Difference between Abstract Class and Interface in C#

To better understand the Abstract Class and Interface Difference in C#, let's take a look at the following comparison table:

Abstract Class	Interface
Can contain both abstract and non-abstract methods, properties and some other members	Can only contain methods (which cannot be implemented inside Interface), property and some other members
Can have access modifiers for its members	C# 8.0 allows access modifiers in an interface. The default level of access for all interface members is public.
Can provide a default implementation for derived classes	Does not provide any implementation or behavior
Can define constructors and destructors	Cannot define constructors or destructors
Can have fields, which can store state	Cannot have fields, only properties
It can be used to define a base class for a family of related classes	Can be used to define a contract for a group of unrelated classes

Now let’s learn the Difference between Interface and Abstract Class C# using some code examples.

Here's a simple example in C# to illustrate some differences:

// Abstract class
public abstract class AbstractClass
{
    protected int myField;
    public int MyProperty { get; set; }

    public void ConcreteMethod()
    {
        Console.WriteLine("This is a concrete method in an abstract    class.");
    }


    public abstract void AbstractMethod(); // This abstract method must be     implemented by class that inherits from this abstract class.
    }
}

// Interface
public interface IMyInterface
{
    int MyProperty { get; set; } // This is a property. Fields cannot be included in an interface.

    void MyMethod(); // This method must be implemented by any class that uses this interface.
}

Now let’s look at a class to see the c# difference between abstract class and interface, that implements our earlier defined "IMyInterface" and another that inherits from "AbstractClass".

public class InterfaceImplementer : IMyInterface
{
    private int _myProperty;
    public int MyProperty
    {
        get { return _myProperty; }
        set { _myProperty = value; }
    }

    public void MyMethod()
    {
        Console.WriteLine("This is a method implementing the interface method.");
    }
}

public class AbstractClassInheritor : AbstractClass
{
    public override void AbstractMethod()
    {
        Console.WriteLine("This is a method implementing the abstract class method.");
    }
}

In the "InterfaceImplementer" class, we are required to provide implementations for all members of the "IMyInterface" interface. In this case, the "MyMethod" method and "MyProperty" property.

In the "AbstractClassInheritor" class, we are required to provide an implementation for the "AbstractMethod" method from "AbstractClass". However, we also automatically inherit the "ConcreteMethod" and "MyProperty" from "AbstractClass" and can use them as is.

It's also worth noting that a class can both inherit from an abstract class and implement multiple interfaces.

public class HybridClass : AbstractClass, IMyInterface
{
    public override void AbstractMethod()
    {
        Console.WriteLine("This is a method implementing the abstract class method.");
    }

    public void MyMethod()
    {
        Console.WriteLine("This is a method implementing the interface method.");
    }
}

In this "HybridClass", we are providing implementations for both the abstract method from "AbstractClass" and the method from "IMyInterface".

Let's consider some more examples in C# difference between abstract class and interface

// Interface
public interface IShape
{
    double Area();
    double Perimeter();
}

public class Rectangle : IShape
{
    public double Width { get; set; }
    public double Height { get; set; }

    public double Area()
    {
        return Width * Height;
    }

    public double Perimeter()
    {
        return 2 * (Width + Height);
    }
}

public class Circle : IShape
{
    public double Radius { get; set; }

    public double Area()
    {
        return Math.PI * Math.Pow(Radius, 2);
    }

    public double Perimeter()
    {
        return 2 * Math.PI * Radius;
    }
}

In this example, the IShape interface defines the Area() and Perimeter() methods that must be implemented by classes that implement the interface. The Rectangle and Circle classes implementing the IShape interface and providing their own implementation of the Area() and Perimeter() methods.

// Abstract class
public abstract class Animal
{
    public string Name { get; set; }
    public abstract void MakeSound();
}

public class Dog : Animal
{
    public override void MakeSound()
    {
        Console.WriteLine("Woof!");
    }
}

public class Cat : Animal
{
    public override void MakeSound()
    {
        Console.WriteLine("Meow!");
    }
}

In this example, the Animal class is an abstract class with an abstract method MakeSound(). The Dog and Cat classes are derived classes that inherit from the Animal class and implement the MakeSound() method.

When it comes to performance, abstract classes, and interfaces are both highly efficient and have negligible differences. The choice between using an abstract class or an interface should be based on the design requirements of the application, rather than on performance considerations.

When choosing between an abstract class and an interface in C#, consider the following factors:

1. Design Requirements: Consider the design requirements of the application and choose the option that best meets those requirements.
2. Reusability: Consider how the abstract class or interface can be reused in the other parts of the application or in future projects.
3. For Just Contract: If you need to define just a contract for the classes, use Interface.
4. Default Implementation: If you need to provide a default implementation for derived classes, or when you want to define a base class for a family of related classes use an abstract class.