An unhandled exception of type 'System.InvalidOperationException' occurred in System.Windows.Forms.dll

Additional information: Cross-thread operation not valid: Control 'btn_Start' accessed from a thread other than the thread it was created on.

运行的时候出现上面的错误提示。
说明：程序的功能是实现文件下载
btn_Start是开始按钮

高手帮帮忙，谢谢~~

但是我好像已经创建线程了啊..那该怎么办？