3b -- introducing an interface

3b.cs

public class Adder {
	private IRandomNumberGenerator _generator;

	public Adder(IRandomNumberGenerator generator) {
		_generator = generator;
	}

	public int AddTwoRandomNumbers() {
		var firstInt = _generator.Generate();
		var secondInt = _generator.Generate();
		return firstInt + secondInt;
	}
}

public interface IRandomNumberGenerator {
	int Generate();
}

public class RandomNumberGenerator : IRandomNumberGenerator {
	public int Generate() {
		Random rnd = new Random();
		return rnd.Next();
	}
}