As of dotnet core 3.1, AddHealthChecks has become incompatible with Azure Functions apps

[danielearwicker]

Describe the bug

We use Azure functions dependency injection, and we call AddHealthChecks() on the service collection.

We then have our own /health function endpoint that injects HealthCheckService and returns the UIHealthReport when requested.

But having upgraded to dotnet core 3.1, this seems to fail a new validation check, described in PR comment as "Adding breaking changes/compatibility analysis"

It lists 6 implementations of IHostedService that are expected, plus 4 more that are optional. If it finds any others, it throws an exception. i.e. this is now a closed system.

The AddHealthChecks() method registers an IHostedService called HealthCheckPublisherHostedService. It attempts to extend what is now closed, and so the validation check throws during startup.

NB. If I add this cursed line of code to knock out the offending registration, it makes the validation happy:

services.Remove(services.Single(s => s.ImplementationType?.Name == "HealthCheckPublisherHostedService"));

(I have to compare the name as a string because the class is internal.) And then our /health function endpoint responds with healthy (2) but I don't know if it's actually doing any healthchecks anymore.

Is there a "right" way to do this now? Looks like this extension now breaks some fundamental assumption recently adopted by the WebJobs/AF hosting code.

To Reproduce

Attempt to use AddHealthChecks() inside Azure Functions service collection registration. It throws the new validation exception during startup.

Expected behavior

The healthcheck capability works, as in 2.x.

[rynowak]

Hi @danielabbatt - have you tried reporting this on azure/azure-functions-host ?

This is their code that's causing the issue.

[danielearwicker]

@rynowak (that's a different Daniel). I already notified them, see link above. It's not so much their code causing the issue. They've merely added a check to make explicit that I've been using an unsupported scenario up to now and there's nothing I can do about that.