In C#, when should you use interfaces and when should you use abstract classes? What can be the deciding factor?
4 Answers
150
votes
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.
105
votes
Abstract classes and interfaces are semantically different, although their usage can overlap.
An abstract class is generally used as a building basis for similar classes. Implementation that is common for the classes can be in the abstract class.
An interface is generally used to specify an ability for classes, where the classes doesn't have to be very similar.
12
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8
votes
The real question is: whether to use interfaces or base classes. This has been covered before.
In C#, an abstract class (one marked with the keyword "abstract") is simply a class from which you cannot instantiate objects. This serves a different purpose than simply making the distinction between base classes and interfaces.
