Public class TypeB : TypeA // in AssemblyB. But implementation provider is independent to choose the algorithm. Now, suppose I decided at a later date that I wish to override Foo() and Boo() in TypeB: What is JVM It is: A specification where working of Java Virtual Machine is specified. In my application which uses both libraries, I have these lines of code:Īnd no prizes for guessing the output of this code: Public class TypeB : TypeA // in AssemblyB.dll Imagine you have two classes TypeA and TypeB, located in AssemblyA and AssemblyB respectively: Here’s a scenario where you will notice the difference: When you override a virtual method the method is resolved at runtime – the CLR determines which method to invoke by looking up a ‘call table’ and find the nearest override of the method.When you hide a base method the method is resolved at compile time – which method is invoked at runtime is predetermined based on what they look like at compile time and baked into the compiled dll.Semantically there’s a very subtle difference between the two approaches because the timing in which the method to invoke is determined differs between the two: It allows you to make a clear statement both in the base and derived class that a method is intended and sometimes expected to be overridden. Overriding a virtual method on the other hand, is very much part and parcel of object oriented design, and an integral part of the Template pattern (one of my favourites :-P) to allow variations in the behaviour of the concrete classes. A pure virtual function does not carry any definition related to its base class. This way, when someone looks at your code it at least offers some indication that Foo() is an override and more importantly, it makes a statement of your intention to hide the Foo() method in the base class (as opposed to it looking like a mistake). To lessen this confusion, you should use the new keyword whenever you absolutely have to hide a method in the base class: It unnecessarily complicates the relationship between the base and derived class as it’s not immediately clear which method is invoked when you call derived.Foo() if Foo() is declared in both the base and derived class. Under normal circumstances, hiding a base method is a code smell. All you need to do is to declare the same method signature in the derived class. Without marking a method in the base class with the virtual keyword it is still possible (albeit not recommended) to provide an override by ‘hiding’ the method defined in the base class. Unlike Java where every method can be overridden by default unless marked with the final keyword, C# employs a safer, opt-in system where methods cannot be overridden unless marked with the virtual keyword.Ī virtual method defined in the base class can be overridden in a derived class by defining a method with the same signature and marking it with the override keyword. A Java Virtual Machine stack stores frames. In C#, there are two ways you can provide an alternative implementation of an inherited method: Each Java Virtual Machine thread has a private Java Virtual Machine stack, created at the same time as the thread.
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