Interface

An interface has the following properties:

• An interface is like an abstract base class: any non-abstract type that implements the interface must implement all its members.
• An interface cannot be instantiated directly.
• Interfaces can contain events, indexers, methods, and properties.
• Interfaces contain no implementation of methods.
• Classes and structs can implement more than one interface.
• An interface itself can inherit from multiple interfaces.
   

Classes and structs implement interfaces in a manner similar to how classes inherit a base class or struct, with two exceptions:

• A class or struct can implement more than one interface.
• When a class or struct implements an interface, it receives only the method names and signatures, because the interface itself contains no implementations, as shown in the following example.
   

Example:

public class Car : IEquatable<Car>
{
    public string Make {get; set;}
    public string Model { get; set; }
    public string Year { get; set; }

    // Implementation of IEquatable<T> interface
    public bool Equals(Car car)
    {
        if (this.Make == car.Make &&
            this.Model == car.Model &&
            this.Year == car.Year)
        {
            return true;
        }
        else
            return false;
    }
}

The IEquatable(Of T) interface announces to the user of the object that the object can determine whether it is equal to other objects of the same type, and the user of the interface does not have to know how this is implemented.