C# Keywords – Why ‘New’ Keyword Is Not Contextual?

ckeywords

Keywords are predefined, reserved identifiers that have special meanings to the compiler.

A contextual keyword is used to provide a specific meaning in the code, but it is not a reserved word in C#.

For Keyword new we have multiple situation for use it, for example:

//One situation for create an instance of class:
StringBuilder sr=new StringBuilder();

//Another situation for method hiding in polymorphism subject of OOP:
public new void Foo()
{
   //Some Code
}

As you can see new keyword has Several meaning Depending on where it's used, then in my openion new must considered as Contextual Keyword and not keyword!

then, the Definition of Contextual keyword is wrong? or considering new as whole keyword? and why?

Best Answer

The question is not how many meanings it has, but whether or not it's reserved.

When Microsoft adds new features to C#, it sometimes need to add new keywords. This can compromise backward compatibility, since it invalidates old code that declares identifiers with the same name. To prevent this, Microsoft makes these new keywords contextual - they only serve as keywords when placed in specific places, places where you can't normally put identifiers.

So - async is a contextual keyword because older code might have declared a variable named async.

new was there from day one, so it doesn't have this problem - if your code declared a variable named new it wouldn't work even in the first version of C#. Therefore, there is no backwards compatibility issue here and new can be a regular keyword.

Related Solutions

C# – Practical Uses of the ‘New’ Modifier for Hiding

You can use it to imitate return type covariance (Note: After this answer was written, C# 9.0 added support for return type covariance). Eric Lippert's Explanation . Eric provides this example code:

abstract class Enclosure
{
    protected abstract Animal GetContents();
    public Animal Contents() { return this.GetContents(); }
}
class Aquarium : Enclosure
{
    public new Fish Contents() { ... }
    protected override Animal GetContents() { return this.Contents(); }
}

This is a work-around. public override Fish Contents() { ... } is not legal, despite being safe.

In general, you should not use method hiding, as it is confusing to consumers of your class (the specific example above does not suffer from this problem). Just name your new method something else if you don't want to override an existing method.

A likely real-world situation where you would need method hiding is if the provider of a base class added a generic method which you had already added to a derived class. Such a program will compile (and give warnings) without the new keyword, but adding new says, "I know my version of this method is replacing the base class's version. This is horrible and confusing, but we're stuck with it." That's still better than forcing the derived class to rename their method.

Just allowing the derived method to be treated as being an override would cause problems. Ignoring any concerns with implementing the compiler, the new method is semantically different from the base method, but polymorphism would cause the new method to be called when asked to call a method with the same name.

This situation is discussed in detail in this post by Eric Lippert.

C# Interfaces – Using Keyword in C# Interface

Visual Studio doesn't know what code you intend to write so includes the most common namespaces for you by default in the "new class" template. This is done so you don't have to resolve all the references for every single line of new code you write.

Once you have written your basic code you can safely remove the ones you don't need. You will have to add any back that you subsequently need to reference in any subsequent code you write.

If you right click and select Organize Usings > Remove and Sort it will delete any that are unused. There are also extensions that will remove and sort the namespaces automatically on saving each file.

Best Answer

Related Solutions

C# – Practical Uses of the ‘New’ Modifier for Hiding

C# Interfaces – Using Keyword in C# Interface

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