Creating an IFunctionProvider with a non-static trigger

Question

I have an IFunctionProvider implementation for an Azure Function that creates a route for /healthcheck. The entrypoint for my function metadata is a method called Somenamespace.TestTrigger.RenderHealthCheck. My code looks like this:

    public class TestTrigger
    {
        public static async Task<IActionResult> RenderHealthCheck(HttpRequest req)
        {
            return new OkObjectResult("OK");
        }        
    }
    
    public class HealthCheckTrigger : IFunctionProvider
    {
        public ImmutableDictionary<string, ImmutableArray<string>> FunctionErrors { get; }
        
        public async Task<ImmutableArray<FunctionMetadata>> GetFunctionMetadataAsync()
        {
            var assembly = Assembly.GetExecutingAssembly();
            
            var functionMetadata = new FunctionMetadata()
            {
                Name = nameof(TestTrigger.RenderHealthCheck),
                FunctionDirectory = null,
                ScriptFile = $"assembly:{assembly.FullName}",
                EntryPoint = $"{typeof(TestTrigger).FullName}.{nameof(TestTrigger.RenderHealthCheck)}",
                Language = "DotNetAssembly"
            };

            var jo = JObject.FromObject(new HttpBindingMetadata()
            {
                Methods = new List<string> { HttpMethods.Get },
                Route = "HealthCheck",
                AuthLevel = AuthorizationLevel.Anonymous,
            });
            
            var binding = BindingMetadata.Create(jo);
            functionMetadata.Bindings.Add(binding);            
            
            var functionMetadataList = new List<FunctionMetadata>
            {
                functionMetadata
            };

            return await Task.FromResult(functionMetadataList.ToImmutableArray()).ConfigureAwait(false);
        }
    }

This works fine, and when I run the function and hit the /HealthCheck endpoint I get my OK message. However, I'd like to make the RenderHealthCheck method non-static, so I can use dependency injection in the constructor and access various services in my application. I changed my code to:

public class TestTrigger
{
    private readonly HealthCheckService healthCheckService;
    
    public TestTrigger(HealthCheckService healthCheckService)
    {
        this.healthCheckService = healthCheckService;
    }

    public async Task<IActionResult> RenderHealthCheck(HttpRequest req)
    {
        return new OkObjectResult("OK");
    }        
}

Now, when I hit the endpoint, I get the following exception:

Executed 'Functions.RenderHealthCheck' (Failed, Id=c02a9ba6-1ed3-4c07-8b38-214d345b6ff1, Duration=488ms)
Microsoft.Azure.WebJobs.Host.FunctionInvocationException: Exception while executing function: Functions.RenderHealthCheck
 ---> System.NullReferenceException: Object reference not set to an instance of an object.
   at lambda_method(Closure , TestTrigger , Object[] )
   at Microsoft.Azure.WebJobs.Host.Executors.TaskMethodInvoker`2.InvokeAsync(TReflected instance, Object[] arguments) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\TaskMethodInvoker.cs:line 21
   at Microsoft.Azure.WebJobs.Script.Description.DotNetFunctionInvoker.MethodInvoker`2.InvokeAsync(Object target, Object[] parameters) in D:\a\1\s\src\WebJobs.Script\Description\DotNet\DotNetFunctionInvoker.cs:line 533
   at Microsoft.Azure.WebJobs.Script.Description.DotNetFunctionInvoker.InvokeCore(Object[] parameters, FunctionInvocationContext context) in D:\a\1\s\src\WebJobs.Script\Description\DotNet\DotNetFunctionInvoker.cs:line 272
   at Microsoft.Azure.WebJobs.Script.Description.FunctionInvokerBase.Invoke(Object[] parameters) in D:\a\1\s\src\WebJobs.Script\Description\FunctionInvokerBase.cs:line 82
   at Microsoft.Azure.WebJobs.Script.Description.FunctionGenerator.Coerce[T](Task`1 src) in D:\a\1\s\src\WebJobs.Script\Description\FunctionGenerator.cs:line 225
   at Microsoft.Azure.WebJobs.Host.Executors.FunctionInvoker`2.InvokeAsync(Object instance, Object[] arguments) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionInvoker.cs:line 52
   at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.InvokeWithTimeoutAsync(IFunctionInvoker invoker, ParameterHelper parameterHelper, CancellationTokenSource timeoutTokenSource, CancellationTokenSource functionCancellationTokenSource, Boolean thro
wOnTimeout, TimeSpan timerInterval, IFunctionInstance instance) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 555
   at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.ExecuteWithWatchersAsync(IFunctionInstanceEx instance, ParameterHelper parameterHelper, ILogger logger, CancellationTokenSource functionCancellationTokenSource) in C:\projects\azure-webjobs-sdk-r
qm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 503
   at Microsoft.Azure.WebJobs.Host.Executors.FunctionExecutor.ExecuteWithLoggingAsync(IFunctionInstanceEx instance, FunctionStartedMessage message, FunctionInstanceLogEntry instanceLogEntry, ParameterHelper parameterHelper, ILogger logger, CancellationToken
 cancellationToken) in C:\projects\azure-webjobs-sdk-rqm4t\src\Microsoft.Azure.WebJobs.Host\Executors\FunctionExecutor.cs:line 281
   --- End of inner exception stack trace --

I didn't see anything in the FunctionMetadata or HttpBindingMetadata that seems to control if the entrypoint is static. Are instance methods simply not supported? Seems like a huge limitation if not. There's basically zero documentation of any sort on IFunctionProvider so not quite sure where to go from here. Any help would be appreciated!

Answer 1

If you really want to be able to inject HealthCheckService into your static function, it is possible to create a custom binding for it and resolve the service from the FunctionExecutionContext.

Based on https://microsoft.github.io/AzureTipsAndTricks/blog/tip247.html, I was able to do the following:

[assembly: WebJobsStartup(typeof(HealthBindingStartup))]

[Binding]
[AttributeUsage(AttributeTargets.Parameter)]
public class HealthAttribute : Attribute
{
}

[Extension(nameof(HealthBinding))]
public class HealthBinding : IExtensionConfigProvider
{
    public void Initialize(ExtensionConfigContext context)
    {
        var rule = context.AddBindingRule<HealthAttribute>();
        rule.BindToInput((HealthAttribute attr, ValueBindingContext context) =>
        {
            var service = context.FunctionContext.CreateObjectInstance<HealthCheckService>();

            return Task.FromResult(service);
        });
    }
}

public static class HealthBindingExtension
{
    public static IWebJobsBuilder AddHealthBinding(this IWebJobsBuilder builder)
    {
        if (builder == null)
        {
            throw new ArgumentNullException(nameof(builder));
        }

        builder.AddExtension<HealthBinding>();
        return builder;
    }
}

public class HealthBindingStartup : IWebJobsStartup
{
    public void Configure(IWebJobsBuilder builder)
    {
        builder.AddHealthBinding();
    }
}

With this, you can update your function and add [Health] HealthCheckService service and have this binding inject the service you want.

The metadata for this binding is:

{ "name": "service", "type": "health", "direction": "in" }

This is what you need when adding functions via the IFunctionProvider which ignores any attributes.