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Double Underscore, __, in Environment Variables

In environment variables, the __ (double underscore) is how ASP.NET Core represents a nested “section” key.

ASP.NET Core follows this convention:

  • MySettings__ApiKey means “section MySettings, key ApiKey”
  • So it maps to what you’d normally write in appsettings.json like:
{
  "MySettings": {
    "ApiKey": "value-here"
  }
}

So when you do:

  • builder.Configuration["MySettings:ApiKey"] you can also supply it via env var:
  • MySettings__ApiKey

Example (bind options)​

If you have:

"MySettings": { "ApiKey": "..." }

You can set:

  • env var: MySettings__ApiKey=123

and bind MySettings using

builder.Services.Configure<MySettings>(...).

: isn’t allowed in environment variable names on many systems, so __ is the standard workaround.