如何在 Bing 地图上设置缩放级别？

Question

我有 19 个菜单项的共享事件处理程序，标记为“1”到“19”，相应的标记值为 1 到 19：

// This event handler is shared by all 19 zoom level menu items
private void toolStripMenuItem_ZoomLevel1_Click(object sender, EventArgs e)
{
    //int zoomLevel = sender.Tag; // "object does not contain a definition for tag"
    //int zoomLevel = Convert.ToInt32(sender.Tag); // " "
    //int zoomLevel = (sender as toolStripMenuItem).Tag; // type or namespace   'toolStripMenuItem' could not be found
    //int zoomLevel = (sender as MenuItem).Tag; // "cannot implicitly convert object to     int"
    int zoomLevel = Convert.ToInt32((sender as MenuItem).Tag);
    this.userControl11.myMap.ZoomLevel = zoomLevel;
}

但是，在运行时，当我 select 这些菜单项之一时，我得到一个空引用：“对象引用未设置为对象的实例”

菜单项（发送者）显然存在，并且有一个标签值。这里的 null 是什么？

更新

使用建议的代码：

this.userControl11.myMap.ZoomLevel = (int)((ToolStripMenuItem)sender).Tag;

...我得到，“指定的转换在运行时无效”。

“故事的其余部分”/血淋淋的细节是：

System.InvalidCastException HResult=0x80004002 Message=Specified cast is not valid. Source=MyMaps StackTrace: at MyMaps.Form1.toolStripMenuItem_ZoomLevel1_Click(Object sender, EventArgs e) in C:\Users\bclay\source\repos\MyMaps\MyMaps\Form1.cs:line 138 at System.Windows.Forms.ToolStripItem.RaiseEvent(Object key, EventArgs e) at System.Windows.Forms.ToolStripMenuItem.OnClick(EventArgs e) at System.Windows.Forms.ToolStripItem.HandleClick(EventArgs e) at System.Windows.Forms.ToolStripItem.HandleMouseUp(MouseEventArgs e)
at System.Windows.Forms.ToolStripItem.FireEventInteractive(EventArgs e, ToolStripItemEventType met) at System.Windows.Forms.ToolStripItem.FireEvent(EventArgs e, ToolStripItemEventType met) at System.Windows.Forms.ToolStrip.OnMouseUp(MouseEventArgs mea) at System.Windows.Forms.ToolStripDropDown.OnMouseUp(MouseEventArgs mea)
at System.Windows.Forms.Control.WmMouseUp(Message& m, MouseButtons button, Int32 clicks) at System.Windows.Forms.Control.WndProc(Message& m) at System.Windows.Forms.ScrollableControl.WndProc(Message& m) at System.Windows.Forms.ToolStrip.WndProc(Message& m) at System.Windows.Forms.ToolStripDropDown.WndProc(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.OnMessage(Message& m) at System.Windows.Forms.Control.ControlNativeWindow.WndProc(Message& m) at System.Windows.Forms.NativeWindow.DebuggableCallback(IntPtr hWnd, Int32 msg, IntPtr wparam, IntPtr lparam) at System.Windows.Forms.UnsafeNativeMethods.DispatchMessageW(MSG& msg)
at System.Windows.Forms.Application.ComponentManager.System.Windows.Forms.UnsafeNativeMethods.IMsoComponentManager.FPushMessageLoop(IntPtr dwComponentID, Int32 reason, Int32 pvLoopData) at System.Windows.Forms.Application.ThreadContext.RunMessageLoopInner(Int32 reason, ApplicationContext context) at System.Windows.Forms.Application.ThreadContext.RunMessageLoop(Int32 reason, ApplicationContext context) at System.Windows.Forms.Application.Run(Form mainForm) at MyMaps.Program.Main() in C:\Users\bclay\source\repos\MyMaps\MyMaps\Program.cs:line 19

Answer 1

根据您的事件处理程序的名称，我假设您正在处理 ToolStrioMenuItem 的点击事件，因此您需要将发件人转换为 ToolStripMenuItem。

如果您不确定类型，请转到设计器并单击您正在处理其单击事件的 control/item，然后查看属性并检查属性 window 在下拉列表中，并在投射发件人时使用该类型：

//To make it a bit more readable:
var item = (ToolStripMenuItem)sender;
var tag = item.Tag;
var value = Convert.ToInt32(tag);
this.userControl11.myMap.ZoomLevel = value;

//Or in a single line:
this.userControl11.myMap.ZoomLevel = Convert.ToInt32(((ToolStripMenuItem)sender).Tag);

这是您其余尝试的问题：

int zoomLevel = sender.Tag;

事件处理程序的发送者参数是对象类型，object 没有标记属性。
int zoomLevel = Convert.ToInt32(sender.Tag);

同上
int zoomLevel = (sender as toolStripMenuItem).Tag;

打字错误！ toolStripMenuItem 应该是 ToolStripMenuItem
int zoomLevel = (sender as MenuItem).Tag;

ToolStripMenuItem 不会驱动 from/isn 无法转换为 MenuItem，因此当您尝试使用 as 运算符将其转换为 MenuItem 时，结果为 null，并尝试获取标记为null，会引发对象空引用异常。

如何在 Bing 地图上设置缩放级别？

How can I set the Zoom Level on my Bing Map?

c#

.net

wpf

winforms

bing-maps

更新