## Update for .NET Core 3.0+

1. You can use [`CreateDefaultBuilder`][1] which will automatically build and pass a configuration object to your startup class:

 ```cs
 WebHost.CreateDefaultBuilder(args).UseStartup<Startup>();
 ```
 
 ```cs
 public class Startup
 {
 public Startup(IConfiguration configuration) // automatically injected
 {
 Configuration = configuration;
 }
 public IConfiguration Configuration { get; }
 /* ... */
 }
 ```

2. `CreateDefaultBuilder` automatically includes the appropriate <code>appsettings.Environment.json</code> file so add a separate appsettings file for each environment:

 [![appsettings.env.json][2]][2]

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:

 [![Visual Studio - Environment Variables][3]][3]


* For **Visual Studio Code**, edit `.vscode/launch.json` > `env`:

 [![Visual Studio Code > Launch Environment][4]][4]


* Using **Launch Settings**, edit `Properties/launchSettings.json` > `environmentVariables`:

 [![Launch Settings][6]][6]

 Which can also be selected from the Toolbar in Visual Studio

 [![Launch Settings Dropdown][7]][7]


* Using **dotnet CLI**, use the appropriate syntax for [setting environment variables per your OS][16]

 > **Note**: When an app is launched with [dotnet run][5], `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`][1] under platform extensions which will provide a default initialization of [`IConfiguration`][8] which provides default configuration for the app in the following order:


> 1. `appsettings.json` using the [JSON configuration provider][9].
> 2. <code>appsettings.Environment.json</code> using the [JSON configuration provider][9]. For example:
> * <code>appsettings.Production.json</code> or
> * <code>appsettings.Development.json</code>
> 4. [App secrets][10] when the app runs in the Development environment.
> 5. Environment variables using the [Environment Variables configuration provider][11].
> 6. Command-line arguments using the [Command-line configuration provider][12].


### Further Reading - MS Docs

* [App startup in ASP.NET Core][13]
* [Configuration in ASP.NET Core][14]
* [Use multiple environments in ASP.NET Core][15]


 [1]: https://learn.microsoft.com/en-us/dotnet/api/microsoft.extensions.hosting.host.createdefaultbuilder
 [2]: https://i.stack.imgur.com/qXihz.png
 [3]: https://i.stack.imgur.com/WSNlA.png
 [4]: https://i.stack.imgur.com/Qy8f5.png
 [5]: https://learn.microsoft.com/en-us/dotnet/core/tools/dotnet-run
 [6]: https://i.stack.imgur.com/JOGFp.png
 [7]: https://i.stack.imgur.com/dVEeO.png
 [8]: https://learn.microsoft.com/en-us/dotnet/api/microsoft.extensions.configuration.iconfiguration
 [9]: https://learn.microsoft.com/en-us/aspnet/core/fundamentals/configuration/#file-configuration-provider
 [10]: https://learn.microsoft.com/en-us/aspnet/core/security/app-secrets
 [11]: https://learn.microsoft.com/en-us/aspnet/core/fundamentals/configuration/#evcp
 [12]: https://learn.microsoft.com/en-us/aspnet/core/fundamentals/configuration/#command-line
 [13]: https://learn.microsoft.com/en-us/aspnet/core/fundamentals/startup
 [14]: https://learn.microsoft.com/en-us/aspnet/core/fundamentals/configuration
 [15]: https://learn.microsoft.com/en-us/aspnet/core/fundamentals/environments
 [16]: https://stackoverflow.com/a/43992754/1366033

I've defined some values in my `appsettings.json` for things like database connection strings, webapi locations and the like which are different for development, staging and live environments.

Is there a way to have multiple `appsettings.json` files (like `appsettings.live.json`, etc, etc) and have the asp.net app just 'know' which one to use based on the build configuration it's running?