SwitchOnDispatcher

DispatcherSwitchTaskAwaitable.cs

using System;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Threading.Tasks;

namespace DeveloperOnTheFlow
{
  public struct DispatcherSwitchTaskAwaitable
  {
    readonly ConfiguredTaskAwaiter configuredTaskAwaiter;

    public DispatcherSwitchTaskAwaitable(Task task, SynchronizationContext synchronizationContext)
    {
      this.configuredTaskAwaiter = new ConfiguredTaskAwaiter(task, synchronizationContext);
    }

    public class ConfiguredTaskAwaiter : INotifyCompletion
    {
      #region Fields

      readonly SynchronizationContext synchronizationContext;
      readonly Task task;

      #endregion

      #region Constructors

      public ConfiguredTaskAwaiter(Task task, SynchronizationContext synchronizationContext)
      {
        this.task = task;
        this.synchronizationContext = synchronizationContext;
      }

      #endregion

      #region Properties, Indexers

      /// <summary>
      /// Returns true if the task is completed and there is no need for the compiler to call <see cref="OnCompleted"/>. This only happens if we're already on the dispatcher thread, otherwise we want <see cref="OnCompleted"/> to run so we can switch threads.
      /// </summary>
      public bool IsCompleted => this.task.IsCompleted && SynchronizationContext.Current == this.synchronizationContext;

      #endregion

      #region Public Methods

      /// <summary>Ends the await on the completed <see cref="System.Threading.Tasks.Task"/>.</summary>
      /// <returns>The result of the completed <see cref="System.Threading.Tasks.Task{TResult}"/>.</returns>
      /// <exception cref="System.NullReferenceException">The awaiter was not properly initialized.</exception>
      /// <exception cref="System.Threading.Tasks.TaskCanceledException">The task was canceled.</exception>
      /// <exception cref="System.Exception">The task completed in a Faulted state.</exception>
      public void GetResult()
      {
        this.task.Wait();
      }

      #endregion

      #region INotifyCompletion Members

      public void OnCompleted(Action continuation)
      {
        this.synchronizationContext.Post(state => continuation(), null);
      }

      #endregion
    }

    /// <summary>Gets an awaiter for this awaitable.</summary>
    /// <returns>The awaiter.</returns>
    public ConfiguredTaskAwaiter GetAwaiter()
    {
      return this.configuredTaskAwaiter;
    }
  }

  public struct DispatcherSwitchTaskAwaitable<TResult>
  {
    readonly ConfiguredTaskAwaiter configuredTaskAwaiter;

    public DispatcherSwitchTaskAwaitable(Task<TResult> task, SynchronizationContext synchronizationContext)
    {
      this.configuredTaskAwaiter = new ConfiguredTaskAwaiter(task, synchronizationContext);
    }

    public class ConfiguredTaskAwaiter : INotifyCompletion
    {
      #region Fields

      readonly SynchronizationContext synchronizationContext;
      readonly Task<TResult> task;

      #endregion

      #region Constructors

      public ConfiguredTaskAwaiter(Task<TResult> task, SynchronizationContext synchronizationContext)
      {
        this.task = task;
        this.synchronizationContext = synchronizationContext;
      }

      #endregion

      #region Properties, Indexers

      /// <summary>
      /// Returns true if the task is completed and there is no need for the compiler to call <see cref="OnCompleted"/>. This only happens if we're already on the dispatcher thread, otherwise we want <see cref="OnCompleted"/> to run so we can switch threads.
      /// </summary>
      public bool IsCompleted => this.task.IsCompleted && SynchronizationContext.Current == this.synchronizationContext;

      #endregion

      #region Public Methods

      /// <summary>Ends the await on the completed <see cref="System.Threading.Tasks.Task"/>.</summary>
      /// <returns>The result of the completed <see cref="System.Threading.Tasks.Task{TResult}"/>.</returns>
      /// <exception cref="System.NullReferenceException">The awaiter was not properly initialized.</exception>
      /// <exception cref="System.Threading.Tasks.TaskCanceledException">The task was canceled.</exception>
      /// <exception cref="System.Exception">The task completed in a Faulted state.</exception>
      public TResult GetResult()
      {
        return this.task.Result;
      }

      #endregion

      #region INotifyCompletion Members

      public void OnCompleted(Action continuation)
      {
        this.synchronizationContext.Post(state => continuation(), null);
      }

      #endregion
    }

    /// <summary>Gets an awaiter for this awaitable.</summary>
    /// <returns>The awaiter.</returns>
    public ConfiguredTaskAwaiter GetAwaiter()
    {
      return this.configuredTaskAwaiter;
    }
  }
}