HttpClient not throwing exception when using await on GetAsync

Question

I'm using the following code to get an endpoint and write it to a cache:

public async Task UpdateCacheFromHttp(string Uri)
{
    if (string.IsNullOrEmpty(Uri))
        return;

    var httpClient = new HttpClient();
    var response = await httpClient.GetAsync(Uri);

    if ((response != null) && (response.IsSuccessStatusCode))
    {
        var responseStream = await response.Content.ReadAsStreamAsync();
        WriteToCache(responseStream);
    }
}

The code is running on IIS.

If the endpoint can't be reached I'd expect GetAsync to throw an exception. Even with a Try-Catch, it never seems to fail. GetAsync never returns (I tried a 5 second timeout on the HttpClient, still didn't return).

This does throw an exception:

public Task UpdateCacheFromHttp(string Uri)
{
    var updateCacheTask = Task.Factory.StartNew(new Action(() =>
    {
        if (string.IsNullOrEmpty(Uri))
            return;

        var httpClient = new HttpClient();
        var response = httpClient.GetAsync(Uri).Result;

        if (response.IsSuccessStatusCode)
        {
            var responseStream = response.Content.ReadAsStreamAsync().Result;
            WriteToCache(responseStream);
        }
    }));

    return updateCacheTask;
}

I get the expected "Unable to connect to the remote server".

I suspect it has something to do with the code running in IIS, but why? How do I get it to properly throw the exception without the need to start a new task?

Answer 1

My intuition tells me that you're calling Wait or Result further up your call stack.

If that is correct, then you're causing a deadlock, as I explain on my blog.