等待在catch块 [英] Await in catch block

问题描述

我有以下的code：

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，当它失败例外，我想从另一个URL下载。这两个时间异步课程。然而，code不能编译，因为，

Basically I want to download from a URL and when it fails with an exception I want to download from another URL. Both time async of course. However the code does not compile, because of

错误CS1985：不能在catch子句的身体等待

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

好吧，这是被禁止无论出于何种原因，但这里有什么正确的code模式？

OK, it's forbidden for whatever reason but what's the correct code pattern here?

编辑：

好消息是， C＃6.0将有可能让等待无论是在渔获物和通话finally块的。

推荐答案

更新：的 C＃6.0支持渔获等待

旧的答案：您可以重写code键移动等待从抓块是用一个标志：

Old Answer: You can rewrite that code to move the await from the catch block using a flag:

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" ) );

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