为什么我不能重用 WebClient 两次发出相同的请求？

Question

所以代码：

        const long testCount = 2;
        const string jsonInput = "{\"blockId\":\"1\",\"userId\":\"{7c596b41-0dc3-45df-9b4c-08840f1da780}\",\"sessionId\":\"{46cd1b39-5d0a-440a-9650-ae4297b7e2e9}\"}";

        Stopwatch watch = Stopwatch.StartNew();

        using (var client = new WebClient())
        {
            client.Headers["Content-type"] = "application/json";
            client.Encoding = Encoding.UTF8;

            for (int i = 0; i < testCount; i++)
            {
                var response = client.UploadString("http://localhost:8080/BlocksOptimizationServices/dataPositions", "POST", jsonInput);
            }

            watch.Stop();
            double speed = watch.ElapsedMilliseconds / (double)testCount; 
            Console.WriteLine("Avg speed: {0} request/ms", testCount);

为了性能测试我只想多次调用client.UploadString("http://localhost:8080/BlocksOptimizationServices/dataPositions", "POST", jsonInput)。但是在第一个请求之后它总是失败

"The remote server returned an error: (400) Bad Request."

如果我处理 WebClient 并重新创建 - 有效，但这会增加额外的性能损失.. 为什么我不能重复使用 WebClient 两次？

Answer 1

WebClient headers 在每次请求后被清除。要亲自查看，您可以添加几个 Debug.Assert() 语句。这与您在第二个请求中出现的“(400) 错误请求”错误一致：

for (int i = 0; i < testCount; i++)
{
      Debug.Assert(client.Headers.Count == 1); // Content-type is set   
      var response = client.UploadString("http://localhost:8080/BlocksOptimizationServices/dataPositions", "POST", jsonInput);    
      Debug.Assert(client.Headers.Count == 0); // Zero!
}

因此，您可以更改代码以每次设置 headers：

using (var client = new WebClient())
{            
    for (int i = 0; i < testCount; i++)
    {
        client.Headers["Content-type"] = "application/json";
        client.Encoding = Encoding.UTF8;

        var response = client.UploadString("http://localhost:8080/BlocksOptimizationServices/dataPositions", "POST", jsonInput);
    }

If I dispose WebClient and recreate - works, but this add extra performance penalty.. why I can't reuse WebClient twice?

这是一个 SO 线程，它似乎不认为每个请求实例化一个 WebClient 会造成很大的性能损失：WebClient construction overhead

祝你好运！

Answer 2

这可能不是您要找的，但这就是我解决这个问题的方法。

public WebClient WebClient
                => new WebClient {Headers = new WebHeaderCollection {{HttpRequestHeader.ContentType, "application/json"}}};

这会在您每次使用时创建一个新的 WebClient。我这样做可能效率不高，但对于我使用它的原因来说，它运行良好并且无需考虑为每个请求设置 headers。

为什么我不能重用 WebClient 两次发出相同的请求？

Why I can't reuse WebClient to make the same request twice?

.net

c#

wcf

webclient