Why can my Azure Function cannot read the environment variable defined in the Startup class?

I want to read the connection string defined in host.json in my Startup class.The problem is when running IConfiguration is always null !

<Project Sdk="Microsoft.NET.Sdk">
  <PropertyGroup>
    <TargetFramework>net8.0</TargetFramework>
    <AzureFunctionsVersion>v4</AzureFunctionsVersion>
    <OutputType>Exe</OutputType>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>
  <ItemGroup>
    <FrameworkReference Include="Microsoft.AspNetCore.App" />
    <PackageReference Include="Azure.Storage.Queues" Version="12.18.0" />
    <PackageReference Include="Microsoft.Azure.Functions.Extensions" Version="1.1.0" />
    <PackageReference Include="Microsoft.Azure.Functions.Worker" Version="1.20.1" />
    <PackageReference Include="Microsoft.Azure.Functions.Worker.Extensions.Http" Version="3.1.0" />
    <PackageReference Include="Microsoft.Azure.Functions.Worker.Extensions.Http.AspNetCore" Version="1.2.0" />
    <PackageReference Include="Microsoft.Azure.Functions.Worker.Sdk" Version="1.16.4" />
    <PackageReference Include="Microsoft.ApplicationInsights.WorkerService" Version="2.22.0" />
    <PackageReference Include="Microsoft.Azure.Functions.Worker.ApplicationInsights" Version="1.1.0" />
    <PackageReference Include="Microsoft.Extensions.DependencyInjection" Version="8.0.0" />
    <PackageReference Include="Newtonsoft.Json" Version="13.0.3" />
  </ItemGroup>
  <ItemGroup>
    <None Update="host.json">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
    </None>
    <None Update="local.settings.json">
      <CopyToOutputDirectory>PreserveNewest</CopyToOutputDirectory>
      <CopyToPublishDirectory>Never</CopyToPublishDirectory>
    </None>
    <None Update="Properties\launchSettings.json">
      <CopyToOutputDirectory>Always</CopyToOutputDirectory>
    </None>
  </ItemGroup>
  <ItemGroup>
    <Using Include="System.Threading.ExecutionContext" Alias="ExecutionContext" />
  </ItemGroup>
  <ItemGroup>
    <Compile Update="Startup.cs">
      <CopyToOutputDirectory>Never</CopyToOutputDirectory>
    </Compile>
  </ItemGroup>
</Project>

This is Function1.csproj

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

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

namespace MyNamespace
{
    public class Startup : FunctionsStartup
    {
        public override void Configure(IFunctionsHostBuilder builder)
        {
            var configuration = new ConfigurationBuilder()
                .SetBasePath(Directory.GetCurrentDirectory())
                .AddJsonFile("host.json", optional: true, reloadOnChange: true)
                .AddEnvironmentVariables().Build();

            IConfiguration conf = configuration;

            builder.Services.AddSingleton(conf);
            builder.Services.AddHttpClient();
        }
    }
}

This is the Startup Class i defined for my dependency injection of the IConfiguration !

using FirstFunction.dto;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.Logging;
using Newtonsoft.Json;
using System.Globalization;
using System.Text;
using Azure.Storage.Queues;
using Azure.Storage.Queues.Models;
using Microsoft.Extensions.Configuration;
using System.Reflection;

namespace FirstFunction
{
    public class Function1
    {
        private readonly ILogger<Function1> _logger;
        private readonly IConfiguration _configuration;
        private readonly HttpClient _httpClient;

        public Function1(ILogger<Function1> logger)
        {
            _logger = logger;
        }

        public Function1(ILogger<Function1> logger, IConfiguration configuration, HttpClient httpClient) : this(logger)
        {
            _configuration = configuration;
            _httpClient = httpClient;
        }

        [Function("Function1")]
        public async Task<IActionResult> Run([HttpTrigger(AuthorizationLevel.Anonymous, "get", "post")] HttpRequest req)
        {
            #region Catch Error
            string requestBody = string.Empty;
            try
            {
                requestBody = await new StreamReader(req.Body).ReadToEndAsync();
            }
            catch (Exception ex)
            {
                _logger.LogError($"Error reading request body: {ex.Message}");
                return new StatusCodeResult(StatusCodes.Status500InternalServerError);
            }
            #endregion

            string myConnectionString = _configuration["ConnectionStrings:QUEUE_CONNECTION_STRING"];

 return new  OkObjectResult("Data was sent to backend");
        }
    }
}

This is my azure Function method. The code is shortened

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

var host = new HostBuilder()
    .ConfigureFunctionsWebApplication()
    .ConfigureServices(services =>
    {
        services.AddApplicationInsightsTelemetryWorkerService();
        services.ConfigureFunctionsApplicationInsights();
    })
    .Build();

host.Run();

This was generated called Programm.cs

The error message is not so exact because of that i let it slide!

thank you for your help !

>Solution :

In Isolated Model , Startup is not used, you have to use Program.cs and to get value from host.json use below code and I have followed SO-Thread:

Program.cs:

using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

var host = new HostBuilder()
    .ConfigureFunctionsWebApplication()
    .ConfigureServices(services =>
    {
        services.AddApplicationInsightsTelemetryWorkerService();
        services.ConfigureFunctionsApplicationInsights();
    })
    .ConfigureAppConfiguration((context, test) =>
    {
        test.SetBasePath(context.HostingEnvironment.ContentRootPath)
          .AddJsonFile("host.json", optional: true, reloadOnChange: true)
          .AddEnvironmentVariables();
    })
    .Build();

host.Run();

Function1.cs:

using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
using Microsoft.Azure.Functions.Worker;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.Logging;

namespace FunctionApp200
{
    public class Function1
    {
        private readonly ILogger<Function1> _logger;
        private readonly IConfiguration ri_con;
        public Function1(ILogger<Function1> logger, IConfiguration config)
        {
            _logger = logger;
            ri_con = config;
        }a

        [Function("Function1")]
        public IActionResult Run([HttpTrigger(AuthorizationLevel.Function, "get", "post")] HttpRequest req)
        {
            _logger.LogInformation("C# HTTP trigger function processed a request.");
            var rith = ri_con["Value"];
            Console.WriteLine(rith);
            return new OkObjectResult($"Hello Rithwik, value is {rith}");
        }
    }
}

host.json:

{
    "version": "2.0",
  "logging": {
    "applicationInsights": {
      "samplingSettings": {
        "isEnabled": true,
        "excludedTypes": "Request"
      },
      "enableLiveMetricsFilters": true
    }
  },
  "Value": "Rithwik"
}

Output: