Question

I need to translate an exception emanating from a Task<T> in the same manner that doing the following would for normal synchronous code:

try {
  client.Call();
} catch(FaultException ex) {
    if (ex.<Some check>)
        throw new Exception("translated");
}

However, I want to do this asynchronously, i.e., Call above is actually Task CallAsync().

So in C# 5 my method would look like this:

async Task CallAndTranslate()
{
    try{
        await client.CallAsync();
    } catch(FaultException ex) {
        if (ex.FaultCode ...)
            throw new Exception("translated");
    }
}

But I’m using C# 4 for now.

So what can I do given that I want to trigger a Task but have the (TPL) Fault be translated and then expose the whole thing once again as a Task<T>?

• originally emanating from a WCF webservice but that’s not important here

EDIT: A slightly more concrete way of saying it:

public class TranslatingExceptions
{
    public Task ApiAsync() // The inner layer exposes it exactly this way
    {
        return Task.Factory.StartNew(()=>{ throw new Exception( "Argument Null" );});
    }

    public Task WrapsApiAsync() // this layer needs to expose it exactly this way
    {
        // async preview pseudocode for what I need to do                            
        try {
            await ApiAsync(  );
        } catch (Exception exception){
            if( exception.Message == "Argument Null"  )
                throw new ArgumentNullException();
        }
    }

    [Fact]
    public void Works()
    {
        var exception = Record.Exception( () => 
            WrapsApiAsync().Wait());
        Assert.IsType<ArgumentNullException>( exception.InnerException);
    }
}

How would you implement WrapsApiAsync() without needing C# 5?

Answer 1

Ok, so now that I’m entirely clear on what you’re looking for, here’s all you would need to do to build the equivalent in 4.0:

public class TranslatingExceptions
{
    public Task ApiAsync() // The inner layer exposes it exactly this way
    {
        return Task.Factory.StartNew(()=>{ throw new Exception( "Argument Null" );});
    }

    public Task WrapsApiAsync() // this layer needs to expose it exactly this way
    {
        // Grab the task that performs the "original" work
        Task apiAsyncTask = ApiAsync();

        // Hook a continuation to that task that will do the exception "translation"
        Task result = aspiAsync.ContinueWith(antecedent =>
        {
            // Check if the antecedent faulted, if so check what the exception's message was
            if ( antecedent.IsFaulted 
              && antecedent.Exception.InnerException.Message == "Argument Null" )
            {
                throw new ArgumentNullException();
            }
        },
        TaskContinuationOptions.ExecuteSynchronously);

        // Now we return the continuation Task from the wrapper method so that the caller of the wrapper method waits on that
        return result;
    }

    [Fact]
    public void Works()
    {
        var exception = Record.Exception(() => 
                                         WrapsApiAsync().Wait());

        Assert.IsType<ArgumentNullException>(exception.InnerException);
    }
}

This should accomplish what you’re looking for. One thing to note is that I use TaskContinuationOptions.ExecuteSynchronously when creating the continuation. This is because this work is small and tight and you don’t want to incur the over head of waiting for a whole other thread having to be picked up from the thread pool by the scheduler just to do this check.