C# virtual Keyword

Use the virtual keyword. When a virtual method is called, the runtime type is used by the compiler.

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Virtual. Consider the runtime types of things. With a virtual call, an object's most derived type is used for a method invocation.

A virtual method can be redefined. The virtual keyword designates a method that is overridden in derived classes. We can add derived types without modifying the rest of the program.

An example. Class A has a public virtual method called Test. Class B, meanwhile, derives from class A and it provides a public override method called Test as well.
Tip: The virtual modifier tells the compiler that when any class derived from class A is used, an override method should be called.
Info: With virtual methods, the runtime type is always used to determine the best method implementation.
In Main: Ref1 has a compile-time and runtime type of A. On the other hand, ref2 has a compile-time type of A but a runtime type of B.
C# program that introduces virtual method using System; class A { public virtual void Test() { Console.WriteLine("A.Test"); } } class B : A { public override void Test() { Console.WriteLine("B.Test"); } } class Program { static void Main() { // Compile-time type is A. // Runtime type is A as well. A ref1 = new A(); ref1.Test(); // Compile-time type is A. // Runtime type is B. A ref2 = new B(); ref2.Test(); } } Output A.Test B.Test

Private virtual. Private virtual methods cannot be declared in the C# language. For private methods, you have already entered the data object.
Thus: No virtualized entry point is ever required. And a private virtual method is not useful.
Here: We see both a program that attempts to use a private virtual method member, as well as the compile-time error.
Note: The error occurs during the static compilation phase. No program that declares a private virtual method will ever be executed.
C# program that uses private virtual method class A { private virtual int Test() { return 1; } } class Program { static void Main() { } } Output Error 1 'A.Test()': virtual or abstract members cannot be private

A discussion. Why would you need to use virtual methods? Your program may be designed in such a way that you do not know all the types of objects that will occur when it is executed.
And: You can provide a standard (base) type and design around that type. Then add methods for the more specific (derived) types.
So: When you call a method on the base type, you invoke the more derived (and useful) method.

Abstract. The term abstract refers to a conceptual type. It has no implementation. An abstract class then is used as a template for derived classes.Abstract

Notes, polymorphism. Virtual methods are an implementation of type-based polymorphism. This gives you a dynamic entry point to the class type.
Note: This enables you to clearly separate the usage of the type from the implementation of each subtype.
Info: If you are already inside the type, you can simply call a regular instance private method.

Notes, performance. Instance and static methods are faster to invoke than interface and virtual methods. Speed is important in many programs.Static

A summary. Virtual specifies that the method can be overridden in derived types. It tells the compiler to generate code that selects an override method.


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