What are the main differences between an interface and a class in object-oriented programming?

The interface and class in object-oriented programming have the following key differences:

• Purpose:

  • An interface defines a set of methods and properties without implementation, serving to describe a contract that a class must fulfill.
  • A class is a concrete implementation that can contain both data (fields) and methods with implementation.

• Implementation:

  • An interface does not contain method implementations (until C# 8.0, after which default implementations appeared).
  • A class contains implementations for all its methods.

• Inheritance:

  • A class can inherit from only one base class but can implement multiple interfaces.
  • An interface can inherit from multiple other interfaces.

• Usage:

  • Interfaces are used to ensure polymorphism and loose coupling.
  • Classes create objects and contain logic.

Example in C#:

interface ILogger
{
    void Log(string message);
}

class ConsoleLogger : ILogger
{
    public void Log(string message)
    {
        Console.WriteLine(message);
    }
}

Here, ILogger is an interface, and ConsoleLogger is a class implementing this interface.