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4. Read from configuration files

ASP.NET Core apps can read configuration from a variety of sources including files, environment variables, command-line parameters, and more.

Because your web server is built using WebHost.CreateDefaultBuilder, application configuration is loaded from appsettings.json and appsettings.<Environment>.json with no additional setup required.

Setup

Let's make use of these settings files. Add a new "Math" section to appsettings.Development.json. Inside it, add a Multiplier property and set it to 10.

{
  "Logging": {
    ...
  },

  "Math": {
    "Multiplier": 10
  }
}

Create a new file named appsettings.Production.json. Big numbers are always more impressive to customers, so let's multiply everything we show them by 1000.

{
  "Math": {
    "Multiplier": 1000
  }
}

Note that you don't have to specify all of your settings in appsettings.Production.json or appsettings.Development.json. The environment-specific appsettings.<Environment>.json files act as overrides for your base set in appsettings.json. The full set of settings applied in an environment will be a combination of the two files, plus any overrides that might come from other configuration sources.

Important: If you use dotnet run or dotnet watch run to start your app, the default profile (in Properties\launchSettings.json) will override the ASPNETCORE_ENVIRONMENT variable. Delete these sections from launchSettings.json or run your app's executable file directly from the bin\Debug\netcoreapp2.1 folder to avoid this.

Near the top of the output after you start your app, you can see the environment that the server thinks it's running in.

Hosting environment: Development

Based on this, you can tell which settings file your app will use. Now, make use of your settings in your app.

Exercise 4

  • Change the code in MathController so that all outputs are multiplied by the multiplier you specify.
  • Settings are read on-the-fly. Update the settings file and make another request to your API to see the new multiplier take effect without restarting your app.
  • Change the ASPNETCORE_ENVIRONMENT environment variable to Development or Production and restart your app to read from the specified settings file.
    • Note: Changing an environment variable from the command line only affects your current command line session. Make sure you restart your app from the same command line session that you used to change the environment variable. Alternatively, set the value globally. See the linked documentation for details.

Tips

  • Use dependency injection to get an IConfiguration object in your constructor and use that to read settings.
  • There are multiple ways of reading settings - you can bind them to a class, read a whole section, or read one at a time. Do whatever makes sense with the hierarchy of your settings.

Verify