Easily switch between foreground and background threads

ThreadSwitcher.cs

using System;
using System.Runtime.CompilerServices;
using System.Threading;
using System.Windows.Threading;

namespace Shared;

/// <summary>
/// Easily switch between foreground and background threads.
/// </summary>
/// <example>
/// ...we're on the foreground thread...
/// await Shared.ThreadSwitcher.ResumeBackgroundAsync();
/// ...do stuff in the background...
/// await Shared.ThreadSwitcher.ResumeForegroundAsync(TheWindow.Dispatcher);
///
/// // Assume we enter on the UI thread.
/// using (var connection = new Connection())
/// {
///     // Initialize on the UI thread since we need information from UI objects.
///     connection.Initialize(SomeParameter);
/// 
///     await ThreadSwitcher.ResumeBackgroundAsync();
///     // Execute on a background thread.
///     connection.Execute();
/// } // connection is disposed here
/// 
/// // Process the results on a background thread.
/// Process(connection.GetResults());
/// 
/// // Get back on the UI thread.
/// await ThreadSwitcher.ResumeForegroundAsync(App.Current.Dispatcher);
/// // ...access UI controls...
/// </example>
/// <see cref="https://devblogs.microsoft.com/oldnewthing/20190329-00/?p=102373" />
/// <seealso cref="https://devblogs.microsoft.com/oldnewthing/20190328-00/?p=102368" />

// For WPF
private struct DispatcherThreadSwitcher : INotifyCompletion
{
	private readonly Dispatcher _dispatcher;

	internal DispatcherThreadSwitcher(Dispatcher dispatcher) => _dispatcher = dispatcher;
	public DispatcherThreadSwitcher GetAwaiter() => this;
	public bool IsCompleted => _dispatcher.CheckAccess();
	public void GetResult() { }

	// Implement INotifyCompletion
	public void OnCompleted(Action continuation) => _dispatcher.BeginInvoke(continuation);
}

// For both WPF and Windows Forms
private struct ThreadPoolThreadSwitcher : INotifyCompletion
{
	public ThreadPoolThreadSwitcher GetAwaiter() => this;
	public bool IsCompleted => (SynchronizationContext.Current is null);
	public void GetResult() { }

	// Implement INotifyCompletion
	public void OnCompleted(Action continuation) => ThreadPool.QueueUserWorkItem(_ => continuation());
}

public class ThreadSwitcher
{
	// For WPF
	static public DispatcherThreadSwitcher ResumeForegroundAsync(Dispatcher dispatcher) => new DispatcherThreadSwitcher(dispatcher);

	// For both WPF and Windows Forms
	static public ThreadPoolThreadSwitcher ResumeBackgroundAsync() => new ThreadPoolThreadSwitcher();
}