這個異步任務方法有什麼問題？

Question

這只是一個簡單的異步任務，但我一直有奇怪的編譯錯誤。此代碼來自ASP.NET 4項目中的Web API服務，使用VS2010創建。

即使ContinueWith（非泛型）隱式返回任務，但此錯誤仍然存​​在。

代碼：

public class TestController : ApiController 
{ 
     public Task<HttpResponseMessage> Test() 
     { 
      string url = "http://www.stackoverflow.com"; 
      var client = new HttpClient(); 

      return client.GetAsync(url).ContinueWith<HttpResponseMessage>((request) => 
      { 
       // Error 361 'System.Threading.Tasks.Task' does not contain a definition 
       // for 'Result' and no extension method 'Result' accepting a first argument 
       // of type 'System.Threading.Tasks.Task' could be found 
       // (are you missing a using directive or an assembly reference?) 
       var response = request.Result; 
       response.EnsureSuccessStatusCode(); 

       // Error 364 Cannot implicitly convert type 'System.Threading.Tasks.Task<System.Net.Http.HttpResponseMessage>' to 'System.Net.Http.HttpResponseMessage' 
       return response.Content.ReadAsStringAsync().ContinueWith<HttpResponseMessage>((read) => 
       { 
        return new HttpResponseMessage(); 
       }); 
      }); 
     } 
} 

Answer 1

的364錯誤是完全正常的，因爲你是返回一個Task<Task<HttpResponseMessage>>而不是Task<HttpResponseMessage>。一旦你確定361錯誤也會消失。

所以，你可以Unwrap結果：

public Task<HttpResponseMessage> Test() 
{ 
    string url = "http://www.stackoverflow.com"; 
    var client = new HttpClient(); 
    return client.GetAsync(url).ContinueWith(request => 
    { 
     var response = request.Result; 
     response.EnsureSuccessStatusCode(); 
     return response.Content.ReadAsStringAsync().ContinueWith(t => 
     { 
      var result = new HttpResponseMessage(); 
      response.CreateContent(t.Result); 
      return response; 
     }); 
    }).Unwrap(); 
}