Using ConfigurationBuilder to add KeyVault as a provider

FunctionsStartup.cs

  public static class ServiceExtensions
  {
      /// <summary>
      /// Creates a custom configuration for easier development of Azure Functions.
      /// When environment is not Development, it adds support for keyvault
      /// </summary>
      /// <param name="builder"></param>
      /// <returns></returns>
      public static IConfiguration GetCustomConfiguration(this IFunctionsHostBuilder builder)
      {
          IConfiguration localConfiguration;

          if (Environment.GetEnvironmentVariable("ASPNETCORE_ENVIRONMENT") == "Development")
          {
              localConfiguration = new ConfigurationBuilder()
                  .SetBasePath(Directory.GetCurrentDirectory())
                  .AddJsonFile("local.settings.json", optional: true, reloadOnChange: true)
                  .AddEnvironmentVariables()
                  .Build() as IConfiguration;

              // we have to replace the rootConfig with the new one
              builder.Services.AddSingleton(localConfiguration);
          }
          else
          {
              var rootConfig = builder.Services.BuildServiceProvider().GetRequiredService<IConfiguration>();
              var keyVaultBaseUrl = rootConfig.GetValue<string>("KeyVaultBaseUrl");

              // add keyvault
              localConfiguration = new ConfigurationBuilder()
                  .AddAzureKeyVault(keyVaultBaseUrl)
                  .AddEnvironmentVariables()
                  .Build() as IConfiguration;

              // we have to replace the rootConfig with the new one that has keyVault
              builder.Services.AddSingleton(localConfiguration);
          }

          return localConfiguration;
      }
  }