{"id":1933,"date":"2011-04-07T23:31:41","date_gmt":"2011-04-07T18:01:41","guid":{"rendered":"http:\/\/JitendraZaa.com\/blog\/?p=1933"},"modified":"2011-04-07T23:31:41","modified_gmt":"2011-04-07T18:01:41","slug":"virtual-override-and-new-keyword-in-c","status":"publish","type":"post","link":"https:\/\/www.jitendrazaa.com\/blog\/microsoft\/csharp\/virtual-override-and-new-keyword-in-c\/","title":{"rendered":"Virtual, Override and new Keyword in C#"},"content":{"rendered":"

Consider following class Hierarchy:<\/p>\n

\"Multilevel<\/a>
Multilevel Inheritance<\/figcaption><\/figure>\n

In this example, we will consider three classes which are TestA, TestB and TestC.<\/p>\n

\nnamespace OOPS_Concept\n{\n    class TestA\n    {\n        public void display() { Console.WriteLine("TestA - display()"); }\n    }\n\n    class TestB : TestA\n    {\n        public void display() { Console.WriteLine("TestB - display()"); }\n    }\n\n    class Test\n    {\n        static void Main(string[] args)\n        {\n            TestA a;\n            TestB b;\n\n            a = new TestA();\n            b = new TestB();\n            a.display();  \/\/ TestA - display()\n            b.display();  \/\/ TestB - display()\n\n            a = new TestB();\n            a.display();  \/\/ TestA - display()\n\n            Console.Read();\n        }\n    }\n}\n<\/pre>\n
output :<\/p>\n
TestA – display()
\nTestB – display()
\nTestA – display()<\/p><\/blockquote>\n
Program will compile and run successfully but<\/p>\n
    \n
  • The Problem of above code is that, third output should be “TestB \u2013display()”\u009d because the variable a have the object of B.<\/li>\n
  • …Test.cs(15,21): warning CS0108: ‘OOPS_Concept.TestB.display()’ hides inherited member ‘OOPS_Concept.TestA.display()’. Use the new keyword if hiding was intended.<\/li>\n<\/ul>\n
    Lets resolve the problem 1.<\/p>\n
    \n
    Keyword Virtual and Override<\/strong>
    \nIn C#, if you like to override the parent class method then you must mark the parent method by keyword “Virtual<\/strong>“\u009d and method in derived class which intended to override the parent method should be marked by keyword “override<\/strong>“\u009d<\/p>\n
    Note:<\/strong><\/p>\n
      \n
    1. If parent method is marked by keyword “Virtual”\u009d but child is not marked<\/strong> by keyword “override”\u009d, then program will compile but the parent method will not be overrided.<\/li>\n
    2. If Child method is marked by keyword “override”\u009d but parent method is not marked by keyword “virtual”\u009d <\/strong>then program will not compile. It will give following error :
      \n‘OOPS_Concept.TestB.display()’: cannot override inherited member ‘OOPS_Concept.TestA.display()’ because it is not marked virtual, abstract, or override.<\/strong><\/li>\n<\/ol>\n
      Example with keyword Virtual and override:<\/p>\n
      \nnamespace OOPS_Concept\n{\n    class TestA\n    {\n        public virtual void display() { Console.WriteLine("TestA - display()"); }\n    }\n\n    class TestB : TestA\n    {\n        public override void display() { Console.WriteLine("TestB - display()"); }\n    }\n\n    class Test\n    {\n        static void Main(string[] args)\n        {\n            TestA a;\n            TestB b;\n\n            a = new TestA();\n            b = new TestB();\n            a.display();  \/\/ TestA - display()\n            b.display();  \/\/ TestB - display()\n\n            a = new TestB();\n            a.display();  \/\/ TestA - display()\n\n            Console.Read();\n        }\n    }\n}\n<\/pre>\n
      output:<\/p>\n
      TestA – display()
      \nTestB – display()
      \nTestB – display()<\/p><\/blockquote>\n
      \n
      Method Hiding \u2013 Keyword new:<\/strong>
      \nAs discussed earlier, second point in output of first program was “Compiler generated warning message”\u009d.
      \nBecause c# also support Method Hiding. To mark method as “hiding”\u009d use keyword “new”\u009d in derived class .
      \nKeyword “new”\u009d can be used with keyword “virtual”\u009d also.
      \nQuick Note :
      \nIf keyword “override”\u009d is used in derive class then its override the parent method.
      \nIf Keyword “new”\u009d is used in derive class then derive method hided by parent method. As shown in below program:<\/p>\n
      \nnamespace OOPS_Concept\n{\n    class TestA\n    {\n        public void display() { Console.WriteLine("TestA - display()"); }\n    }\n\n    class TestB : TestA\n    {\n        public virtual new void display() { Console.WriteLine("TestB - display()"); }\n        \/\/Instead of virtual new, new can also be written.\n    }\n\n    class TestC : TestB\n    {\n        public new void display() { Console.WriteLine("TestC - display()"); }\n    }\n\n    class Test\n    {\n        static void Main(string[] args)\n        {\n            TestB b;\n            TestC c;\n\n            b = new TestB();\n            b.display();  \/\/ TestB - display()\n\n            b = new TestC();\n            b.display();  \/\/ TestB - display() instead of TestC - display()\n\n            Console.Read();\n        }\n    }\n}\n<\/pre>\n
      Output:<\/p>\n
      TestB – display()
      \nTestB – display()<\/p><\/blockquote>\n","protected":false},"excerpt":{"rendered":"
      Virtual, Override and new Keyword : Polymorphism , Method Hiding and Overriding in C#<\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":false,"template":"","format":"standard","meta":{"advanced_seo_description":"","jetpack_seo_html_title":"","jetpack_seo_noindex":false,"jetpack_post_was_ever_published":false,"_jetpack_newsletter_access":"","_jetpack_dont_email_post_to_subs":false,"_jetpack_newsletter_tier_id":0,"_jetpack_memberships_contains_paywalled_content":false,"jz_research_post":"","_jetpack_memberships_contains_paid_content":false,"footnotes":""},"categories":[22],"tags":[51],"class_list":["post-1933","post","type-post","status-publish","format-standard","hentry","category-csharp","tag-c"],"jetpack_featured_media_url":"","jetpack_sharing_enabled":true,"jetpack-related-posts":[{"id":1356,"url":"https:\/\/www.jitendrazaa.com\/blog\/microsoft\/csharp\/final-keyword-in-c-sealed-with-const-and-readonly\/","url_meta":{"origin":1933,"position":0},"title":"Final Keyword in C# – sealed with const and readonly","author":"Jitendra","date":"April 7, 2011","format":false,"excerpt":"Example of Final Keyword in C# - sealed with const and readonly","rel":"","context":"In "c#"","block_context":{"text":"c#","link":"https:\/\/www.jitendrazaa.com\/blog\/category\/microsoft\/csharp\/"},"img":{"alt_text":"","src":"","width":0,"height":0},"classes":[]},{"id":702,"url":"https:\/\/www.jitendrazaa.com\/blog\/java\/difference-between-interfaceinheritance-abstract-class\/","url_meta":{"origin":1933,"position":1},"title":"Interface, Inheritance and abstract class","author":"Jitendra","date":"August 6, 2010","format":false,"excerpt":"What is abstract class, interface and inheritance in java. 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