Waiting for Events with Tasks in .NET

Would you like to just await the next time that an event fires? It takes a little setup, but you can!

Migrating your code from using an Event-based Asynchronous Pattern to an Task-based Asynchronous Pattern can be very tricky. You can use the TaskCompletionSource to manage your Task, and then you just need to create the wire up around registering and unregistering your event handler. Unfortunately this process is not nearly as generic as I would like.

Event-based Asynchronous Pattern Tests

Here is a way of waiting for an event to fire by simply sleeping while we wait.

This is a terrible solution because we can not know how long we will have to wait for the event. This means we have to wait for one long time period, or we have to periodically poll to see if the event has fired.

public delegate void SingleParamTestDelegate(int key, string value);

Injectable Dataflow Blocks

I really enjoy working with Dataflows, but I always want to resolve my blocks with dependency injection. Thus I have created some abstract wrapper classes around the sealed ActionBlock and TransformBlock classes. This way your can put my logic into the superclass and inject it's dependencies via constructor injection. Additionally, the action method is public, making it even easier to test your code!

Update: I refactored to ditch the constructor parameters in favor of a new abstract BlockOptions.

Throttling Datafow and the Task Parallel Library

The Task Parallel Library is an amazingly powerful and versatile library. However, knowledge of how dataflow blocks process their data is vital to using them correctly. Trying to link source and target blocks to each other without fully understanding them is like throwing a live grenade into your app domain; at some point it will tear it down!

I recently experienced a bug where linking two blocks together without managing their Bounded Capacity caused them queue actions at an unsustainable rate, and eventually the dataflow literally eat all of the available memory on the server. This could have been easily avoided by throttling the source and target blocks.

How do you throttle your source block based on your target block?

Once linked together, a source block will produce messages as fast as it's target block can consume them. To prevent a source block from being too greedy, you want to restrict the bounded capacity for both it and it's consumer. Even then, you still need to understand that setting a bounded capacity could cause message producers to either block or fail to post.

...I'm sorry, but this subject is very complex, but I think code will explain these details best! Below are a detailed set of tests to take you through all the basic scenarios for throttling a dataflow: