错误CS1061说字符串不包含定义

Question

此C＃代码给我一个错误消息，该字符串不包含AddItem的定义。我尝试添加其他标识符，但我不知道是哪个标识符。

C＃：

const string RIBBON_TAB = "HILLAM TAB";
const string RIBBON_PANEL = "Ribbon button";

public object ToolTip {
    get;
    private set;
}

public BitmapSource Png {
    get;
    private set;
}

// Both OnStartup and OnShutdown must be implemented as public method
public Result OnStartup(UIControlledApplication a) {

    //get the ribbon tab
    try {
        a.CreateRibbonTab(RIBBON_TAB);
    }

    catch(Exception) {} //tab already exists
    // Add a new ribbon panel
    RibbonPanel panel = null;
    List < RibbonPanel > panels = a.GetRibbonPanels(RIBBON_TAB);
    foreach(RibbonPanel pnl in panels) {
        if (pnl.Name == RIBBON_PANEL) {
            panel = pnl;
            break;
        }
    }
    if (panel == null) {
        panel = a.CreateRibbonPanel(RIBBON_TAB, RIBBON_PANEL);
    }

    // Create a push button to trigger a command add it to the ribbon panel.
    string thisAssemblyPath = Assembly.GetExecutingAssembly().Location;
    PushButtonData buttonData = new PushButtonData("cmdHelloWorld", "Hello World", thisAssemblyPath, "RevitAddin1.HelloWorld");
    PushButton pushButton = RIBBON_PANEL.AddItem(buttonData) as PushButton;

    // Optionally, other properties may be assigned to the button
    // a) tool-tip
    pushButton.ToolTip = "Say hello world.";

我希望单击面板，它会打开一个新窗口，并显示以下消息：“问好世界”。