Demonstrates using a .NET semaphore as a way to synchronize access to non-thread-safe code

MultiThreadedCallsSingleThreadedViaSemaphore.cs

public class ThreadSafeDictionary<K, V>
{
  private readonly SemaphoreSlim semaphore = new SemaphoreSlim(1);
  private readonly Dictionary<K, V> inner = new Dictionary<K, V>();

  public async Task AddAsync(K key, V value, CancellationToken cancellationToken = default(CancellationToken)) {
    await this.semaphore.WaitAsync(cancellationToken);
    try {
      this.inner.Add(key, value);
    } finally {
      this.semaphore.Release();
    }
  }
}