异步调用中的异常会扰乱执行

Question

I am making two httpClient.GetAsync() calls.

According to the requirement, one of two calls will always throw a "No host known" exception and one will return a proper HttpResponseMessage object.

My problem is determining the bool value only after both async httpClient calls finish.

public async Task<bool> BothHttpCallsTask(string hostName)
{
    bool response1 = false;
    bool response2 = false;
    try
    {
        //httpClient and the GetAsync() are inside this method
        response1 = await CheckRedirectionBool(url1); 
        response2 = await CheckRedirectionBool(url2);
    }
    catch(Exception e)
    {
        //when exception is caught here, the method evaluates as false
        //as response2  is still not executed
    }
    return response1 || response2 ;
}

How do I make the execution only evaluate when both async calls complete successfully (keeping in mind the mandatory exception makes the return statement evaluate before response 2 can get a value from its execution) I need to return true if even one of the two http calls are successful.

Answer 1

Could you simply wrap each in its own exception handler? For instance:

try{
 response1 = ...
}
catch(Exception e){
 //set some flag here
}

try{
 response2 = ...
}
catch(Exception e){
 //set some flag here
}

This way you know which one past vs which one didn't and set some flags based on that condition, etc.

Answer 2

My problem is determining the bool value only after both async httpClient calls finish.

If you want to treat an exception the same as returning false , then I recommend writing a little helper method:

async Task<bool> TreatExceptionsAsFalse(Func<Task<bool>> action)
{
  try { return await action(); }
  catch { return false; }
}

Then it becomes easier to use Task.WhenAll :

public async Task<bool> BothHttpCallsTask(string hostName)
{
  var results = await Task.WhenAll(
      TreatExceptionsAsFalse(() => CheckRedirectionBool(url1)),
      TreatExceptionsAsFalse(() => CheckRedirectionBool(url2))
  );
  return results[0] || results[1];
}