SendAsync正在阻止UI线程

尝试将我的程序的一部分转换为异步http客户端调用. 拿出部分代码(下面)进行测试. 基本上是一个具有非阻塞的异步按钮(据我所知)SendAsync().它不应该阻止UI线程，对吗?仍然由于目前无法看到的原因而将其阻止.

Trying to convert part of my program into asynchronous http client call. Took out part of code (which is below) to test. Basically an async'ed button with non-blocking (should be to my knowledge) SendAsync(). It should not block UI thread, am I correct? It still blocks it for a reason I cannot currently see.

我花了最后两天的时间来找出问题所在.我实现了非阻塞文件写入日志记录和电子邮件发送功能，它们可以正常工作.

I've spent last 2 days trying to figure out whats wrong. I implemented non-blocking file write logging and email send features and they work correctly.

有人可以指出我在做什么错吗?

Could someone point out what am I doing wrong please?

private async void button2_Click(object sender, EventArgs e)
    {
    NetworkCredential differentCredToPass = new NetworkCredential("user", "*****", "domain");
    WebProxy wcProxy = new WebProxy("1.1.1.1", 8080);
    wcProxy.UseDefaultCredentials = false;
    wcProxy.Credentials = differentCredToPass;
    var httpHandler = new HttpClientHandler();
    httpHandler.UseProxy = true;
    httpHandler.UseDefaultCredentials = false;
    httpHandler.Proxy = wcProxy;
    using(HttpClient httpClient = new HttpClient(httpHandler) )
        {
        try
            {
            HttpRequestMessage request = new HttpRequestMessage(HttpMethod.Get, "http://cisco.Com");
            HttpResponseMessage response = await httpClient.SendAsync(request);
            textBox1.AppendText(response.StatusCode.ToString()   Environment.NewLine);
            }
        catch (Exception ex)
            {
            textBox1.AppendText(ex.Message.ToString()   Environment.NewLine);
            throw;
            }
        }
    }