What's the difference between Foo().Result and Task.Run(() => Foo()).Result in C#?

Question

In C# what is the difference between these two statements? If I use the first one in my constructor in my test classes I get a deadlock, or something similar, and the tests never finish. With the second one the code works.

// Deadlock.
var r = MyMethod().Result;

// Works.
var r = Task.Run(() => MyMethod()).Result;

Update: There is a bit more context in this commit: https://github.com/webCRMdotcom/erp-integrations/pull/92/commits/dd8af89899ce1de837ef6e34f0688a685a5cea3b.

Answer 1

The difference is the starting thread context.

Here a simple sample

using System;
using System.Threading.Tasks;

public class Program
{
    public static void Main()
    {
        string r;
        OutputThreadInfo("Main");
        r = MyMethod().Result;
        r = Task.Run( () => MyMethod() ).Result;
    }

    public static async Task<string> MyMethod()
    {
        OutputThreadInfo("MyMethod");
        await Task.Delay(50);
        return "finished";
    }

    private static void OutputThreadInfo(string context)
    {
        Console.WriteLine("{0} {1}",context,System.Threading.Thread.CurrentThread.ManagedThreadId);
    }
}

.net fiddle

which will output

Main 32
MyMethod 32
MyMethod 63

The first call of MyMethod will start at the same thread as Main and if started from a thread with a synchronization context it will block.

The second call of MyMethod will start from a different thread (worker thread from thread pool) as Main which does not have a synchronization context and therefor will not block.

PS You should keep in mind that Console applications do not have a synchronization context as default but WinForms, WPF, UWP application do have and so will behave somehow different on async/await