Articles → .NET DESIGN PATTERN → Interface Segregation Principal C#

Interface Segregation Principal C#

In this article, we will discuss the interface segregation principle.

Scenario

Consider a scenario in which you (as the programmer) are working for an insurance company. The insurance company is providing car insurance, medical insurance, and term insurance to everyone in the city. While planning, you have categorized people into four categories: -

Children
Serviceman
Businessman
Old age people

You have decided to make an interface IInsurance that contains the following code.

public interface IInsurance {
	void CarInsurance();
	void MedicalInsurance();
	void TermInsurance();
}

Now you create four classes, one for each category, and implement the IInsurance interface. So, your classes will be: -

public class Children: IInsurance {
	public void CarInsurance() {
		throw new NotImplementedException();
	}

	public void MedicalInsurance() {
		throw new NotImplementedException();
	}

	public void TermInsurance() {
		throw new NotImplementedException();
	}
}
public class ServiceMan: IInsurance {
	public void CarInsurance() {
		throw new NotImplementedException();
	}

	public void MedicalInsurance() {
		throw new NotImplementedException();
	}

	public void TermInsurance() {
		throw new NotImplementedException();
	}
}

public class BusinessMan: IInsurance {
	public void CarInsurance() {
		throw new NotImplementedException();
	}

	public void MedicalInsurance() {
		throw new NotImplementedException();
	}

	public void TermInsurance() {
		throw new NotImplementedException();
	}
}

public class OldAge: IInsurance {
	public void CarInsurance() {
		throw new NotImplementedException();
	}

	public void MedicalInsurance() {
		throw new NotImplementedException();
	}

	public void TermInsurance() {
		throw new NotImplementedException();
	}
}

However, the insurance company has certain rules that are specific to a category: -

There is no relevance of car insurance for children
There is no relevance in providing medical insurance to a serviceman because their employer has already opted for the insurance for their employees (because of the government policy)
No term insurance for old age people because according to the insurance company, the maximum age should be 40

So, from the preceding rules: -

CarInsurance method is not relevant for the Children's class
MedicalInsurance method is not relevant to the ServiceMan class
TermInsurance method is not relevant for the OldAge class

But you need to add these methods to the class because you have implemented the interface. This is called a polluted interface.

Interface Segregation Principle For Rescue

According to the interface segregation principle, we can split the IInsurance interface into multiple interfaces. See the following code.

public interface ICarInsurance {
	void CarInsurance();
}

public interface IMedicalInsurance {
	void MedicalInsurance();
}
public interface ITermInsurance {
	void TermInsurance();
}

And classes will only implement those interfaces that are relevant. In this way, they are not forced to implement any method that is not relevant. So according to this principle, the classes can be modified as: -

public class Children: IMedicalInsurance {

	public void MedicalInsurance() {
		throw new NotImplementedException();
	}
}
public class ServiceMan: ICarInsurance,
ITermInsurance {

	public void CarInsurance() {
		throw new NotImplementedException();
	}

	public void TermInsurance() {
		throw new NotImplementedException();
	}
}

public class BusinessMan: ICarInsurance,
IMedicalInsurance,
ITermInsurance {

	public void CarInsurance() {
		throw new NotImplementedException();
	}

	public void MedicalInsurance() {
		throw new NotImplementedException();
	}

	public void TermInsurance() {
		throw new NotImplementedException();
	}
}

public class OldAge: ICarInsurance {

	public void CarInsurance() {
		throw new NotImplementedException();
	}
}

So, the interface segregation principle requires additional effort to implement but it makes your system more flexible.

Posted By -	Karan Gupta

Posted On -	Thursday, June 4, 2015

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