5

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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2
  • 2
    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
  • 1
    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
3

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.
0

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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