WebClient DownloadFileCompleted事件Lambda中await报错：如何添加async修饰符？

API 探路者

2026-8-20

解决C#异步Lambda表达式的await报错问题

你遇到的错误是因为绑定DownloadFileCompleted事件的Lambda表达式中使用了await，但该Lambda未标记为异步。

解决方法

只需在Lambda的参数列表前添加async修饰符即可，修改后的事件绑定代码如下：

webClient.DownloadFileCompleted += new AsyncCompletedEventHandler(async (sender, e) => await Completed(sender, e, sw));

也可以省略显式的委托类型声明，编译器会自动推断，写法更简洁：

webClient.DownloadFileCompleted += async (sender, e) => await Completed(sender, e, sw);

修改后的完整代码

public async Task DownloadFile(string urlAddress, string location)
{
    Stopwatch sw = new Stopwatch();
    using (WebClient webClient = new WebClient())
    {
        webClient.Headers.Add("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:67.0) Gecko/20100101 Firefox/67.0 Chrome");
        // 添加async修饰符修复报错
        webClient.DownloadFileCompleted += async (sender, e) => await Completed(sender, e, sw);
        webClient.DownloadProgressChanged += (sender, e) => ProgressChanged(sender, e, sw);

        Uri URL = new Uri(urlAddress);
        
        sw.Start();
        try
        {
            await webClient.DownloadFileTaskAsync(URL, location);
        }
        catch (Exception ex)
        {
            MessageBox.Show(ex.Message);
        }
    }
}