Understanding context in C# 5 async/await

Question

Am I correct that async/await itself has nothing to do with concurrency/parallelism and is nothing more than continuation-passing style (CPS) implementation? And the real th

Answer 1

No, the async/await keywords have everything to do with concurrency. async/await basically wrap your method code in to a task and continuation. To see the exact translation that the compiler produces (using the Task Parallel Library) disassemble some code snippet. This translation of async/await usage is 'similar' (but not identical!) to the example below

async Task TaskOfTResult_MethodAsync()
{
    int hours;
    // . . .
    // Return statement specifies an integer result.
    return hours;
}

// Calls to TaskOfTResult_MethodAsync
Task returnedTaskTResult = TaskOfTResult_MethodAsync();
int intResult = await returnedTaskTResult;
// or, in a single statement
int intResult = await TaskOfTResult_MethodAsync();

this is approxiamtely converted to

private int Result()
{
    int hours;
    // . . .
    // Return statement specifies an integer result.
    return hours;
}

where you wait for the return outside of the method like

int? hours = null;
Task task = null;
task = Task.Factory.StartNew(() => Result());
task.ContnueWith(cont => 
{
    // Some task completion checking...
    hours = task.Result;
}, CancellationToken.None, 
   TaskCreationOptions.None, 
   TaskScheduler.Current);

Or, you could place the TPL code into the Result method

private int ResultAsync()
{
    int? hours = null;
    Task task = null;
    task = Task.Factory.StartNew(() => 
    {
        int hours;
        // . . .
        // Return statement specifies an integer result.
        return hours;
    }, CancellationToken.None, 
       TaskCreationOptions.None, 
       TaskScheduler.Current);
    try
    {
        return task.Result;
    }
    catch (AggregateException aggEx)
    {
        // Some handler method for the agg exception.
        aggEx.Handle(HandleException); 
    }
}

SynchronizationContext does not guarantee that continuation will be executed on the same thread for the async/awate code. However, you can set the context using TPL code, via the SynchronisationContex keyword.