== Why is this an issue?

Making blocking calls to ``++async++`` methods transforms code that was intended to be asynchronous into a blocking operation. Doing so can cause deadlocks and unexpected blocking of context threads.


According to the MSDN documentation:

____
The root cause of this deadlock is due to the way ``++await++`` handles contexts. By default, when an incomplete ``++Task++`` is awaited, the current “context” is captured and used to resume the method when the ``++Task++`` completes. This “context” is the current ``++SynchronizationContext++`` unless it’s null, in which case it’s the current ``++TaskScheduler++``. GUI and ASP.NET applications have a ``++SynchronizationContext++`` that permits only one chunk of code to run at a time. When the ``++await++`` completes, it attempts to execute the remainder of the ``++async++`` method within the captured context. But that context already has a thread in it, which is (synchronously) waiting for the ``++async++`` method to complete. They’re each waiting for the other, causing a deadlock.

____

[frame=all]
[cols="^1,^1,^1"]
|===
|To Do This …|Instead of This …|Use This

|Retrieve the result of a background task|``++Task.Wait++``, ``++Task.Result++`` or ``++Task.GetAwaiter.GetResult++``|``++await++``
|Wait for any task to complete|``++Task.WaitAny++``|``++await Task.WhenAny++``
|Retrieve the results of multiple tasks|``++Task.WaitAll++``|``++await Task.WhenAll++``
|Wait a period of time|``++Thread.Sleep++``|``++await Task.Delay++``
|===

=== Noncompliant code example

[source,text]
----
public static class DeadlockDemo
{
    private static async Task DelayAsync()
    {
        await Task.Delay(1000);
    }

    // This method causes a deadlock when called in a GUI or ASP.NET context.
    public static void Test()
    {
        // Start the delay.
        var delayTask = DelayAsync();
        // Wait for the delay to complete.
        delayTask.Wait(); // Noncompliant
    }
}
----


=== Compliant solution

[source,text]
----
public static class DeadlockDemo
{
    private static async Task DelayAsync()
    {
        await Task.Delay(1000);
    }

    public static async Task TestAsync()
    {
        // Start the delay.
        var delayTask = DelayAsync();
        // Wait for the delay to complete.
        await delayTask;
    }
}
----


=== Exceptions

* Main methods of Console Applications are not subject to this deadlock issue and so are ignored by this rule.
* ``++Thread.Sleep++`` is also ignored when it is used in a non-``++async++`` method.
* Calls chained after ``++Task.Run++`` or ``++Task.Factory.StartNew++`` are ignored because they don't suffer from this deadlock issue


== Resources

* https://msdn.microsoft.com/en-us/magazine/jj991977.aspx[Async/Await - Best Practices in Asynchronous Programming]