C#: What is the difference between abstract class and interface in C#?

Abstract class — can have implementation. Interface — only contract (until C# 8).

Abstract class:

• Can have implemented methods.
• Can have fields.
• Can have constructors.
• Single inheritance only.
• Can have access modifiers.

1public abstract class Animal {
2    public string Name { get; set; }
3
4    // Implemented method
5    public void Eat() {
6        Console.WriteLine("Eating...");
7    }
8
9    // Abstract method
10    public abstract void MakeSound();
11}

Interface:

• Only method signatures (traditionally).
• No fields.
• No constructors.
• Multiple inheritance.
• All members public.

1public interface IAnimal {
2    string Name { get; set; }
3    void MakeSound();
4}

C# 8.0+ interface changes:

• Can have default implementations.
• Can have static methods.
• Can have private methods.