Override App.config value with an environment variable

The ConfigurationManager class doesn't do that for you, it will only read from your app config. To fix this, you can use a function to get the variable and use that instead of calling ConfigurationManager.AppSettings directly. This is good practice to do anyway as it means you can easily move your config into a JSON file or a database and you won't need to update every usage of the old method.

For example:

public string GetSetting(string key)
{
    var value = Environment.GetEnvironmentVariable(key);

    if(string.IsNullOrEmpty(value))
    {
        value = ConfigurationManager.AppSettings[key];
    }

    return value;
}

In .net 4.7.1 you can use ConfigurationBuilders to accomplish this.

See https://learn.microsoft.com/en-us/dotnet/api/system.configuration.configurationbuilder?view=netframework-4.8