Interface Segregation Principle (ISP)

Introduction#

The principle states that no client should be forced to depend on methods that it doesn’t use. A client should never be forced to implement an interface that it doesn’t use or client shouldn’t be forced to depend on methods that they don’t use.

Interface Segregation Principle C#

Here we give an example of ISP violation and then refactor that violation. Without talking unnecessary things let’s jump into the code.

ISP violation :

public interface IMessage{
 IList<string> ToAddress {get; set;}
 IList<string> BccAddresses {get; set;}
 string MessageBody {get; set;}
 string Subject {get; set;}
 bool Send();
}
 
public class SmtpMessage : IMessage{
 public IList<string> ToAddress {get; set;}
 public IList<string> BccAddresses {get; set;}
 public string MessageBody {get; set;}
 public string Subject {get; set;}
 public bool Send(){
  // Code for sending E-mail.
 }
}
 
public class SmsMessage : IMessage{
 public IList<string> ToAddress {get; set;}
 public IList<string> BccAddresses {
  get { throw new NonImplementedException(); }
  set { throw new NonImplementedException(); } 
 }
 public string MessageBody {get; set;}
 public string Subject {
  get { throw new NonImplementedException(); }
  set { throw new NonImplementedException(); } 
 }
 public bool Send(){
  // Code for sending SMS.
 }
}

In the SmsMessage we don’t need BccAddresses and Subject, but we forced to implement it because of IMessage interface . So it’s violate the ISP principle.

Remove violation according to ISP:

public interface IMessage{
 bool Send(IList<string> toAddress, string messageBody);
}
 
public interface IEmailMessage : IMessage{
 string Subject {get; set;}
 IList<string> BccAddresses {get; set;}
}
 
public class SmtpMessage : IEmailMessage{
 public IList<string> BccAddresses {get; set;}
 public string Subject {get; set;}
 public bool Send (IList<string> toAddress, string messageBody){
  // Code for sending E-mail.
 }
}
 
public class SmsMessage : IMessage{
 public bool Send (IList<string> toAddress, string messageBody){
  // Code for sending SMS.
 }
}

SmsMessage need only toAddress and messageBody, so now we can use IMessage interface to avoid unnecessary implementations.