Features of Abstract Class
An abstract class cannot be instantiated.
An abstract class contain abstract members as well as non-abstract members.
An abstract class cannot be a sealed class because the sealed modifier prevents a class from being inherited and the abstract modifier requires a class to be inherited.
A non-abstract class which is derived from an abstract class must include actual implementations of all the abstract members of parent abstract class.
An abstract class can be inherited from a class and one or more interfaces.
An Abstract class can has access modifiers like private, protected, internal with class members. But abstract members cannot have private access modifier.
An Abstract class can has instance variables (like constants and fields).
An abstract class can has constructors and destructor.
An abstract method is implicitly a virtual method.
Abstract properties behave like abstract methods.
An abstract class cannot be inherited by structures.
An abstract class cannot support multiple inheritance.
Common design guidelines for Abstract Class
Don't define public constructors within abstract class. Since abstract class cannot be instantiate and constructors with public access modifiers provides visibility to the classes which can be instantiated.
Define a protected or an internal constructor within an abstract class. Since a protected constructor allows the base class to do its own initialization when sub-classes are created and an internal constructor can be used to limit concrete implementations of the abstract class to the assembly which contains that class.
When to use
Need to create multiple versions of your component since versioning is not a problem with abstract class. You can add properties or methods to an abstract class without breaking the code and all inheriting classes are automatically updated with the change.
Need to to provide default behaviors as well as common behaviors that multiple derived classes can share and override.
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