C# - new keyword in method signature

Question

While performing a refactoring, I ended up creating a method like the example below. The datatype has been changed for simplicity's sake.

I previous had an assignment statement like this:

MyObject myVar = new MyObject();

It was refactored to this by accident:

private static new MyObject CreateSomething()
{
  return new MyObject{"Something New"};
}

This was a result of a cut/paste error on my part, but the new keyword in private static new is valid and compiles.

Question: What does the new keyword signify in a method signature? I assume it's something introduced in C# 3.0?

How does this differ from override?

Answer 1

No, it's actually not "new" (pardon the pun). It's basically used for "hiding" a method. IE:

public class Base
{
   public virtual void Method(){}
}

public class Derived : Base
{
   public new void Method(){}
}

If you then do this:

Base b = new Derived();
b.Method();

The method in the Base is the one that will be called, NOT the one in the derived.

Some more info: http://www.akadia.com/services/dotnet_polymorphism.html

Re your edit: In the example that I gave, if you were to "override" instead of using "new" then when you call b.Method(); the Derived class's Method would be called because of Polymorphism.

Answer 2

From the docs:

If the method in the derived class is preceded with the new keyword, the method is defined as being independent of the method in the base class.

What this means in practice:

If you inherit from another class and you have a method that shares the same signature you can define it as 'new' so that it independent from the parent class. This means that if you have a reference to the 'parent' class then that implementation will be executed, if you have a reference to the child class then that implementation will be executed.

Personally I try to avoid the 'new' keyword as it normally means I've got my class hierarchy wrong, but there are times when it can be useful. One place is for versioning and backwards compatibility.

There's lot of information in the MSDN for this (http://msdn.microsoft.com/en-us/library/6fawty39(VS.80).aspx)

Answer 3

http://msdn.microsoft.com/en-us/library/51y09td4(VS.71).aspx

From MSDN:

Use the new modifier to explicitly hide a member inherited from a base class. To hide an inherited member, declare it in the derived class using the same name, and modify it with the new modifier.

Answer 4

It means the method replaces a method by the same name inherited by the base class. In your case, you probably don't have a method by that name in the base class, meaning the new keyword is totally superfluous.

Answer 5

New keyword reference from MSDN:

MSDN Reference

Here is an example I found on the net from a Microsoft MVP that made good sense: Link to Original

public class A
{
   public virtual void One();
   public void Two();
}

public class B : A
{
   public override void One();
   public new void Two();
}

B b = new B();
A a = b as A;

a.One(); // Calls implementation in B
a.Two(); // Calls implementation in A
b.One(); // Calls implementation in B
b.Two(); // Calls implementation in B

Override can only be used in very specific cases. From MSDN:

You cannot override a non-virtual or static method. The overridden base method must be virtual, abstract, or override.

So the 'new' keyword is needed to allow you to 'override' non-virtual and static methods.

Answer 6

As others explained, it is used to hide an existing method. It is useful for overriding a method that isn't virtual in the parent class.

Keep in mind that creating a "new" member is not polymorphic. If you cast the object to the base type, it will not use the derived type's member.

If you have a base class:

public class BaseClass
{
    public void DoSomething() { }
}

And then the derived class:

public class DerivedType
{
    public new DoSomething() {}

}

If you declare a type of DerivedType and then cast it, the method DoSomething() isn't polymorphic, it will call the base class' method, not the derived one.

BaseClass t = new DerivedType();
t.DoSomething();// Calls the "DoSomething()" method of the base class.