A file I used to understand how async / await works behind-the-scenes in C#.

ExampleAsync.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Runtime.CompilerServices;
using System.Threading.Tasks;

namespace Tests
{
    /// <summary>
    /// Shows an example of code generated by the compiler for an <see langword="async"/> method.
    /// </summary>
    public sealed class ExampleAsync
    {
        #region Example method
        /// <summary>
        /// Asynchronously returns the answer to Life, the Universe, and
        /// Everything. Fortunately, this answer is already known, and all
        /// this method does is return a cached version of it,
        /// after complating it a few moments.
        /// </summary>
        /// 
        /// <remarks>
        /// This method will be compiled to an entirely different method by the
        /// compiler, and an <see cref="IAsyncStateMachine"/> will be generated
        /// for it.
        /// The methods below show what the result looks like.
        /// </remarks>
        public async Task<int> GetAnswerAsync(int contemplationTime)
        {
            await Task.Delay(contemplationTime);

            if (contemplationTime < 1000)
                return await GetAnswerAsync(1000);

            return 42;
        }
        #endregion

        [CompilerGenerated]
        private Task<int> GetAnswerAsync_CompilerGenerated(int contemplationTime)
        {
            // This method will simply:
            // - Create the generated class (ExampleStateMachine)
            // - Initialize it with local variables, such as "this", and the
            //   "contemplationTime" parameter
            // - Set its initial state (-1)
            // - Initialize its AsyncTaskMethodBuilder (whose generic argument
            //   is the same as the return type)
            // - Start the builder
            // - Return the task created by the builder
            ExampleStateMachine asm = new ExampleStateMachine
            {
                @this = this,
                comtemplationTime = contemplationTime,
                builder = AsyncTaskMethodBuilder<int>.Create(),
                state = -1
            };

            AsyncTaskMethodBuilder<int> taskBuilder = asm.builder;
            taskBuilder.Start(ref asm);

            return asm.builder.Task;
        }

        #region Actual ASM implementation
        /// <summary>
        /// Example state machine written by a human to demonstrate how
        /// the compiler generates <see cref="IAsyncStateMachine"/>s.
        /// </summary>
        [CompilerGenerated]
        public sealed class ExampleStateMachine : IAsyncStateMachine
        {
            // Typically, fields will have different names:
            // - Variables and parameters will keep the same name
            // - @this, builder, and state will be prefixed with an identifier, ie. "<>__1builder"
            public ExampleAsync @this;
            public int comtemplationTime;
            public AsyncTaskMethodBuilder<int> builder;
            public int state;

            // The private fields can be:
            // - The result of the awaiters used everytime an "await ..." call is made in the method body
            // - Temporary variables used in chain calls
            private TaskAwaiter awaiter1;
            private TaskAwaiter<int> awaiter2;
            private int result1;

            // 

            void IAsyncStateMachine.MoveNext()
            {
                // This method does *all the job*.
                // It uses an integer named "state" to know where it is in the method,
                // and if / else / goto statements to go back to a previous state.
                // That way, the method can return and be called back,
                // whilst keeping the same state.
                // Here, state can mean a few things:
                //  [-2]: The result of the method is computed, or it has thrown;
                //        we can really return now, and never come back.
                //  [-1]: Start "await Task.Delay(contemplationTime)", and:
                //         - If it completed instantly, or if it done, keep going.
                //         - If it hasn't completed, wait till it ends, and return.
                //  [ 0]: Wait till "await Task.Delay(contemplationTime)" is done.
                //  [ 1]: Wait till "await GetAnswerAsync(1000)" is done, and returns something.

                // Represents a local variable of the state (faster access)
                int locState = this.state;

                // Represents the return value of the whole method
                int returnValue;

                try
                {
                    TaskAwaiter taskAwaiter1;
                    TaskAwaiter<int> taskAwaiter2;

                    // The following block attempts to get the return value of the
                    // "await Task.Delay(contemplationTime)" invocation.
                    if (locState != 0)
                    {
                        if (locState != 1)
                        {
                            taskAwaiter1 = Task.Delay(this.comtemplationTime).GetAwaiter();

                            if (!taskAwaiter1.IsCompleted)
                            {
                                this.state = 0;
                                this.awaiter1 = taskAwaiter1;
                                ExampleStateMachine stateMachine = this;

                                this.builder.AwaitUnsafeOnCompleted(ref taskAwaiter1, ref stateMachine);
                                return;
                            }
                        }
                        else
                        {
                            taskAwaiter2 = this.awaiter2;
                            this.awaiter2 = default(TaskAwaiter<int>);
                            this.state = -1;
                            goto label_10;
                        }
                    }
                    else
                    {
                        taskAwaiter1 = this.awaiter1;
                        this.awaiter1 = default(TaskAwaiter);
                        this.state = -1;
                    }

                    // At this point the result should have been computed
                    // by builder.AwaitUnsafeOnCompleted().
                    taskAwaiter1.GetResult();

                    if (this.comtemplationTime < 1000)
                    {
                        taskAwaiter2 = [email protected](1000).GetAwaiter();

                        if (!taskAwaiter2.IsCompleted)
                        {
                            this.state = 1;
                            this.awaiter2 = taskAwaiter2;
                            ExampleStateMachine stateMachine = this;

                            this.builder.AwaitUnsafeOnCompleted(ref taskAwaiter2, ref stateMachine);
                            return;
                        }
                    }
                    else
                    {
                        returnValue = 42;
                        goto label_11;
                    }

                    label_10:
                    returnValue = result1 = taskAwaiter2.GetResult();
                    taskAwaiter2 = default(TaskAwaiter<int>);
                    label_11:
                    returnValue = 42;
                }
                catch (Exception ex)
                {
                    this.state = -2;
                    this.builder.SetException(ex);

                    return;
                }

                this.state = -2;
                this.builder.SetResult(returnValue);
            }

            void IAsyncStateMachine.SetStateMachine(IAsyncStateMachine stateMachine)
            {
                // This method is never used, but if it gets called, we should relay it to the builder.
                builder.SetStateMachine(stateMachine);
            }
        }
        #endregion
        
        // Dig deeper:
        // - How the compiler does it: http://source.roslyn.io/#Microsoft.CodeAnalysis.CSharp/Lowering/AsyncRewriter/AsyncMethodToStateMachineRewriter.cs
    }
}