved,
            StringBuilder commandstring,
            int cch);
    }

Notes

I found that GetCommandString gave me problems when defining the variable commandstring as a StringBuilder. (It had an unexplainable limit of 16 characters.) I found that by changing my definition to IntPtr, and then copying a string into this pointer by using lstrcpy works for the true buffer size designated by cch.