C# Interface without members

public interface ISetting {  }  public class ApplicationSetting:ISetting {     string Setting1 {get;set;}     string Setting2 {get;set;} }   public class AnotherSetting : ISetting {      string AnotherSetting1{get;set;}     string AnotherSetting2{get;set;} } 

I guess this is called a marker interface, isn’t it? I asked because I only know how to use interface with members or properties inside of them. So for example, I do have this code.

public interface IData  {      List<string> Data { get;} } 

I could derive a class from this and use a pattern like strategy but I really don’t have an idea how to use an interface without members.

I also did some research about marker interfaces and I saw this code.

public interface IContainSensitiveInformation {  }      public class Customer : IContainSensitiveInformation     {         public string LastName { get; set; }         public string SocialSecurityName { get; set; }         public string CreditCardNumber { get; set; }     }  public void SaveCustomer(Customer customer) {     if (customer is IContainSensitiveInformation)         _secureService.SaveCustomer(customer);     else         _regularService.SaveCustomer(customer); } 

Source : Marker Interface

Yes, I can understand the code sample above but I really don’t have an idea when to use it on a real-world project. Looking at the example above, why would you check if a class implemented IContainSensitiveInformation?. Would love to have someone explain things up for me to better understand when to use a marker interface.