In C#, interfaces are used to define behavior contracts that need to be implemented by multiple unrelated classes, suitable for multiple inheritance, dependency injection and unit testing; abstract classes are used for closely related classes of shared logic, supporting fields, access modifiers and constructors. 1. The interface defines behavior contracts, supports default implementations, but is mainly used for structural constraints; 2. The abstract class contains abstract and concrete methods, providing shared logic; 3. The interface allows multiple implementations, without fields and constructors, and members are exposed by default; 4. The abstract class can only inherit a single one, and can have private members and constructors; 5. The interface is suitable for plug-in architecture and API design, and the abstract class is suitable for "is-a" relationship modeling; 6. It can be used in combination, and the abstract class implements the interface to provide basic implementation. The choice depends on the design goal: the interface focuses on ability, and the abstract class focuses on identity and common logic.

In C#, interfaces and abstract classes are both used to achieve abstraction, but they serve different purposes and have distinct use cases. Knowing when to use each can help you design more flexible and maintainable code.

What an Interface Does

An interface in C# defines a contract that implementing classes must follow. It contains method signatures, properties, events, or indexers without any implementation. Starting from C# 8.0, interfaces can also include default implementations, but their main purpose remains to enforce structure rather than behavior.

When to Use an Interface:

• When you need multiple unrelated classes to implement the same functionality.
• When you want to support multiple inheritance of behavior (C# doesn't allow multiple base classes).
• When working with dependency injection or unit testing — interfaces make it easier to mock dependencies.

For example:

 public interface ILogger
{
    void Log(string message);
}

A class can then implement this interface and provide its own version of Log . This makes it easy to swap out logging mechanisms without changing dependent code.

What an Abstract Class Does

An abstract class is a class that cannot be instantiated on its own and may contain both implemented methods and abstract methods (which must be implemented by derived classes). It's useful for sharing common logic among related classes while still enforcing some implementation details.

When to Use an Abstract Class:

• When you have closely related classes that share some common implementation.
• When you want to define a base class with some default behavior and require subclasses to implement specific parts.
• When you need to control versioning better — adding a new method to an abstract class can be done without breaking existing derived classes (unlike with interfaces before C# 8).

Example:

 public abstract class Animal
{
    public abstract void MakeSound();

    public void Sleep()
    {
        Console.WriteLine("Sleeping...");
    }
}

Here, all animals must implement MakeSound() , but they can reuse the Sleep() method.

Key Differences Between Interface and Abstract Class

Here's a quick breakdown:

• Implementation: Interfaces (before C# 8) had no implementation; abstract classes always could.
• Multiple Inheritance: A class can implement multiple interfaces but inherit from only one abstract class.
• Access Modifiers: Interfaces don't allow access modifiers on members (they're implicitly public); abstract classes can have protected or internal members.
• Fields: Interfaces can't have fields; abstract classes can.
• Constructors: Abstract classes can have constructors; interfaces cannot.

These differences influence how and where to use each. For instance, if you're designing plug-in architectures or APIs that might change over time, interfaces give more flexibility. If you're building a family of related objects with shared logic, abstract classes are often the better choice.

Choosing Between Them Isn't Always Clear-Cut

Sometimes it comes down to design preference or project requirements. But here are a few rules of thumb:

• Favor interfaces when defining behaviors that cross-cut unrelated classes.
• Use abstract classes when modeling "is-a" relationships with shared implementation.
• Consider using both — an abstract class can implement an interface to offer a base implementation.

You'll find that many well-designed systems use a mix of both depending on what part of the system you're working in.

Basically, interfaces are about capability, abstract classes are about identity and shared logic. Choosing the right one helps keep your code organized and scalable without unecessary complexity.

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