Ignore the Tasks throwing Exceptions at Task.WhenAll and get only the completed results

Question

I am working on a Task parallel problem that I have many Tasks that may or may not throw Exception.

I want to process all the tasks that finishes properly and log the rest. The Task.WhenAll propage the Task exception without allowing me to gather the rest results.

    static readonly Task<string> NormalTask1 = Task.FromResult("Task result 1");
    static readonly Task<string> NormalTask2 = Task.FromResult("Task result 2");
    static readonly Task<string> ExceptionTk = Task.FromException<string>(new Exception("Bad Task"));
    var results = await Task.WhenAll(new []{ NormalTask1,NormalTask2,ExceptionTk});

The Task.WhenAll with throw the Exception of ExcceptionTk ignoring the rest results. How I can get the results ignoring the Exception and log the exception at same time?

I could wrap the task into another task that try{...}catch(){...} the internal exception but I don't have access to them and I hope I will not have to add this overhead.

Answer 1

You can create a method like this to use instead of Task.WhenAll:

public Task<ResultOrException<T>[]> WhenAllOrException<T>(IEnumerable<Task<T>> tasks)
{    
    return Task.WhenAll(
        tasks.Select(
            task => task.ContinueWith(
                t => t.IsFaulted
                    ? new ResultOrException<T>(t.Exception)
                    : new ResultOrException<T>(t.Result))));
}


public class ResultOrException<T>
{
    public ResultOrException(T result)
    {
        IsSuccess = true;
        Result = result;
    }

    public ResultOrException(Exception ex)
    {
        IsSuccess = false;
        Exception = ex;
    }

    public bool IsSuccess { get; }
    public T Result { get; }
    public Exception Exception { get; }
}

Then you can check each result to see if it was successful or not.

---

EDIT: the code above doesn't handle cancellation; here's an alternative implementation:

public Task<ResultOrException<T>[]> WhenAllOrException<T>(IEnumerable<Task<T>> tasks)
{    
    return Task.WhenAll(tasks.Select(task => WrapResultOrException(task)));
}

private async Task<ResultOrException<T>> WrapResultOrException<T>(Task<T> task)
{
    try
    {           
        var result = await task;
        return new ResultOrException<T>(result);
    }
    catch (Exception ex)
    {
        return new ResultOrException<T>(ex);
    }
}