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I am trying to understand how inheritance works in C#. I wrote the following code:

class Program { static void Main(string[] args) { Animal animal = new Dog(); animal.OverRideMe(); //animal.NewMethod(); Dog dog = (Dog)animal; dog.OverRideMe(); dog.NewMethod(); Console.Read(); } } public abstract class Animal { public Animal() { Console.WriteLine("Base Constructor"); } public virtual void OverRideMe() { Console.WriteLine("In Base Class's OverRideMe"); Console.Read(); } } public class Dog : Animal { public Dog() { Console.WriteLine("Derived Constructor"); } public override void OverRideMe() { Console.WriteLine("In Derived Class's OverRideMe"); Console.Read(); } public void NewMethod() { Console.WriteLine("In Derived Class's NewMethod"); Console.Read(); } }

The CIL(Common Intermediate Language) code for the Main() looks like the following:

.method private hidebysig static void Main ( string[] args ) cil managed { // Method begins at RVA 0x2050 // Code size 42 (0x2a) .maxstack 1 .entrypoint .locals init ( [0] class ConsoleApplication1.Animal animal, [1] class ConsoleApplication1.Dog dog ) IL_0000: nop IL_0001: newobj instance void ConsoleApplication1.Dog::.ctor() IL_0006: stloc.0 IL_0007: ldloc.0 IL_0008: callvirt instance void ConsoleApplication1.Animal::OverRideMe() IL_000d: nop IL_000e: ldloc.0 IL_000f: castclass ConsoleApplication1.Dog IL_0014: stloc.1 IL_0015: ldloc.1 IL_0016: callvirt instance void ConsoleApplication1.Animal::OverRideMe() IL_001b: nop IL_001c: ldloc.1 IL_001d: callvirt instance void ConsoleApplication1.Dog::NewMethod() IL_0022: nop IL_0023: call int32 [mscorlib]System.Console::Read() IL_0028: pop IL_0029: ret } // end of method Program::Main

The lines in CIL that are troubling me are:

IL_000f: castclass ConsoleApplication1.Dog IL_0014: stloc.1 IL_0015: ldloc.1 IL_0016: callvirt instance void ConsoleApplication1.Animal::OverRideMe() IL_001b: nop IL_001c: ldloc.1 IL_001d: callvirt instance void ConsoleApplication1.Dog::NewMethod()

After the castclass of animal to Dog type the code executes dog.OverRideMe();. This is translated to CIL as

IL_0016: callvirt instance void ConsoleApplication1.Animal::OverRideMe()

I had cast the animal object to Dog type. Why should dog.OverRideMe(); be translated to the above statement in CIL? The output of the above code is:

enter image description here

This output has nothing to do with Base class Animal but CIL still places a call to it.

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    I do not understand the need for downvoting this question. At the least leave a comment so that I may improve it. Commented Aug 12, 2013 at 14:56
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    Your question is fine, and frankly a good one. Ignore the haters, they are out of control on the site these days IMO. Commented Aug 12, 2013 at 15:00

2 Answers 2

Reset to default
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You're invoking a virtual method. Virtual method invocation is determined by the runtime type of an object. You can call it a Dog all you want, but the compiler is still going to emit instructions to determine the appropriate method to invoke at runtime. Starting with the compile-time type of dog, it walks up the inheritance chain until it finds the "top-level" definition1 of OverRideMe and it emits a virtual method invocation for that. In this case, the highest place in the inheritance chain that OverRideMe is defined is in Animal; thus, it emits a virtual method invocation for Animal.OverRideMe.

Here's a previous answer that might help you understand what is going on a little better.

1: The highest place in the inheritance chain where the method is defined. Some care has to be taken here to understand how method hiding and what not impact this.

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2 Comments

You maybe need to take a look at the new keyword in method signature. This will call the method on the actual type not as override.
@Sebi: There is no keyword new used to hide any methods in the OP's code.
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It says "callvirt" - the virtual table is associated with "Animal" class so that's where the call should be placed. After resolving the virtual table, during runtime, the intended method will be called.

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