DeferredAction.cs

using System;
using System.Threading;

internal sealed class DeferredAction
{
    private static readonly Action SentinelNextSetShouldPost = () => { };

    private readonly SynchronizationContext synchronizationContext;
    private Action scheduledAction = SentinelNextSetShouldPost;

    public DeferredAction()
    {
        synchronizationContext = SynchronizationContext.Current
            ?? throw new InvalidOperationException($"{nameof(DeferredAction)} requires a current synchronization context.");
    }

    public void Set(Action action)
    {
        var previouslyScheduledAction = Interlocked.Exchange(ref scheduledAction, action);

        if (ReferenceEquals(previouslyScheduledAction, SentinelNextSetShouldPost))
            synchronizationContext.Post(OnSynchronizationContextPost, state: this);
    }

    public void Clear() => Set(null);

    private static void OnSynchronizationContextPost(object state)
    {
        var @this = (DeferredAction)state;

        Interlocked.Exchange(ref @this.scheduledAction, SentinelNextSetShouldPost)?.Invoke();
    }
}