async/await in C#

async/await syntax should be used for I/O bound asynchronous operations only.

public async Task<int> GetUrlContentLengthAsync()
    var client = new HttpClient();

    Task<string> getStringTask =


    string contents = await getStringTask;

    return contents.Length;

Compiler transforms code into a state machine that to track yielding execution for await and resuming execution when a background job is finished.

Things to keep in mind:

  • async method needs await
  • add Async suffix to asynchronous methods
  • async void for ‘Fire and Forget’ work i.e. event Handlers
  • be aware of mixing LINQ with async lambdas
  • wait for tasks in a non-blocking manner (do not use Task.Result i.e.)
  • for tight loops use ValueTasks
  • Consider using ConfigureAwait(false) for performance and for avoiding deadlocks

Further read:

CPU bound vs I/O bound

There are two types of asynchronous operations in .NET.

  • I/O bound
  • CPU bound

For I/O bound async/await syntax should be used. TPL should not be used for this. async methods does not spawns new thread to hand le I/O work asynchronously.

For CPU bound operations TPL should be used (Task.Run). This spawns new thread. Performance testing should be done to check if overhead of new thread and context switching is not greater than actual work to be done.

  • CPU bound should be done in thread pool (Task.Run or Parallel.ForEach)
  • I/O bound should not
  • UI thread should do minimal work (mostly update UI or react to user actions)
  • extensive computations should be done on different thread (CPU bound).
  • I/O waits should be awaited (less overhead since ThreadPool have some overhead)
  • if I/O task are awaited then it is easier to update UI with new data (only one thread is allowed to update UI – main one)

Further read:

SupressFinalize in IDisposable interface

  • informs GC that object does not need to be added to finalizer queue
  • finalizer execution is not deterministic and objects can live for long time in queue – this is optimization technique
  • to make sure that object disposes of any unamanaged resources even if Dispose will be not called we need to implement finalizer

Basically finalizer execution time is not deterministic. We cannot predict when this will be called and till that time – object is existing and still taking memory space. Since we are controlling creation and disposal of those resources we can say to GC: ‘Do not worry about that! It already had been taken care of!’. So this way object will be destroyed sooner, leaving this space to be used by something else possibly.

The Dispose() method must call Dispose(true) and should suppress finalization for performance.


Further read:

IDisposable interface pattern

Example implementation of IDisposable

public class MyResource : IDisposable
    public MyResource(IntPtr handle)
        this.handle = handle;

    public void Dispose()
        Dispose(disposing: true);

    protected virtual void Dispose(bool disposing)
            handle = IntPtr.Zero;
            disposed = true;

        Dispose(disposing: false);

MyResource object have pointer which is unmanaged. This is why this class should implement IDisposable interface. Implementation should be correct according to recommended pattern (see below for documentation).

There a re few points to highlight here:

  • IDisposable is meant to be used with using keyword.
  • using is calling IDisposable.Dispose method in the background
  • This method should be public then
  • protected Dispose(bool) method is meant to be called from finalizer via GC
  • SuppressFinalize disables this object from finalization mechanism in GC to prevent finalization code to be executed second time (see here)
  • Finalization is called from GC *only* if whoever is using this object will forget to use using or otherwise fail to call Dispose manually
  • Dispose must be save to call multiple times (if(!this.disposed))
  • Dispose(true) is meant to dispose also any other managed resources this implementation relies on
  • Dispose(false) is meant to be called from GC finalization mechanism. All managed resource are most likely already disposed so we cannot dispose them again

Further read: