Abstract vs virtual vs new

The <a href=https://msdn.microsoft.com/en-us/library/aa645615(v=vs.71).aspx> abstract modifier is used to indicate that a class is incomplete and that it is intended to be used only as a base class. An abstract class differs from a non-abstract class in the following ways:

• An abstract class cannot be instantiated directly, and it is a compile-time error to use the new operator on an abstract class. While it is possible to have variables and values whose compile-time types are abstract, such variables and values will necessarily either be null or contain references to instances of non-abstract classes derived from the abstract types.
• An abstract class is permitted (but not required) to contain abstract members.
• An abstract class cannot be sealed.
• A non-abstract class derived from an abstract class must include actual implementations of all inherited abstract methods and accessors.
• An abstract class must provide implementation for all interface members.

When a non-abstract class is derived from an abstract class, the non-abstract class must include actual implementations of all inherited abstract members, thereby overriding those abstract members.For example: ```C# abstract class A { public abstract void F(); } abstract class B: A { public void G() {} } class C: B { public override void F() { // actual implementation of F } } ``` The abstract class A introduces an abstract method F. Class B introduces an additional method G, but since it doesn't provide an implementation of F, B must also be declared abstract. Class C overrides F and provides an actual implementation. Since there are no abstract members in C, C is permitted (but not required) to be non-abstract.

####An abstract class that implements an interface might map the interface methods onto abstract methods. For example:

interface I
{
   void M();
}
abstract class C : I
{
  public abstract void M();
}

####The abstract keyword enables you to create classes and class members that are incomplete and must be implemented in a derived class. The sealed keyword enables you to prevent the inheritance of a class or certain class members that were previously marked virtual.

###It is an error to use the static or virtual modifiers in an abstract method declaration.

Polymorphism, Method Hiding and Overriding

override

Virtual methods and Overriding

Method Hiding and Overriding