title: C# static service or implement interface
body: <p>I have two case:</p>
  
  <p>-Use static service</p>
  
  <pre><code>public class TestService {
     public static bool FunctionA(int b) {
        return b &gt; 0;
     }
  }
  </code></pre>
  
  <p>-Use interface</p>
  
  <pre><code>public interface ITestSerice {
      bool FunctionA(int b);
  }
  
  public class TestService : ITestService {
      public bool FunctionA(int b) {
          return b &gt; 0;
      }
  }
  </code></pre>
  
  <p>Static class is very simple. But I often see more people using the interface (or higher than Dependency Injection). Please explain to me why and when to use the interface? (which is better?)</p>
  
  <p>Sorry if my english is too bad :D</p>