Question

检查网址/网站名称的简单代码是否有效。这段代码（同步代码）非常完美。当我评论同步代码并取消注释async代码时，代码无法正常工作。我的async编程有什么问题？

class Program
{
    static StringBuilder urlStatus = new StringBuilder("");
    //public static async Task<string> GetHeadersAsync(string url)     //Async Code
    public static string GetHeadersAsync(string url)    ////Sync Code
    {
        var request = (HttpWebRequest)WebRequest.Create(url);
        request.Method = "HEAD";
        try
        {
            //using (var response = await request.GetResponseAsync() as HttpWebResponse)   //Async Code
            using (var response = request.GetResponse() as HttpWebResponse)     ////Sync Code
            {
                if (response != null && response.StatusCode == HttpStatusCode.OK)
                    return "HTTP_OK";
                return "Error";
            }
        }
        catch (WebException ex)
        {
            return ex.ToString();
        }
        catch (Exception ex)
        {
            return ex.ToString();
        }
    }
    static void Main(string[] args)
    {
        string lineData;
        //content of URLList.txt
        //http://www.oracle.com
        //http://www.oracletest.com
        //http://www.cnn.com
        //http://www.cnntest.com
        using (StreamReader sr = new StreamReader("URLList.txt"))
        {
            while ((lineData = sr.ReadLine()) != null)
            {
                var urlRetStatus = GetHeadersAsync(lineData);
                urlStatus.Append(lineData + " " + urlRetStatus.ToString() + System.Environment.NewLine);
            }
        }
        System.IO.File.WriteAllText(@"ErrorURL.txt", urlStatus.ToString());
    }
}

Answer 1

当您调用async方法时，您将返回表示异步操作的任务。要从该任务中提取实际结果，请使用await关键字：

Task<string> task = GetHeadersAsync(lineData);
string result = await task;

在您的情况下，您从Main调用该方法，该方法可能不是async方法且无法使用await，因此您需要使用Task.Result属性：

Task<string> task = GetHeadersAsync(lineData);
string result = task.Result;

请记住，这将阻止调用线程，直到async操作完成，因此不建议这样做。

异步等待代码。为什么不工作？

1 个答案: