Automatically set appsettings.json for dev and release environments in asp.net core?

I have added screenshots of a working environment, because it cost me several hours of R&D.

1. First, add a key to your launch.json file.

   See the below screenshot, I have added Development as my environment.

   

2. Then, in your project, create a new appsettings.{environment}.json file that includes the name of the environment.

   In the following screenshot, look for two different files with the names:

   • appsettings.Development.Json
   • appSetting.json

   
   

3. And finally, configure it to your StartUp class like this:

   public Startup(IHostingEnvironment env)
   {
       var builder = new ConfigurationBuilder()
           .SetBasePath(env.ContentRootPath)
           .AddJsonFile("appsettings.json", optional: false, reloadOnChange: true)
           .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true)
           .AddEnvironmentVariables();
   
       Configuration = builder.Build();
   }
   

4. And at last, you can run it from the command line like this:

   dotnet run --environment "Development"
   

   where "Development" is the name of my environment.

Update for .NET Core 3.0+

1. You can use CreateDefaultBuilder which will automatically build and pass a configuration object to your startup class:

   WebHost.CreateDefaultBuilder(args).UseStartup<Startup>();
   

   public class Startup
   {
       public Startup(IConfiguration configuration) // automatically injected
       {
           Configuration = configuration;
       }
       public IConfiguration Configuration { get; }
       /* ... */
   }
   

2. CreateDefaultBuilder automatically includes the appropriate appsettings.Environment.json file so add a separate appsettings file for each environment:

   

3. Then set the ASPNETCORE_ENVIRONMENT environment variable when running / debugging

How to set Environment Variables

Depending on your IDE, there are a couple places dotnet projects traditionally look for environment variables:

• For Visual Studio go to Project > Properties > Debug > Environment Variables:

  

• For Visual Studio Code, edit .vscode/launch.json > env:

  

• Using Launch Settings, edit Properties/launchSettings.json > environmentVariables:

  

  Which can also be selected from the Toolbar in Visual Studio

  

• Using dotnet CLI, use the appropriate syntax for setting environment variables per your OS

  Note: When an app is launched with dotnet run, launchSettings.json is read if available, and environmentVariables settings in launchSettings.json override environment variables.

How does Host.CreateDefaultBuilder work?

.NET Core 3.0 added Host.CreateDefaultBuilder under platform extensions which will provide a default initialization of IConfiguration which provides default configuration for the app in the following order:

1. appsettings.json using the JSON configuration provider.
2. appsettings.Environment.json using the JSON configuration provider. For example:
   • appsettings.Production.json or
   • appsettings.Development.json
3. App secrets when the app runs in the Development environment.
4. Environment variables using the Environment Variables configuration provider.
5. Command-line arguments using the Command-line configuration provider.

Further Reading - MS Docs

• App startup in ASP.NET Core
• Configuration in ASP.NET Core
• Use multiple environments in ASP.NET Core

In ASP.NET Core you should rather use Environment Variables instead of build configuration for proper appsettings.json

1. Right click on you project > Properties > Debug > Environment Variables

   

2. ASP.NET Core will use the appropriate appsettings.json file:

   

3. Now you can use that Environment Variable like this:

   public Startup(IHostingEnvironment env)
   {
       var builder = new ConfigurationBuilder()
           .SetBasePath(env.ContentRootPath)
           .AddJsonFile("appsettings.json", optional: true, reloadOnChange: true)
           .AddJsonFile($"appsettings.{env.EnvironmentName}.json", optional: true)
           .AddEnvironmentVariables();
   
       Configuration = builder.Build();
   }
   

Note: If you use @Dmitry's answer, you can run into problems eg. when overriding appsettings.json values on Azure.