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new Modifier (C# Reference)

When used as a declaration modifier, the new keyword explicitly hides a member that is inherited from a base class. When you hide an inherited member, the derived version of the member replaces the base class version. Although you can hide members without using the new modifier, you get a compiler warning. If you use new to explicitly hide a member, it suppresses this warning.

To hide an inherited member, declare it in the derived class by using the same member name, and modify it with the new keyword. For example:

public class BaseC
{
    public int x;
    public void Invoke() { }
}
public class DerivedC : BaseC
{
    new public void Invoke() { }
}

In this example, BaseC.Invoke is hidden by DerivedC.Invoke. The field x is not affected because it is not hidden by a similar name.

Name hiding through inheritance takes one of the following forms:

  • Generally, a constant, field, property, or type that is introduced in a class or struct hides all base class members that share its name. There are special cases. For example, if you declare a new field with name N to have a type that is not invocable, and a base type declares N to be a method, the new field does not hide the base declaration in invocation syntax. See the C# language specification for details (see section "Member Lookup" in section "Expressions").

  • A method introduced in a class or struct hides properties, fields, and types that share that name in the base class. It also hides all base class methods that have the same signature.

  • An indexer introduced in a class or struct hides all base class indexers that have the same signature.

It is an error to use both new and override on the same member, because the two modifiers have mutually exclusive meanings. The new modifier creates a new member with the same name and causes the original member to become hidden. The override modifier extends the implementation for an inherited member.

Using the new modifier in a declaration that does not hide an inherited member generates a warning.

In this example, a base class, BaseC, and a derived class, DerivedC, use the same field name x, which hides the value of the inherited field. The example demonstrates the use of the new modifier. It also demonstrates how to access the hidden members of the base class by using their fully qualified names.

public class BaseC
{
    public static int x = 55;
    public static int y = 22;
}

public class DerivedC : BaseC
{
    // Hide field 'x'. 
    new public static int x = 100;

    static void Main()
    {
        // Display the new value of x:
        Console.WriteLine(x);

        // Display the hidden value of x:
        Console.WriteLine(BaseC.x);

        // Display the unhidden member y:
        Console.WriteLine(y);
    }
}
/*
Output:
100
55
22
*/

In this example, a nested class hides a class that has the same name in the base class. The example demonstrates how to use the new modifier to eliminate the warning message and how to access the hidden class members by using their fully qualified names.

public class BaseC 
{
    public class NestedC 
    {
        public int x = 200;
        public int y;
    }
}

public class DerivedC : BaseC 
{
    // Nested type hiding the base type members. 
    new public class NestedC   
    {
        public int x = 100;
        public int y; 
        public int z;
    }

    static void Main() 
    {
        // Creating an object from the overlapping class:
        NestedC c1  = new NestedC();

        // Creating an object from the hidden class:
        BaseC.NestedC c2 = new BaseC.NestedC();

        Console.WriteLine(c1.x);
        Console.WriteLine(c2.x);   
    }
}
/*
Output:
100
200
*/

If you remove the new modifier, the program will still compile and run, but you will get the following warning:

The keyword new is required on 'MyDerivedC.x' because it hides inherited member 'MyBaseC.x'.

For more information, see the C# Language Specification. The language specification is the definitive source for C# syntax and usage.

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