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

Interface — contract. Abstract class — partial implementation.

Interface:

• Multiple implementation.
• No state.
• Contract.

Abstract class:

• Single inheritance.
• State (fields).
• Partial implementation.

1// Interface
2public interface IAnimal
3{
4    void Speak();
5}
6
7// Abstract class
8public abstract class Animal
9{
10    public string Name { get; set; }
11    public abstract void Speak();
12}

Key: Interface for contracts, abstract for shared code.