c# - 在捕获 block 中等待

Question

我有以下代码:

WebClient wc = new WebClient();
string result;
try
{
  result = await wc.DownloadStringTaskAsync( new Uri( "http://badurl" ) );
}
catch
{
  result = await wc.DownloadStringTaskAsync( new Uri( "http://fallbackurl" ) );
}

基本上我想从一个 URL 下载，当它因异常而失败时，我想从另一个 URL 下载。当然，两者都是异步的。但是代码无法编译，因为

error CS1985: Cannot await in the body of a catch clause

好吧，无论出于何种原因，它都是被禁止的，但是这里正确的代码模式是什么？

编辑:

好消息是C# 6.0 will likely allow await calls both in catch and finally blocks .

Answer 1

更新: C# 6.0 supports await in catch

---

旧答案:您可以重写该代码以使用标志从 catch block 中移动 await:

WebClient wc = new WebClient();
string result = null;
bool downloadSucceeded;
try
{
  result = await wc.DownloadStringTaskAsync( new Uri( "http://badurl" ) );
  downloadSucceeded = true;
}
catch
{
  downloadSucceeded = false;
}

if (!downloadSucceeded)
  result = await wc.DownloadStringTaskAsync( new Uri( "http://fallbackurl" ) );