dependency-injection-based-on-request-headers.cs

public class DefaultMessageService : IMessageService
{
    public string GetMessage() => "Hello world!";
}

public class HiddenMessageService : IMessageService
{
    private readonly ISecretKey _key;

    public HiddenMessageService(ISecretKey key)
    {
            _key = key;
    }

    public string GetMessage() => 
        "The answer to life the universe and everything: " + 
        _key.GetKey();
}

public interface ISecretKey
{
    public string GetKey();
}

public class SecretKey : ISecretKey
{
    public string GetKey() => "42";
}