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Interface Segregation Principal C#

This article describes the Interface Segregation Principle in C#.

Scenario

Imagine a scenario where you, as the programmer, work for an insurance company. The company offers car insurance, health insurance, and term life insurance to all residents in the city.

While planning, you have divided people into four categories: -

Children
Serviceman
Businessman
Old age people

You have decided to create an interface named IInsurance that contains the following members: -

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

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 specific rules that apply to a particular category: -

Car insurance is not relevant for children.
There is no term insurance for elderly people because, according to the insurance company, the maximum age should be 40.

So, from the previous rules: -

The CarInsurance method is not relevant for the Children's class.
The TermInsurance method is not relevant for the Old Age class.

However, you need to add these methods to the class because you have implemented the interface. This is known as 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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