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.

Hint:

You may have to explicitly call lstrcpynW or lstrcpynA based uFlags

Sample Code:

void IContextMenu.GetCommandString(int idCmd, uint uFlags, int reserved, IntPtr commandString, int cchMax)

{

    string commandStr = "" ;
    if ((uFlags & (uint)GCS.VERB) != 0)
    {
        //obviously you could insert some cases here
        switch (idCmd)
        {
            default:
                commandStr = "...";
                break;
        }
    }
    if((uFlags  & (uint)GCS.HELPTEXT) != 0)
    {
        switch (idCmd)
        {
            case 0:
                commandStr = "Menu Item 1. And a brief description of what it does.";
                break;
            case 1:
                commandStr = "Menu Item 2. And a brief description of what it does." ;
                break;
            case 2:
                commandStr = "Menu Item 3. And a brief description of what it does.";
                break;
            case 3:
                commandStr = "Menu Item 4. And a brief description of what it does.";
                break;
            default:
                commandStr = "...";
                break;
        }
    }

    // must limit the return string to cchMax
    if (commandStr.Length >= cchMax) commandStr = commandStr.Substring(0, cchMax-1) ;

    // now return unicode or ansi based on uFlags
    if((uFlags  & (uint)GCS.UNICODE) != 0 )
    {
        str = Marshal.StringToHGlobalUni(commandStr);
        Helpers.lstrcpynW(commandString, str, cchMax);
        Marshal.FreeHGlobal(str);
    }
    else
    {
        str = Marshal.StringToHGlobalAnsi(commandStr);
        Helpers.lstrcpynA(commandString, str, cchMax);
        Marshal.FreeHGlobal(str);
    }

}