sample for understanding the behavior of 'await' in C#. inspired by Jake Archibald's interesting quiz: https://twitter.com/jaffathecake/status/999269332889763840

async_test.cs

using System;
using System.Threading.Tasks;

class Program {
    const bool UseCompletedTask = true; // You can change this.
    //If you use the completed task, "2 3" must be printed, because the program is run synchronously.
    //If you use the incompleted task, "1 2", "2 1", "2 3", ... may be printed, because the program is run asynchronously.

    static void Main() {
        var x = 0;

        async Task Test() {
            var task = UseCompletedTask ? Task.FromResult(2) : Task.Run(() => 2);
            x += await task;
            Console.WriteLine(x);
        }

        Test();
        x += 1;
        Console.WriteLine(x);
        Task.Delay(100).Wait();
    }
}

This is the result when I use the completed task.

This is the result when I use the incompleted task.