Dependency injection on azure functions

This document summarizes the microsoft's dependency injection tutorial. Following steps must be followed to achieve IoC(Inversion of control) in Azure functions.

1. Install prerequisites
2. Register services

Prerequisites

• Microsoft.Azure.Functions.Extensions
• Microsoft.NET.Sdk.Functions: Don't install if you already have sdK installed.
• Microsoft.Extensions.DependencyInjection

Registering Services

using Microsoft.Azure.Functions.Extensions.DependencyInjection;
using Microsoft.Extensions.DependencyInjection;

[assembly: FunctionsStartup(typeof(MyNamespace.Startup))]

namespace MyNamespace
{
    public class Startup : FunctionsStartup
    {
        public override void Configure(IFunctionsHostBuilder builder)
        {
            builder.Services.AddSingleton<IMyService>((s) => {
                return new MyService();
            });
        }
    }
}

Using dependencies

GetMessage method can be invoked from injected class "IMyService"

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.Extensions.Logging;
using System.Net.Http;
using System.Threading.Tasks;

namespace MyNamespace
{
    public class MyHttpTrigger
    {
        private readonly HttpClient _client;
        private readonly IMyService _service;

        public MyHttpTrigger(IMyService service)
        {
            this._service = service;
        }

        [FunctionName("MyHttpTrigger")]
        public async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest req,
            ILogger log)
        {
            var message = _service.GetMessage();

            return new OkObjectResult("Response from function with injected dependencies.");
        }
    }
}

Issues

Should you get an error like this “Could not load file or assembly 'Microsoft.Extensions.DependencyInjection.Abstractions, Version=6.0.0.0, Culture=neutral, PublicKeyToken=adb9793829ddae60'. The system cannot find the file specified”

You need to downgrade Microsoft.Extensions* to 3.1.0

https://stackoverflow.com/questions/64809716/could-not-load-file-or-assembly-microsoft-extensions-configuration-abstractions

Conclusion

It's important to note available service lifeline

• Transient: Created upon resolution of each service.
• Scope: Created once per function execution. Further requests during execution uses the same instance.
• Singleton: matches the lifetime of the host and it's reused across multiple executions

References

#1: Use dependency injection in .NET Azure Functions