Parallelism in .NET – Part 17, Think Continuations, not Callbacks
- by Reed
In traditional asynchronous programming, we’d often use a callback to handle notification of a background task’s completion. The Task class in the Task Parallel Library introduces a cleaner alternative to the traditional callback: continuation tasks.
Asynchronous programming methods typically required callback functions. For example, MSDN’s Asynchronous Delegates Programming Sample shows a class that factorizes a number. The original method in the example has the following signature:
public static bool Factorize(int number, ref int primefactor1, ref int primefactor2)
{
//...
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However, calling this is quite “tricky”, even if we modernize the sample to use lambda expressions via C# 3.0.
Normally, we could call this method like so:
int primeFactor1 = 0;
int primeFactor2 = 0;
bool answer = Factorize(10298312, ref primeFactor1, ref primeFactor2);
Console.WriteLine("{0}/{1} [Succeeded {2}]", primeFactor1, primeFactor2, answer);
If we want to make this operation run in the background, and report to the console via a callback, things get tricker. First, we need a delegate definition:
public delegate bool AsyncFactorCaller(
int number,
ref int primefactor1,
ref int primefactor2);
Then we need to use BeginInvoke to run this method asynchronously:
int primeFactor1 = 0;
int primeFactor2 = 0;
AsyncFactorCaller caller = new AsyncFactorCaller(Factorize);
caller.BeginInvoke(10298312, ref primeFactor1, ref primeFactor2,
result =>
{
int factor1 = 0;
int factor2 = 0;
bool answer = caller.EndInvoke(ref factor1, ref factor2, result);
Console.WriteLine("{0}/{1} [Succeeded {2}]", factor1, factor2, answer);
}, null);
This works, but is quite difficult to understand from a conceptual standpoint. To combat this, the framework added the Event-based Asynchronous Pattern, but it isn’t much easier to understand or author.
Using .NET 4’s new Task<T> class and a continuation, we can dramatically simplify the implementation of the above code, as well as make it much more understandable. We do this via the Task.ContinueWith method. This method will schedule a new Task upon completion of the original task, and provide the original Task (including its Result if it’s a Task<T>) as an argument. Using Task, we can eliminate the delegate, and rewrite this code like so:
var background = Task.Factory.StartNew(
() =>
{
int primeFactor1 = 0;
int primeFactor2 = 0;
bool result = Factorize(10298312, ref primeFactor1, ref primeFactor2);
return new {
Result = result,
Factor1 = primeFactor1,
Factor2 = primeFactor2
};
});
background.ContinueWith(task => Console.WriteLine("{0}/{1} [Succeeded {2}]",
task.Result.Factor1,
task.Result.Factor2,
task.Result.Result));
This is much simpler to understand, in my opinion. Here, we’re explicitly asking to start a new task, then continue the task with a resulting task. In our case, our method used ref parameters (this was from the MSDN Sample), so there is a little bit of extra boiler plate involved, but the code is at least easy to understand.
That being said, this isn’t dramatically shorter when compared with our C# 3 port of the MSDN code above. However, if we were to extend our requirements a bit, we can start to see more advantages to the Task based approach. For example, supposed we need to report the results in a user interface control instead of reporting it to the Console. This would be a common operation, but now, we have to think about marshaling our calls back to the user interface. This is probably going to require calling Control.Invoke or Dispatcher.Invoke within our callback, forcing us to specify a delegate within the delegate. The maintainability and ease of understanding drops. However, just as a standard Task can be created with a TaskScheduler that uses the UI synchronization context, so too can we continue a task with a specific context. There are Task.ContinueWith method overloads which allow you to provide a TaskScheduler. This means you can schedule the continuation to run on the UI thread, by simply doing:
Task.Factory.StartNew(
() =>
{
int primeFactor1 = 0;
int primeFactor2 = 0;
bool result = Factorize(10298312, ref primeFactor1, ref primeFactor2);
return new {
Result = result,
Factor1 = primeFactor1,
Factor2 = primeFactor2
};
}).ContinueWith(task => textBox1.Text = string.Format("{0}/{1} [Succeeded {2}]",
task.Result.Factor1,
task.Result.Factor2,
task.Result.Result),
TaskScheduler.FromCurrentSynchronizationContext());
This is far more understandable than the alternative. By using Task.ContinueWith in conjunction with TaskScheduler.FromCurrentSynchronizationContext(), we get a simple way to push any work onto a background thread, and update the user interface on the proper UI thread. This technique works with Windows Presentation Foundation as well as Windows Forms, with no change in methodology.
