HttpClient 在 GetStringAsync 方法上冻结

Question

我有这个 c# MVC 代码，它与 GetAsync 和 GetPost 方法一起工作得很好，但是当使用 GetStringAsync 时，它在该行冻结：

version = await client.GetStringAsync("/API/Version");

驱动程序代码：

Task<string>[] tasks = new Task<string>[count];
for (int i = 0; i < count; i++)
{
    tasks[i] = MyHttpClient.GetVersion(port, method);
}
Task.WaitAll(tasks);
string[] results = new string[tasks.Length];
for(int i=0; i<tasks.Length; i++)
{
    Task<string> t = (Task<string>)(tasks[i]);
    results[i] = (string)t.Result;
}

HttpCilent 代码：

public static async Task<string> GetVersion(int port, string method)
{
    try
    {
        var client = new HttpClient();
        client.BaseAddress = new Uri("http://localhost:" + port);
        string version = null;
        if (method.ToUpper().Equals("GETSTR"))
        {
            version = await client.GetStringAsync("/API/Version");
        }
        else if (method.ToUpper().Equals("GET"))
        {
            HttpResponseMessage res = client.GetAsync("/API/Version").Result;
            version = res.Content.ReadAsStringAsync().Result;
        }
        else if (method.ToUpper().Equals("POST"))
        {
            var content = new FormUrlEncodedContent(new[] 
            {
                new KeyValuePair<string, string>("name", "jax")
            });
            HttpResponseMessage res = client.PostAsync("/API/Version", content).Result;
            version = res.Content.ReadAsStringAsync().Result;
        }
        client.Dispose();
        return version;
    }
    catch (Exception ex)
    {
        return "Error: " + ex.Message;
    }
}
}

任何想法为什么？

Answer 1

通常，您不应将async/await与.Result和混合使用.Wait()。首先修复GetVersion方法：

public static async Task<string> GetVersion(int port, string method)
{
    try
    {
        using (var client = new HttpClient())
        {
            client.BaseAddress = new Uri("http://localhost:" + port);
            if (method.ToUpper().Equals("GETSTR"))
            {
                return await client.GetStringAsync("/API/Version")
                    .ConfigureAwait(false);
            }
            else if (method.ToUpper().Equals("GET"))
            {
                var res = await client.GetAsync("/API/Version").ConfigureAwait(false);
                return await res.Content.ReadAsStringAsync().ConfigureAwait(false);
            }
            else if (method.ToUpper().Equals("POST"))
            {
                var content = new FormUrlEncodedContent(new[] 
                {
                    new KeyValuePair<string, string>("name", "jax")
                });
                var res = await client.PostAsync("/API/Version", content)
                    .ConfigureAwait(false);
                return await res.Content.ReadAsStringAsync().ConfigureAwait(false);
            }
        }
    }
    catch (Exception ex)
    {
        return "Error: " + ex.Message;
    }
}

我在这里所做的就是<task>.Result用await <task>. 对于每个await， 后面的方法部分await将被注册为Task正在等待的继续- 如果await遇到每个表达式 - 而不是在 的情况下阻塞线程.Result。

现在在你的调用代码中你需要做同样的事情：

var tasks = new Task<string>[count];
for (int i = 0; i < count; i++)
{
    tasks[i] = MyHttpClient.GetVersion(port, method);
}
var results = new string[tasks.Length];
for (int i = 0; i < tasks.Length; i++)
{
    results[i] = await tasks[i];
}

或者更简单：

var tasks = new Task<string>[count];
for (int i = 0; i < count; i++)
{
    tasks[i] = MyHttpClient.GetVersion(port, method);
}
var results = await Task.WhenAll(tasks);