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

Abstract class — partial implementation. Interface — contract only.

Abstract class:

• Can have implementation.
• State (fields).
• Single inheritance.

Interface:

• Contract only (C# 8+ can have default).
• No state.
• Multiple implementation.

1// Abstract class
2public abstract class Shape
3{
4    public string Color { get; set; }
5    public abstract double Area();
6}
7
8// Interface
9public interface IShape
10{
11    double Area();
12    string Color { get; set; }
13}

Key: Abstract for shared code, interface for contracts.