Azure Functions: Inherit HttpTrigger functions / routes from base class - azure

Question

I've got some common functionality in a number of Azure Function Apps. Each Function App exposes the exact same query operations on a common resource that each Function App has. Same endpoints, same dependancies etc. Up until now, there's been a lot of copy paste keeping this going.
So I refactored it out into a base abstract class that lives in a common project consumed by all services. It looks like this:
public abstract class DeadLetterApiBaseFunction : BaseFunction
{
private readonly IDeadLetterBlobClient _deadLetterBlobClient;
private readonly string ROLE_READ;
private readonly string ROLE_WRITE;
private readonly ILogger _logger;
public DeadLetterApiBaseFunction(
ILogger logger,
IMiddlewareService middleware,
IDeadLetterBlobClient deadLetterBlobClient,
string ROLE_READ,
string ROLE_WRITE) : base(logger, middleware)
{
_logger = logger;
_deadLetterBlobClient = deadLetterBlobClient;
this.ROLE_READ = ROLE_READ;
this.ROLE_WRITE = ROLE_WRITE;
}
[FunctionName(nameof(GetDeadLetterBlobItemsForContainer))]
[ProducesResponseType(typeof(ResponsePayload<List<BlobItemWithContent>>), Status200OK)]
[ProducesResponseType(Status500InternalServerError)]
[ProducesResponseType(Status400BadRequest)]
[QueryStringParameter("container", "Name of the container", DataType = typeof(string))]
public async Task<ActionResult<ResponsePayload<List<BlobItemWithContent>>>> GetDeadLetterBlobItemsForContainer(
[HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "v1/dead-letters/blobs-for-container")]
HttpRequest request)
{
// .... do stuff
}
}
}
The idea being that each service will subclass this like so:
public class DeadLetterApiFunction : DeadLetterApiBaseFunction
{
public DeadLetterApiFunction(
ILogger<DeadLetterApiFunction> logger,
IMiddlewareService middleware,
IDeadLetterBlobClient deadLetterBlobClient) : base(
logger: logger,
middleware: middleware,
deadLetterBlobClient: deadLetterBlobClient,
ROLE_READ: "blarg",
ROLE_WRITE: "blarg2")
{
}
}
Looks great right? However, when you run the function, the endpoints are not being exposed - the functions are not being detected.
Is there a way to get Azure Functions HttpTrigger functions exposed from a base class? Or is that just not supported? We're using .NET Core 3.1 and Azure Functions 3.
There's a related question here, but the answer took a different path:
Use Azure Functions in abstract base class?

Answer 1

As an HTTP trigger is a single point of entry for a specific route, I would abstract out the HTTP trigger in a separate class and then inject it in the DeadLetterApiFunction class upon creation as a Singleton via the Startup.cs file. This is possible as Azure functions support instance functions to enable DI.
public class HttpTrigger
{
public HttpTrigger()
{
}
[FunctionName(nameof(GetDeadLetterBlobItemsForContainer))]
[ProducesResponseType(typeof(ResponsePayload<List<BlobItemWithContent>>), Status200OK)]
[ProducesResponseType(Status500InternalServerError)]
[ProducesResponseType(Status400BadRequest)]
[QueryStringParameter("container", "Name of the container", DataType = typeof(string))]
public async Task<ActionResult<ResponsePayload<List<BlobItemWithContent>>>> GetDeadLetterBlobItemsForContainer(
[HttpTrigger(AuthorizationLevel.Anonymous, "get", Route = "v1/dead-letters/blobs-for-container")]
HttpRequest request)
{
// .... do stuff
}
}