Read custom settings from local.settings.json in Azure functions

When using Environment.GetEnvironmentVariable in an .Net Core Azure Function I had to add the EnvironmentVariableTarget.Process parameter to retrieve the connection string

 string connectionString= Environment.GetEnvironmentVariable("DBConnectionString", 
 EnvironmentVariableTarget.Process);

My local.settings.json looked something like this:

 {"IsEncrypted": false,
 "Values": {
"AzureWebJobsStorage": "UseDevelopmentStorage=true",
"DBConnectionString": "<conn string here>"
 }
}

@Kirk and @Slava have helped you get rid of confusion. Just add some details for you to refer.

By default, publication neither upload local.settings.json to Azure, nor makes modification on Application settings based on that local file, hence we need to update them manually on Azure portal. We can also do that on VS publish panel.(Create profile first if we need to change settings before publish.)

About how to get parameters in app settings, one thing to note is that ConfigurationManager is no long supported in v2 function(runtime beta), may only get null or exception with it. For v1 function(runtime ~1), it still works.

1. For v1 function

   To read Application settings on Azure(also Values in local.settings.json), System.Environment.GetEnvironmentVariable($"{parameterName}") is recommended.

   Turn to Connection strings, unfortunately GetEnvironmentVariable only works on Azure because Connection strings(ConnectionStrings in local.settings.json) are not imported into Environment Variables. So we need ConfigurationManager, which works in both Azure and local env. Of course it can read Application settings as well.

2. For v2 function, two choices for both Application settings and Connection strings.

   One is to use GetEnvironmentVariable. We can refer to this list for Prefixes of Connection String on Azure.

   // Get Application settings
   var appParameter= "AzureWebJobsStorage";
   System.Environment.GetEnvironmentVariable($"{appParameter}");
   
   // Get Connection strings(put local and Azure env together)
   var connParameter= "MySqlAzureConnection";
   var Prefix = "SQLAZURECONNSTR_";
   var connectionString = System.Environment.GetEnvironmentVariable($"ConnectionStrings:{connParameter}");
   if(string.IsNullOrEmpty(connectionString )){
      connectionString = System.Environment.GetEnvironmentVariable($"{Prefix}{connParameter}");
   }
   

   Another one is to use ConfigurationBuilder. Add ExecutionContext parameter, which is used to locate function app directory.

   [FunctionName("FunctionName")]
   public static void Run(...,ExecutionContext context)
   {
      //"Values" and "Connection" sections are injected into EnvironmentVariables automatically hence we don't need to load Json file again. 
      //Hence SetBasePath and AddJsonFile are only necessary if you have some custom settings(e.g. nested Json rather than key-value pairs) outside those two sections. It's recommended to put those setting to another file if we need to publish them.
      //Note that Function binding settings(e.g. Storage Connection String) must be EnvironmentVariables, i.e. must be stored in "Values" section.
       var config = new ConfigurationBuilder()
           .SetBasePath(context.FunctionAppDirectory)
           .AddJsonFile("local.settings.json", optional: true, reloadOnChange: true)
           .AddEnvironmentVariables()
           .Build();
   
       // Get Application Settings
       var appParameter= "AzureWebJobsStorage";
       string appsetting = config[$"{appParameter}"];
   
       // Get Connection strings
       var connParameter= "MySqlAzureConnection";
       string connectionString = config.GetConnectionString($"{connParameter}");
   }
   

For those who are trying to use the EnvironmentVariable in the Functions Startup, And find the Environment null, You can check this code:

using System.IO;
using System.Net;
using System.Net.Mail;
using Microsoft.Azure.Functions.Extensions.DependencyInjection;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.FileProviders;

[assembly: FunctionsStartup(typeof(Startup))]
namespace Company.Domain.Project;

internal class Startup : FunctionsStartup
{
    public override void Configure(IFunctionsHostBuilder builder)
    {
        var configBuilder = new ConfigurationBuilder()
            .AddEnvironmentVariables();
        
        IConfiguration configuration = configBuilder.Build();
        builder.Services.AddSingleton(configuration);
    }
}

after that you can use IConfiguration.GetValue<TType>("key"); in your startup and configuration.

The full described guidance can be found here:

quickstart azure functions - Microsoft Documentation