c# 异步调用之异步模式
Func<string, IImageRequest, string> downloadString = (address, req) =>
            {
                var client = new WebClient();
                foreach (var header in req.Headers)
                {
                    client.Headers.Add(header.Key, header.Value);
                }
                client.Credentials = req.Credentials;
                client.Encoding = System.Text.Encoding.UTF8;
                return client.DownloadString(address);
            };

            Action<SearchItemResult> addItem = item => _searchInfo.List.Add(item);

            foreach (var req in GetSearchRequests())
            {
                downloadString.BeginInvoke(req.Url, req, ar =>//BeginInvoke：线程池中一个线程异步调用
                {
                    try
                    {
                        string resp = downloadString.EndInvoke(ar);//downloadString异步方法执行结束，执行ar委托
                        var images = req.Parse(resp);
                        foreach (var image in images)
                        {
                            this.Dispatcher.Invoke(addItem, image);//当前线程回到主线程同步执行
                        }
                    }
                    catch (WebException ex) when (ex.Message.Contains("401"))
                    {
                        MessageBox.Show(errorMessage, "Registration Needed");
                    }
                }, null);
            }

c#异步调用之基于事件
foreach (var req in GetSearchRequests())
            {
                var client = new WebClient();
                foreach (var header in req.Headers)
                {
                    client.Headers.Add(header.Key, header.Value);
                }
                client.Credentials = req.Credentials;
                client.DownloadStringCompleted += (sender1, e1) =>
                {
                    try
                    {
                        string resp = e1.Result;
                        var images = req.Parse(resp);
                        foreach (var image in images)
                        {
                            _searchInfo.List.Add(image);//事件处理程序可以直接访问UI
                        }
                    }
                    catch (Exception ex) when (ex.InnerException?.Message.Contains("401") ?? false)
                    {
                        MessageBox.Show(errorMessage, "Registration Needed");
                    }
                };
                client.DownloadStringAsync(new Uri(req.Url));//异步变体方法 以“async”后缀；结束之后会调用事件，此方法对应DownloadStringCompleted事件
            }