15.3 Task 异常

 1         async Task<string> FetchFirstSuccessfulAsync(IEnumerable<string> urls)
 2         {
 3             //验证url
 4             foreach (string url in urls)
 5             {
 6                 try
 7                 {
 8                     using (var client=new HttpClient())
 9                     {
10                         return await client.GetStringAsync(url);
11                     }
12                 }
13                 catch (System.Net.WebException)
14                 {
15                     //记录日志
16                 }
17             }
18             throw new System.Net.WebException("No urls Success");
19         }