Interfaces vs. abstract classes

Question

In C#, when should you use interfaces and when should you use abstract classes? What can be the deciding factor?

Answer 1

Update: C# 8.0 New Feature: Beginning with C# 8.0, an interface may define a default implementation for members, including properties. Defining a default implementation for a property in an interface is rare because interfaces may not define instance data fields.

The advantages of an abstract class are:

• Ability to specify default implementations of methods
• Added invariant checking to functions
• Have slightly more control in how the "interface" methods are called
• Ability to provide behavior related or unrelated to the interface for "free"

Interfaces are merely data passing contracts and do not have these features. However, they are typically more flexible as a type can only be derived from one class, but can implement any number of interfaces.