c# - HttpRequest 将参数传递给 AsyncCallback 委托(delegate)

Question

我正在异步使用 HttpWebRequest 类，如下所示(它是一个 Windows 应用程序)

private void StartWebRequest(string url)
{
   HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
   request.BeginGetResponse(new AsyncCallback(FinishWebRequest), request);
}

private void FinishWebRequest(IAsyncResult result)
{
   HttpWebResponse response = (result.AsyncState as HttpWebRequest).EndGetResponse(result) as HttpWebResponse;
   Stream responseStream = response.GetResponseStream();
   int num = 100000;
   byte[] buffer = new byte[num];
   int offset = 0;

   while ((num2 = responseStream.Read(buffer, offset, 1000)) != 0)
   {
      offset += num2;
   }

   MemoryStream stream = new MemoryStream(buffer, 0, offset);
   Bitmap bitmap = (Bitmap)Image.FromStream(stream);
   bitmap.Save(@"z:\new.jpg");
   response.Close();
   responseStream.Close();
   stream.Close();
}

但是我想从 StartWebRequest 方法向 AsyncCallback 委托(delegate)提供一些参数，这可能吗？因为我想将图片名称作为参数，例如:

bitmap.Save(@"z:\MYPARAMATERVALUE.jpg");

Answer 1

这里最简单的方法是使用匿名函数(匿名方法或 lambda 表达式)，如下所示:

private void StartWebRequest(string url, string filename)
{
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(url);
    request.BeginGetResponse(result => {
        FinishWebRequest(result, request, filename);
    }, null); // Don't need the state here any more
}

private void FinishWebRequest(IAsyncResult result,
                              HttpWebRequest request,
                              string filename)
{
    using (HttpWebResponse response =
             (HttpWebResponse) request.EndGetResponse(result))
    {
        // Use filename here
    }
}

当然，您可以手动完成所有这些操作，但您需要创建一个新类来捕获文件名，创建该类的实例，然后使用该类中的方法作为回调委托(delegate)。这正是编译器通过匿名函数为您所做的事情。