你好，
可以使用WebRequest的HEAD方法来判断，
WebRequest request = WebRequest.Create(new Uri("http://download.microsoft.com/download/B/A/4/BA4A7E71-2906-4B2D-A0E1-80CF16844F5F/dotNetFx45_Full_setup.exe"));
request.Method = "HEAD";
using (WebResponse response = request.GetResponse())
{
Console.WriteLine("{0} {1}", response.ContentLength, response.ContentType);
}
Console.ReadKey();
如果URI是一个可以下载的地址，则程序运行正常，并且能够输出ContentLength。如果不是一个下载地址，则抛出异常。
具体方式请参考：
How to check if a file exists on a server using c# and the WebClient class
How to check if System.Net.WebClient.DownloadData is downloading a binary file?
谢谢！
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