C#: dependency injection makes code testable

Dependency injection means a class receives the objects it depends on instead of creating them. If Greeter called DateTime.Now itself, its output would change with the time of day, and a test could not control it. Depending on an interface lets callers supply any implementation.

using System;

interface IClock {
  int Hour { get; }
}

class SystemClock : IClock {
  public int Hour { get { return DateTime.Now.Hour; } }
}

class FixedClock : IClock {
  public FixedClock(int hour) {
    Hour = hour;
  }

  public int Hour { get; }
}

class Greeter {
  private readonly IClock clock;

  public Greeter(IClock clock) {
    this.clock = clock;
  }

  public string Greet() {
    return clock.Hour < 12 ? "Good morning" : "Good afternoon";
  }
}

class Program {
  static void Main() {
    Console.WriteLine(new Greeter(new FixedClock(9)).Greet());
    Console.WriteLine(new Greeter(new FixedClock(15)).Greet());
  }
}

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