How to use custom dependency injection.

AzureFunction.cs

using Microsoft.ApplicationInsights;
using Microsoft.ApplicationInsights.Extensibility;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.Functions.Extensions.DependencyInjection;
using Microsoft.Azure.WebJobs;
using Microsoft.Azure.WebJobs.Extensions.Http;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Newtonsoft.Json.Linq;
using System.IO;
using System.Net.Http;
using System.Threading.Tasks;

// https://docs.microsoft.com/en-us/azure/azure-functions/functions-dotnet-dependency-injection
// https://docs.microsoft.com/en-us/azure/azure-functions/functions-monitoring#configure-sampling
// https://docs.microsoft.com/en-us/aspnet/core/fundamentals/http-requests?view=aspnetcore-2.2#named-clients

[assembly: FunctionsStartup(typeof(FunctionApp1.Startup))]

namespace FunctionApp1
{
    public class Startup : FunctionsStartup
    {
        public override void Configure(IFunctionsHostBuilder builder)
        {
            builder.Services.AddScoped<IUtility, Utility>();
        }
    }

    public interface IUtility
    {
        bool TryRead(HttpRequest request, string key, out string value);
        IActionResult OkayResult(string value = null);
        IActionResult BadResult(string value = null);
    }

    public class Utility : IUtility
    {
        public bool TryRead(HttpRequest request, string key, out string value)
        {
            return request.TryReadQueryString(key, out value) || request.TryReadBody(key, out value);
        }
        public IActionResult OkayResult(string value = null)
        {
            return value == null
                ? new OkResult()
                : (IActionResult)new OkObjectResult(value);
        }
        public IActionResult BadResult(string value = null)
        {
            return value == null
                ? new BadRequestResult()
                : (IActionResult)new BadRequestObjectResult(value);
        }
    }

    public class MyFunction
    {
        private readonly IUtility _utility;

        public MyFunction(IUtility common)
        {
            _utility = common;
        }

        [FunctionName("MyFunction")]
        public async Task<IActionResult> Run(
            [HttpTrigger(AuthorizationLevel.Function, "get", "post", Route = null)] HttpRequest request,
            ExecutionContext context,
            ILogger log)
        {
            log.LogInformation($"C# HTTP trigger function processed a request: {context.InvocationId}");

            if (_utility.TryRead(request, "name", out var query_value))
            {
                return _utility.OkayResult($"Hello, {query_value}.");
            }
            else
            {
                return _utility.BadResult("Please pass a name on the query string or in the request body");
            }
        }
    }

    public static class HttpRequestExtensions
    {
        public static bool TryReadQueryString(this HttpRequest request, string key, out string value)
        {
            value = request.Query.ContainsKey(key) ? request.Query[key].ToString() : null;
            return value != null;
        }

        public static bool TryReadBody(this HttpRequest request, string key, out string value)
        {
            var json = new StreamReader(request.Body).ReadToEndAsync().Result;
            var jobj = JObject.Parse(json);
            value = jobj.ContainsKey(key) ? jobj.Value<string>(key) : null;
            return value != null;
        }
    }
}

Prerequisites

Before you can use dependency injection, you must install the following NuGet packages:

• Microsoft.Azure.Functions.Extensions
• Microsoft.NET.Sdk.Functions package version 1.0.28 or later
• Optional: Microsoft.Extensions.Http Only required for registering HttpClient at startup

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>netcoreapp2.1</TargetFramework>
    <AzureFunctionsVersion>v2</AzureFunctionsVersion>
  </PropertyGroup>
  <ItemGroup>
    <PackageReference Include="Microsoft.ApplicationInsights" Version="2.10.0" />
    <PackageReference Include="Microsoft.Azure.Functions.Extensions" Version="1.0.0" />
    <PackageReference Include="Microsoft.Extensions.Http" Version="2.2.0" />
    <PackageReference Include="Microsoft.NET.Sdk.Functions" Version="1.0.29" />
  </ItemGroup>
  <ItemGroup>
    <None Update="host.json">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    </None>
    <None Update="local.settings.json">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
      <CopyToPublishDirectory>Never</CopyToPublishDirectory>
    </None>
  </ItemGroup>
</Project>