Overriding appsettings.json via Environment Variables

In a previous post, Flexible Leveraging Of appsettings.json In .NET Applications, we talked about how to use the .NET configuration infrastructure via the options pattern to set and load setting.

In a subsequent post, Setting The Staging & Production Environments During Development, we talked about how to set the environment of an application so as to determine the settings to be loaded.

In the last post, Overriding appsettings.json Via Command Line, we looked at overriding settings via the command line.

In this post, we will look at how to override the settings in the json files via environment variables.

As a recap, the test application has the following settings:

{
  "Logging": {
    "LogLevel": {
      "Default": "Information",
      "Microsoft.AspNetCore": "Warning"
    }
  },
  "AllowedHosts": "*",
  "SystemSettings": {
    "ForegroundColour": "White",
    "BackgroundColour": "Black"
  }
}

And we have the following web API:

using ApplicationSettings.Properties;
using Microsoft.Extensions.Options;

var builder = WebApplication.CreateBuilder(args);

// Configure options DI
builder.Services.AddOptions<SystemSettings>()
    .Bind(builder.Configuration.GetSection(nameof(SystemSettings)));

var app = builder.Build();

app.MapGet("/", (IOptions<SystemSettings> options) =>
{
    var settings = options.Value;
    return settings;
});

app.Run();

If we run this, we get the following result:

It is possible to override this at runtime, by passing the required settings as parameters via environment variables.

Depending on your environment, there are various ways to pass environment variables:

1. Bash (Unix / Linux / macOS)
2. PowerShell (Unix / Linux / macOS / Windows)
3. Cmd (Windows)

Bash

For bash, you can set the variables as follows:

export SystemSettings__ForegroundColour=Neon
export SystemSettings__BackgroundColour=Mauve

PowerShell

For PowerShell:

$env:SystemSettings__ForegroundColour = "Neon"
$env:SystemSettings__BackgroundColour = "Mauve"

Cmd

For Cmd:

set SystemSettings__ForegroundColour=Neon
set SystemSettings__BackgroundColour=Mauve

Once you have set the variables, run the application as normal.

dotnet run

Here we are setting the ForegroundColour and BackgroundColour at the command line.

Of note is that for hierarchical settings, we denote the hierarchy with the colon __ , as shown above.

This should result in the following:

TLDR

You can override settings in appsettings.json by passing new ones via command line.

The code is in my GitHub.

Happy hacking!