A class must be declared abstract if it contains variant functionality. Variant functionality is functionality that is defined in a base class but implemented in a derived (or child) class. This is indicated by using the C# ‘abstract’ keyword (or in VB.NET, the ‘MustOverride’ keyword) before a method or property name.
Abstract classes cannot be instantiated themselves since they do not provide a complete implementation all their functionality. They are very useful when a base class needs to provide both invariant and variant functionality.
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