Azure Functions .NET 5 Isolated HttpTrigger Path Variable Input Binding - azure

Question

I'm just trying to figure out how to do something in .NET 5 that worked in 3.1 and before.
In 3.1, the route variable binds correctly to the Guid parameter of the same name:
[FunctionName("Function1")]
public static async Task Run(
[HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "records/{clientId:Guid}")] HttpRequest req,
Guid clientId,
ILogger log)
{
return new OkObjectResult(clientId);
}
A comparable .NET 5 version of this same function fails to bind the path variable:
[Function("Function1")]
public static HttpResponseData Run([HttpTrigger(AuthorizationLevel.Function, "get", Route = "records/{clientId:Guid}")] HttpRequestData req,
Guid clientId,
FunctionContext executionContext)
{
var response = req.CreateResponse(HttpStatusCode.OK);
response.WriteString(clientId.ToString());
return response;
}
The error that is thrown is as follows:
Exception:
Microsoft.Azure.Functions.Worker.Diagnostics.Exceptions.FunctionInputConverterException:
Error converting 1 input parameters for Function 'Function1': Cannot
convert input parameter 'clientId' to type 'System.Guid' from type
'System.String'.
I can change the type of the parameter to string and then parse it into a Guid after the fact, of course, but I'd like to know if it's still possible to do it the aforementioned way.

Answer 1

There are two things:
Why isn't the route constraint taken into account (i.e. why does a conversion need to occur)
I found a related git issue. That doesn't seem to be fixed, though it makes me wonder how you managed to make it work with .NET Core 3.1 :)
Why isn't the input string automatically converted to a Guid
I had a look at the code to understand where the exception is raised.
The model binding is using a list of IConverter to convert between the input type and the binding type. In your case, the input type is string and the binding type is Guid, and there's no built-in converter that can do that. You can't even create your own IConverter, because it's an internal interface.
Note: here's an example of IConverter that converts a string to a byte array: StringToByteConverter
So basically, there's nothing you can do apart from your suggestion to parse the Guid yourself.