C# and method hiding

Question

Per MSDN, the \"new\" keyword when used for method hiding only suppresses a warning.

http://msdn.microsoft.com/en-us/library/435f1dw2.aspx

Do I really need

Answer 1

As far as I know, the only difference is the one you suggest: new hides the warning. I tried out the following code:

class Base
{
    public void Test(){}
}

class ChildA : Base
{
    public void Test(){}
}


class ChildB : Base
{
    public new void Test(){}
}

The test method for both classes look identical in IL:

.method public hidebysig instance void  Test() cil managed
{
  // Code size       1 (0x1)
  .maxstack  8
  IL_0000:  ret
} // end of method ChildA::Test

.method public hidebysig instance void  Test() cil managed
{
  // Code size       1 (0x1)
  .maxstack  8
  IL_0000:  ret
} // end of method ChildB::Test

The compiler issued a warning for Test in ClassA, but not in ClassB. But as other answers has stated; don't confuse the concepts of method hiding and method overriding; not the same thing.

Answer 2

You get method hiding whether or not you specify "new", but it's not the same as overriding. Here's an example where they're different:

using System;

class Base
{
    public virtual void OverrideMe()
    {
        Console.WriteLine("Base.OverrideMe");
    }

    public virtual void HideMe()
    {
        Console.WriteLine("Base.HideMe");
    }
}

class Derived : Base
{
    public override void OverrideMe()
    {
        Console.WriteLine("Derived.OverrideMe");
    }

    public new void HideMe()
    {
        Console.WriteLine("Derived.HideMe");
    }
}

class Test
{
    static void Main()
    {
        Base x = new Derived();
        x.OverrideMe();
        x.HideMe();
    }
}

The output is:

Derived.OverrideMe
Base.HideMe

Even though the base HideMe method is virtual, it isn't overridden in Derived, it's just hidden - so the method is still bound to the virtual method in Base, and that's what gets executed.

Member hiding is generally a bad idea, making the code harder to understand. The fact that it's available is beneficial in terms of versioning, however - it means adding a method to a base class doesn't potentially let derived classes override it unintentionally, and they can keep working as before. That's why you get the warning.

Answer 3

Use of new keyword while hiding a method is to "make the intent of the programmer clear". This avoids accidental hidings by the programmers.

If the new keyword is not present then the compiler issues a warning and treats it as if it was present.

You can read more about it Versioning with the Override and New Keywords (C# Programming Guide)