title: How to add member variable to an interface in c#
body: <p>I know this may be basic but I cannot seem to add a member variable to an interface.
  I tried inheriting the interface to an abstract class and add member variable to the abstract class but it still does not work. Here is my code:</p>
  
  <pre><code>public interface IBase {
    void AddData();
    void DeleteData();
  }
  
  public abstract class AbstractBase : IBase {
    string ErrorMessage;
    public abstract void AddData();
    public abstract void DeleteData();
  }
  
  public class AClass : AbstractBase {
    public override void AddData();
    public override void DeleteData();
  }
  </code></pre>
  
  <p>updated base on comment of Robert Fraser</p>