c# - Why does this work? Method overloading + method overriding + polymorphism

Question

In the following code:

public abstract class MyClass
{
public abstract bool MyMethod(
        Database database,
        AssetDetails asset,
        ref string errorMessage);
}

public sealed class MySubClass : MyClass
{
    public override bool MyMethod(
        Database database,
        AssetDetails asset,
        ref string errorMessage)
    {
        return MyMethod(database, asset, ref errorMessage);
    }

    public bool MyMethod(
        Database database,
        AssetBase asset,
        ref string errorMessage)
    {
    // work is done here
}
}

where AssetDetails is a subclass of AssetBase.

Why does the first MyMethod call the second at runtime when passed an AssetDetails, rather than getting stuck in an infinite loop of recursion?

Answer 1

C# will resolve your call to your other implementation because calls to a method on an object, where the class for that object has its own implementation will be favored over an overridden or inherited one.

This can lead to subtle and hard-to-find problems, like you've shown here.

For instance, try this code (first read it, then compile and execute it), see if it does what you expect it to do.

using System;

namespace ConsoleApplication9
{
    public class Base
    {
        public virtual void Test(String s)
        {
            Console.Out.WriteLine("Base.Test(String=" + s + ")");
        }
    }

    public class Descendant : Base
    {
        public override void Test(String s)
        {
            Console.Out.WriteLine("Descendant.Test(String=" + s + ")");
        }

        public void Test(Object s)
        {
            Console.Out.WriteLine("Descendant.Test(Object=" + s + ")");
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Descendant d = new Descendant();
            d.Test("Test");
            Console.In.ReadLine();
        }
    }
}

Note that if you declare the type of the variable to be of type Base instead of Descendant, the call will go to the other method, try changing this line:

Descendant d = new Descendant();

to this, and re-run:

Base d = new Descendant();

So, how would you actually manage to call Descendant.Test(String) then?

My first attempt looks like this:

public void Test(Object s)
{
    Console.Out.WriteLine("Descendant.Test(Object=" + s + ")");
    Test((String)s);
}

This did me no good, and instead just called Test(Object) again and again for an eventual stack overflow.

But, the following works. Since, when we declare the d variable to be of the Base type, we end up calling the right virtual method, we can resort to that trickery as well:

public void Test(Object s)
{
    Console.Out.WriteLine("Descendant.Test(Object=" + s + ")");
    Base b = this;
    b.Test((String)s);
}

This will print out:

Descendant.Test(Object=Test)
Descendant.Test(String=Test)

you can also do that from the outside:

Descendant d = new Descendant();
d.Test("Test");
Base b = d;
b.Test("Test");
Console.In.ReadLine();

will print out the same.

But first you need to be aware of the problem, which is another thing completely.