Interface Segregation Principle (ISP)

=> 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.

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 :

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.