你好：
你可增加你的textbox的BorderThickness来增加你的textbox的宽度：
比如： BorderThickness="5"
除此以外：你还可以试着把你的拖动操作与GotFocus事件结合。
更多的你可以看看textbox相关的属性和事件：
http://msdn.microsoft.com/en-us/library/system.windows.controls.textbox(v=vs.95).aspx
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