Set environment variables

Environment variables are name–value pairs passed to your application when it starts. They let you change configuration without publishing the application again, and they are a good place for secrets such as connection strings and API keys, because they are not stored in the files you deploy.

Add a variable

Each variable belongs to one website and is visible only to that website. Values that look like secrets are masked in the list; click Show to reveal them.

Use them in ASP.NET Core

ASP.NET Core reads environment variables into IConfiguration automatically, and they override the values from appsettings.json.

Use a double underscore __ for nested sections. A variable named ConnectionStrings__Default overrides this value:

{
  "ConnectionStrings": {
    "Default": "..."
  }
}

In the same way Mail__Host maps to Mail:Host. A whole configuration section can be moved to environment variables like this:

{
  "Mail": {
    "Host": "smtp.local",
    "Port": 25,
    "Username": "test",
    "Password": "test123"
  }
}

The same application can then run in development, testing and production without any change to appsettings.json. More about the naming rules is in the Microsoft documentation.

A value from an environment variable is read as usual:

var connectionString = builder.Configuration.GetConnectionString("Default");

Any variable can also be read directly:

var apiKey = Environment.GetEnvironmentVariable("MY_API_KEY");

ASPNETCORE_ENVIRONMENT

The variable ASPNETCORE_ENVIRONMENT set to Production, Staging or Development decides which appsettings.{Environment}.json file your application loads and how it behaves.

Example: keep the connection string out of your files

Other options

• A Node.js application can also read variables from a .env file placed in the application folder.
• Variables can be defined in web.config as well, but values set there are stored in a deployed file.