Using Async/Await with Disposable Objects

This article will show by example how to dispose of disposable objects when using the async/ await pattern. When using await, Microsoft recommends using the ConfigureAwait method as shown below:

await this._channel.Writer.WriteAsync(item, token).ConfigureAwait(false);

Using ConfigureAwait(false) avoids forcing the callback to be invoked on the original context or scheduler as Steven Toub explains in this article: ConfigureAwait FAQ – .NET Blog (microsoft.com). This can improve performance and avoids deadlocks. But it’s a bit tricky when creating the object using the using pattern since ConfigureAwait() returns ConfiguredAsyncDisposable.

How To Implement

Let’s change some code in my open-source project Spargine. The code below converts a string to a Brotli compressed string.

await using (var inputStream = new MemoryStream(Encoding.Unicode.GetBytes(input)))
{
     await using (var outputStream = new MemoryStream())
     {
          await using (var brotliStream = new BrotliStream(outputStream, level))
          {
               await inputStream.CopyToAsync(brotliStream).ConfigureAwait(false);
               await brotliStream.FlushAsync().ConfigureAwait(false);

               return Convert.ToBase64String(outputStream.ToArray());
          }
     }
}

When I add ConfigureAwait(false) to the creation of MemoryStream, an error will appear:

The error appears since inputStream is now actually a ConfiguredAsyncDisposable type so CopyToAsync() is not available. To fix this, we need create the disposable type, before the using statement as shown below:

var inputStream = new MemoryStream(Encoding.Unicode.GetBytes(input));

await using (inputStream.ConfigureAwait(false))
{
     var outputStream = new MemoryStream();

     await using (outputStream.ConfigureAwait(false))
     {
          var brotliStream = new BrotliStream(outputStream, level);

          await using (brotliStream.ConfigureAwait(false))
          {
               await inputStream.CopyToAsync(brotliStream).ConfigureAwait(false);
               await brotliStream.FlushAsync().ConfigureAwait(false);

               return Convert.ToBase64String(outputStream.ToArray());
          }
     }
}

Unfortunately, this code will then cause the following error:

CA2000: Call System.IDisposable.Dispose on object created by 'new MemoryStream()' before all references to it are out of scope

To fix it, add this attribute to the method:

[SuppressMessage("Microsoft.Build", "CS2000")]

I do wish that Microsoft solved this issue without this many code changes on our part. Maybe someday… fingers crossed.

Summary

Hidden dispose issues are more difficult to solve, like this one. I highly recommend adding the IDisposableAnalyzers NuGet package to all your projects to uncover disposable issues like this. I believe this should be part of .NET, not a package that has to be added since virtual memory issues are a big deal if your codebase has any and I can say most of the projects out there do!

Do you have any questions or comments? Please make them below.

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