C# SmartDispatcher class by Jeff Wilcox for WPF MVVM

App.xaml.cs

public App()
{
    this.Startup += this.Application_Startup;
    this.Exit += this.Application_Exit;
    this.UnhandledException += this.Application_UnhandledException;
 
    SmartDispatcher.Initialize(Deployment.Current.Dispatcher);
 
    InitializeComponent();
}

PropertyChangedBase.cs

// (c) Copyright Microsoft Corporation.
// This source is subject to the Microsoft Public License (Ms-PL).
// Please see http://go.microsoft.com/fwlink/?LinkID=131993 for details.
// All other rights reserved.
 
using System;
using System.Collections.Generic;
using System.Windows.Threading;
 
namespace System.ComponentModel
{
    /// <summary>
    /// A base class for data objects that implement the property changed 
    /// interface, offering data binding and change notifications.
    /// </summary>
    public class PropertyChangedBase : INotifyPropertyChanged
    {
        /// <summary>
        /// A static set of argument instances, one per property name.
        /// </summary>
        private static Dictionary<string, PropertyChangedEventArgs> _argumentInstances = new Dictionary<string, PropertyChangedEventArgs>();
 
        /// <summary>
        /// The property changed event.
        /// </summary>
        public event PropertyChangedEventHandler PropertyChanged;
 
        /// <summary>
        /// Notify any listeners that the property value has changed.
        /// </summary>
        /// <param name="propertyName">The property name.</param>
        protected void NotifyPropertyChanged(string propertyName)
        {
            PropertyChangedEventHandler handler = PropertyChanged;
            if (handler != null)
            {
                PropertyChangedEventArgs args;
                if (!_argumentInstances.TryGetValue(propertyName, out args))
                {
                    args = new PropertyChangedEventArgs(propertyName);
                    _argumentInstances[propertyName] = args;
                }
 
                // Fire the change event. The smart dispatcher will directly
                // invoke the handler if this change happened on the UI thread,
                // otherwise it is sent to the proper dispatcher.
                SmartDispatcher.BeginInvoke(delegate
                {
                    handler(this, args); 
                });
            }
        }
    }
}

SmartDispatcher.cs

// (c) Copyright Microsoft Corporation.
// This source is subject to the Microsoft Public License (Ms-PL).
// Please see http://go.microsoft.com/fwlink/?LinkID=131993 for details.
// All other rights reserved.
 
using System.ComponentModel;
 
namespace System.Windows.Threading
{
    /// <summary>
    /// A smart dispatcher system for routing actions to the user interface
    /// thread.
    /// </summary>
    public static class SmartDispatcher
    {
        /// <summary>
        /// A single Dispatcher instance to marshall actions to the user
        /// interface thread.
        /// </summary>
        private static Dispatcher _instance;
 
        /// <summary>
        /// Backing field for a value indicating whether this is a design-time
        /// environment.
        /// </summary>
        private static bool? _designer;
         
        /// <summary>
        /// Requires an instance and attempts to find a Dispatcher if one has
        /// not yet been set.
        /// </summary>
        private static void RequireInstance()
        {
            if (_designer == null)
            {
                _designer = DesignerProperties.IsInDesignTool;
            }
 
            // Design-time is more of a no-op, won't be able to resolve the
            // dispatcher if it isn't already set in these situations.
            if (_designer == true)
            {
                return;
            }
 
            // Attempt to use the RootVisual of the plugin to retrieve a
            // dispatcher instance. This call will only succeed if the current
            // thread is the UI thread.
            try
            {
                _instance = Application.Current.RootVisual.Dispatcher;
            }
            catch (Exception e)
            {
                throw new InvalidOperationException("The first time SmartDispatcher is used must be from a user interface thread. Consider having the application call Initialize, with or without an instance.", e);
            }
 
            if (_instance == null)
            {
                throw new InvalidOperationException("Unable to find a suitable Dispatcher instance.");
            }
        }
 
        /// <summary>
        /// Initializes the SmartDispatcher system, attempting to use the
        /// RootVisual of the plugin to retrieve a Dispatcher instance.
        /// </summary>
        public static void Initialize()
        {
            if (_instance == null)
            {
                RequireInstance();
            }
        }
 
        /// <summary>
        /// Initializes the SmartDispatcher system with the dispatcher
        /// instance.
        /// </summary>
        /// <param name="dispatcher">The dispatcher instance.</param>
        public static void Initialize(Dispatcher dispatcher)
        {
            if (dispatcher == null)
            {
                throw new ArgumentNullException("dispatcher");
            }
 
            _instance = dispatcher;
 
            if (_designer == null)
            {
                _designer = DesignerProperties.IsInDesignTool;
            }
        }
 
        /// <summary>
        /// 
        /// </summary>
        /// <returns></returns>
        public static bool CheckAccess()
        {
            if (_instance == null)
            {
                RequireInstance();
            }
 
            return _instance.CheckAccess();
        }
 
        /// <summary>
        /// Executes the specified delegate asynchronously on the user interface
        /// thread. If the current thread is the user interface thread, the
        /// dispatcher if not used and the operation happens immediately.
        /// </summary>
        /// <param name="a">A delegate to a method that takes no arguments and 
        /// does not return a value, which is either pushed onto the Dispatcher 
        /// event queue or immediately run, depending on the current thread.</param>
        public static void BeginInvoke(Action a)
        {
            if (_instance == null)
            {
                RequireInstance();
            }
 
            // If the current thread is the user interface thread, skip the
            // dispatcher and directly invoke the Action.
            if (_instance.CheckAccess() || _designer == true)
            {
                a();
            }
            else
            {
                _instance.BeginInvoke(a);
            }
        }
    }
}