Azure Functions tips and tricks 1, parameter parsing in http routes

I’m writing this one while waiting on Gatwick at 3:47 AM, with what has to be the worst avocado sandwich I’ve ever seen in my life sooooooooooooooo, bear with me.

(MS Ignite has been a blast, stalk me on twitter to know more about my not-so-well handled life.)

Introduction

This is going to be a series of posts about little tricks and tips for dotnet core functions, short and to the point.

Parameter parsing in Http routes

The Functions webjobs host includes routing functionality for http requests, meaning that we can define and bind parameters in the http route…just like a classic kestrel API.

This is a neat feature to write more API-like Functions projects but can produce some headaches.

The default binder works great as long as the input is correct but if you feed it a malformed request route…it will crash with a pretty nasty exception and 500 error code:

[18/01/2020 4:01:04] Executed 'GetAuthorComments' (Failed, Id=892d85bc-0a97-4800-b13f-ffdf7457f9a4)
[18/01/2020 4:01:04] System.Private.CoreLib: Exception while executing function: GetAuthorComments. Microsoft.Azure.WebJobs.Host: One or more errors occurred. (Exception binding parameter 'req') (Exception binding parameter 'author'). Exception binding parameter 'req'. System.Private.CoreLib: Input string was not in a correct format.

Although this can be considered “ok” behaviour (you malformed a request, here is your error dude!) there might be some cases where we want a default fallback value if the user messes up the request.

Also throwing and returning the exception is pretty computationally expensive compared to returning a nice and tidy 400 error and a “Request malformed, bla bla bla” response.

To avoid the binding exceptions just bind every parameter as a string and then perform any needed castings inside the function code.

Example endpoint that retrieves comments based on a provided Guid value:

        [FunctionName(nameof(GetAuthorComments))]
        public async Task<IActionResult> GetAuthorComments(
           [HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "Comment/Author/{author}")] HttpRequest req, int author, ILogger log)
        {
            if (!Guid.TryParse(author, out var authorGuid))
            {
                return new BadRequestObjectResult("Malformed query uri");
            }

            return new OkObjectResult(this.myDummyService.GetComments(authorGuid));
        }