One of fundamentals of The Twelve-Factor is the strict separation of config from code and storing them in environment variables.

A common way is to store default values in JSON or XML files and have the possibility to override them with environment variables.

That's why in ASP.NET Core, we usually find this kind of builder:

var builder = new ConfigurationBuilder()
    .SetBasePath(env.ContentRootPath)
    .AddJsonFile("appsettings.json")
    .AddJsonFile($"appsettings.{env.EnvironmentName}.json")
    .AddEnvironmentVariables();
Configuration = builder.Build();

1. appsettings.json contains all the default configuration.
2. appsettings.{env}.json is merged with the default one. In this case the environment name is required as an environment variable.
3. Finally the merged values are overridden by environment variables having the same name.

But what if we have to support System.Configuration.ConfigurationManager?

ConfigurationManager

With ConfigurationManager there are instead XML files, such as app.config and app.{env}.config.
Unfortunately the artefact cannot be immutable and be shipped with all environments conf.
During the build process, the final config file is transformed with a XDT tool (Xml Document Transformation).

Since .NET Framework 4.7.1, there is a handy NuGet that allows us to override the default values by environment variables.

• Add Microsoft.Configuration.ConfigurationBuilders.Environment NuGet.
• Add the following markup in the config file:

   <configuration>

      <configSections>
        <section name="configBuilders" type="System.Configuration.ConfigurationBuildersSection, System.Configuration" restartOnExternalChanges="false" requirePermission="false" />
      </configSections>

      <configBuilders>
        <builders>
          <add name="EnvConfigBuilder" type="Microsoft.Configuration.ConfigurationBuilders.EnvironmentConfigBuilder, Microsoft.Configuration.ConfigurationBuilders.Environment" />
        </builders>
      </configBuilders>

      <appSettings configBuilders="EnvConfigBuilder">
        ...
      </appSettings>

      <connectionStrings configBuilders="EnvConfigBuilder">
        ...
      </connectionStrings>

   </configuration>

• If an environment variable with the same key/name exists, then it will be overridden during execution.

💡 For more details please follow the documentation.

Azure Key Vault

It's best to stick with the Twelve-Factor methodology for your App Service or Azure Functions and to not use other ConfigurationBuilders libraries.

For this you can setup Key Vault references.
So without requiring any code changes, all secrets will be added as environment variables and consumed by EnvironmentConfigBuilder.

Tips

If environment variable names cannot match the config, it's possible to customize like this:

using Microsoft.Configuration.ConfigurationBuilders;

public class EnvConfigBuilder : EnvironmentConfigBuilder
{
    public override string GetValue(string key) => key switch
    {
        "MyConfName" => base.GetValue("MyEnvVarName"),
        _ => null // means do not override the default value
    }
}

<configBuilders>
  <builders>
    <add name="EnvConfigBuilder" type="MyNamespace.EnvConfigBuilder, MyAssemblyName" />
  </builders>
</configBuilders>

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