Using Polly to retry after HttpStatusCode.Unauthorized

Question

I'm making calls to an external API and want to deal with the event that a call returns an Unauthorized HttpResponseMessage. When this happens I want to refresh the access token and make the call again.

I'm trying to use Polly with the following code:

    public async Task<HttpResponseMessage> MakeGetRequestAsync()
    {
        var retryPolicy = Policy
            .HandleResult<HttpResponseMessage>(r => r.StatusCode == HttpStatusCode.Unauthorized)
            .Retry(1, (exception, retryCount) =>
            {
                RefreshAccessToken();
            });

        var result = await retryPolicy.ExecuteAsync(() => CallApiAsync());

        return result;
    }


    private async Task<HttpResponseMessage> CallApiAsync()
    {
        var url = Options.ResourceSandboxUrl;

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

        response.StatusCode = HttpStatusCode.Unauthorized;
        return response;
    }

I put breakpoints on the ExecuteAsync statement and in DoSomethingAsync - when I step over ExecuteAsync DoSomethingAsync is not called and control is returned to the function that called MakeGetRequestAsync

I don't understand why DoSomethingAsync is not called - can anyone help me with what I'm trying to achieve?

I've looked at the Polly documentation & Polly questions on Stack Overflow but I can't figure out what's going on..

Answer 1

The Polly wiki officially documents this pattern. In particular this is the code snippet suggested:

var authorisationEnsuringPolicy = Policy
    .HandleResult<HttpResponseMessage>(r => r.StatusCode == HttpStatusCode.Unauthorized) 
    .RetryAsync(
       retryCount: 1, // Consider how many retries. If auth lapses and you have valid credentials, one should be enough; too many tries can cause some auth systems to blacklist. 
       onRetryAsync: async (outcome, retryNumber, context) => FooRefreshAuthorizationAsync(context), 
       /* more configuration */); 

var response = authorisationEnsuringPolicy.ExecuteAsync(context => DoSomethingThatRequiresAuthorization(context), cancellationToken);

The FooRefreshAuthorizationAsync(...) method can obtain a new authorization token and pass it to the delegate executed through the policy using Polly.Context.