WhenAll for .NET 3.5

Question

I am trying to write a version of Task.WhenAll for .NET 3.5 using the "Task Parallel Library for .NET 3.5". This is what I came up with. Is there a better way of doing this?

    public static Task WhenAll(IEnumerable<Task> tasks)
    {
        var tcs = new TaskCompletionSource<object>();

        var remainingTasks = tasks.ToList();
        int count = remainingTasks.Count();
        var exceptions = new List<Exception>();

        foreach (var task in remainingTasks)
        {
            task.ContinueWith(t =>
            {
                if (Interlocked.Decrement(ref count) == 0)
                {
                    foreach (var task1 in remainingTasks)
                    {
                        if (task1.IsFaulted)
                        {
                            exceptions.Add(task1.Exception);
                        }
                    }

                    if (exceptions.Any())
                    {
                        tcs.SetException(new AggregateException(exceptions));
                    }
                    else
                    {
                        tcs.SetResult(null);
                    }
                }
            });
        }

        return tcs.Task;
    }

Answer 1

int count = remainingTasks.Count();

Since remainingTasks is a List, you can use the Count property here.

---

var exceptions = new List<Exception>();

This variable should be declared where it's used: in the if Interlocked.Decrement block.

---

if (task1.IsFaulted)

This means you're treating canceled tasks the same as successfully completed ones. Instead, you can check whether Exception is null.

You could also rewrite the inner foreach using LINQ:

var exceptions = remainingTasks
    .Select(task => task.Exception)
    .Where(e => e != null)
    .ToList();

Answer 2

I like how you've written this code. Just a couple of nitpicks:

Consistency with var

    var remainingTasks = tasks.ToList();
    int count = remainingTasks.Count();
    var exceptions = new List<Exception>();

Why not use var to declare count? Seems pretty obvious to me that remainingTasks.Count() would be an int, and it's the only explicitly typed variable in your method.

Naming

I don't like task1; you should prefer a meaninfgul name here, such as remainingTask. Also I'm not sure about tcs, I might have called it completionSource.

Answer 3

I'm a fan of the fail-fast methodology, so I'd flip around the conditional to:

if (Interlocked.Decrement(ref count) > 0)
{
    return;
}