Friday, October 19, 2018

.Net execution of async methods synchronously

Sometimes we need to execute an async method synchronously. And before you bristle up and try to deny it think of the following scenarios:
  • event handlers are by default synchronous
  • third party software might require you to implement a synchronous method (like a lot of Microsoft code, like Windows services)

All I want is to take public async Task ExecuteStuff(param1) and run it with ExecuteSynchronously(ExecuteStuff, param1).

"What's the problem?", you might ask, "Just use SomeMethod().Result instead of await SomeMethod()". Well, it doesn't work because of the dreaded deadlock (a dreadlock). Here is an excerpt of the .Net C# code for Task<T>.InternalWait (used by Result and Wait):
// Alert a listening debugger that we can't make forward progress unless it slips threads.
// We call NOCTD for two reasons:
//    1. If the task runs on another thread, then we'll be blocked here indefinitely.
//    2. If the task runs inline but takes some time to complete, it will suffer ThreadAbort with possible state corruption,
//       and it is best to prevent this unless the user explicitly asks to view the value with thread-slipping enabled.
Debugger.NotifyOfCrossThreadDependency();
As you can see, it only warns the debugger, it doesn't crash it just freezes. And this is just in case it catches the problem at all.

What about other options? Task has a method called RunSynchronously, it should work, right? What about Task<T>.GetAwaiter().GetResult() ? What about .ConfigureAwait(false)? In my experience all of these caused deadlocks.

Now, I am giving you two options, one that I tried and works and one that I found on a Microsoft forum. I don't have the time and brain power to understand how this all works behind the scenes, so I am asking for assistance in telling me why this works and other things do not.

First of all, two sets of extensions methods I use to execute synchronously an async method or func:
/// <summary>
/// Execute an async function synchronously
/// </summary>
public static TResult ExecuteSynchronously<TResult>(this Func<Task<TResult>> asyncMethod)
{
    TResult result = default(TResult);
    Task.Run(async () => result = await asyncMethod()).Wait();
    return result;
}
 
/// <summary>
/// Execute an async function synchronously
/// </summary>
public static TResult ExecuteSynchronously<TResult, TParam1>(this Func<TParam1, Task<TResult>> asyncMethod, TParam1 t1)
{
    TResult result = default(TResult);
    Task.Run(async () => result = await asyncMethod(t1)).Wait();
    return result;
}
 
/// <summary>
/// Execute an async function synchronously
/// </summary>
public static void ExecuteSynchronously(this Func<Task> asyncMethod)
{
    Task.Run(async () => await asyncMethod()).Wait();
}
 
/// <summary>
/// Execute an async function synchronously
/// </summary>
public static void ExecuteSynchronously<TParam1>(this Func<TParam1,Task> asyncMethod, TParam1 t1)
{
    Task.Run(async () => await asyncMethod(t1)).Wait();
}

I know, the ones returning a result would probably be better as return Task.Run(...).Result or even Task.Run(...).GetAwaiter().GetResult(), but I didn't want to take any chances. Because of how the compiler works, you can't really use them as extension methods like SomeMethod.ExecuteSynchronously(param1) unless SomeMethod is a variable defined as Func<TParam1, TResult>. Instead you must use them like a normal static method: MethodExtensions.ExecuteSynchronously(SomeMethod, param1).

Second is the solution for this proposed by slang25 :

public static T ExecuteSynchronously<T>(Func<Task<T>> taskFunc)
{
    var capturedContext = SynchronizationContext.Current;
    try {
        SynchronizationContext.SetSynchronizationContext(null);
        return taskFunc.Invoke().GetAwaiter().GetResult();
    }
    finally {
        SynchronizationContext.SetSynchronizationContext(capturedContext);
    }
}

I haven't tried this, so I can only say that it looks OK.

A third options, I am told, works by configuring every await to run with ConfigureAwait(false). There is a NuGet package that does this for you. Again, untried.

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