How to Release a Semaphore with a Using Block

I love that .NET has so many useful utilities available in the base framework. I often use the SemaphoreSlim, and I love that it supports async await. However, I don't like always having to create a try finally block around every use call to ensure that the release method gets called.

Below is a simple little extension method that will allow you to place the result of the Semaphore wait into a using block, thus ensuring that the dispose will always release the lock.

SemaphoreSlim Extensions

public static class SemaphoreSlimExtensions

{

    public static async Task<IDisposable> UseWaitAsync(

        this SemaphoreSlim semaphore, 

        CancellationToken cancelToken = default(CancellationToken))

    {

        await semaphore.WaitAsync(cancelToken).ConfigureAwait(false);

        return new ReleaseWrapper(semaphore);

    }

    private class ReleaseWrapper : IDisposable

    {

        private readonly SemaphoreSlim _semaphore;

        private bool _isDisposed;

        public ReleaseWrapper(SemaphoreSlim semaphore)

        {

            _semaphore = semaphore;

        }

        public void Dispose()

        {

            if (_isDisposed)

                return;

            _semaphore.Release();

            _isDisposed = true;

        }

    }

}

Unit Tests

public class SemaphoreSlimExtensionsTests

{

    [Fact]

    public async Task UseWaitAsync()

    {

        var semaphore = new SemaphoreSlim(1);

        Assert.Equal(1, semaphore.CurrentCount);

        using (await semaphore.UseWaitAsync())

        {

            Assert.Equal(0, semaphore.CurrentCount);

        }

        Assert.Equal(1, semaphore.CurrentCount);

    }

}

Enjoy,
Tom