c# - Async method in lock-statement block

Question

I want to place an authentication token in a cache where it can be used in multiple app domains. This token will expire every hour. When a client is unable to authorize with the token, it asks the token generation service to generate a new one.

I only want this regeneration to occur for the first client that does not authenticate sucessfully, so I've employed a lock object like this:

public async Task<Token> GenerateToken(Token oldToken)
{
    Token token;
    lock (lockObject)
    {
        var cachedToken = GetTokenFromCache();
        if (cachedToken == oldToken)
        {
            var authClient = new AuthClient(id, key);
            token = await authClient.AuthenticateClientAsync(); //KABOOM
            PutTokenInCache(token);
        }
        else
        {
            token = cachedToken;
        }
    }
    return token;
}

My issue is AuthClient only has async methods, and async methods are not allowed in lock-statement blocks. I don't have any control over AuthClient, is there some other strategy I can employ here?

Answer 1

You can use SemaphoreSlim as a basic async-ready lock replacement:

private readonly SemaphoreSlim lockObject = new SemaphoreSlim(1);
public async Task<Token> GenerateToken(Token oldToken)
{
  Token token;
  await lockObject.WaitAsync();
  try
  {
    var cachedToken = GetTokenFromCache();
    if (cachedToken == oldToken)
    {
      var authClient = new AuthClient(id, key);
      token = await authClient.AuthenticateClientAsync();
      PutTokenInCache(token);
    }
    else
    {
      token = cachedToken;
    }
  }
  finally
  {
    lockObject.Release();
  }
  return token;
}