What effect does the "new" keyword have in C# and why is it only a warning when not employed?

Question

Consider the following code:

public abstract class Test1
{
    public object Data { get; set; }
}

public abstract class Test2<T> : Test1
{
    public T Data { get; set; }
}

This will generate the following warning:

'Test2.Data' hides inherited member 'Test1.Data'. Use the new keyword if hiding was intended.

Why is this only a warning and what effect will adding the "new" keyword have?

According to my testing I cannot find any difference once the "new" keyword is added.

Don't get me wrong, I'm all for being explicit, but I was curious as to the benefit of adding "new".

My only thoughts on what it might be are:

• Improved human readability
• Some saving at run-time when the compiler isn't left to figure the inevitable out the long way around

Answer 1

The only effect the new keyword has is to remove the warning. The purpose of getting the warning when not using the new keyword is to prevent you from accidentally shadowing the method when you really meant to override it.

Answer 2

new is used to hide a method or property of the base class. This is not like overriding it : the member doesn't need to have the same signature as the hidden base class member, and is not involved in polymorphism.

An example :

class A
{
    public virtual void Test()
    {
        Console.WriteLine("A.Test");
    }
}

class B : A
{
    public new void Test()
    {
        Console.WriteLine("B.Test");
    }
}

class B : A
{
    public override void Test()
    {
        Console.WriteLine("C.Test");
    }
}

public static void Main(string[] args)
{
    A aa = new A();
    aa.Test(); // Prints "A.Test"

    A ab = new B();
    ab.Test(); // Prints "A.Test" because B.Test doesn't overrides A.Test, it hides it

    A ac = new C();
    ac.Test(); // Prints "C.Test" because C.Test overrides A.Test

    B b = new B();
    b.Test(); // Prints "B.Test", because the actual type of b is known at compile to be B
}