async task vs async void sample

This repro contains the sample code for my question on Stack Overflow.

Background

We have Web API controllers that look like this:

public class HomeController : ApiController
{
    Logger logger = new Logger();

    [HttpGet, Route("")]
    public IHttpActionResult Index()
    {
        logger.Info("Some info logging");

        return Ok();
    }
}

Internally, our loggers use the HttpClient to POST data to an Azure Event Hub. This means that the logger has a synchronous method which internally calls an asynchronous method.

The broken implementation

When our logger has a private method with an async void signature like this:

public class Logger
{
    public void Info(string message)
    {
        LogAsync(message);
    }

    private async void LogAsync(string message)
    {
        await PostAsync(message);
    }
}

We see the following error:

The working implementation

If we change our logger to use an async Task signature like this it works:

public class Logger
{
    public void Info(string message)
    {
        LogAsync(message);
    }

    private async Task LogAsync(string message)
    {
        await PostAsync(message);
    }
}

Running the sample app

The sample app in this repro is an stripped down Web API application.

The following requests use the AsyncVoidLogger and will result in the InvalidOperationException:

GET http://localhost:7144
GET http://localhost:7144/async-void

This request uses the AsyncTaskLogger and will return an HTTP 200 result:

GET http://localhost:7144/async-task

The question

In both cases our logging messages are sent to the Event Hub. I'd like to know why async Task allows the controller to return the expected result, and async void results in an InvalidOperationException?