Virtual vs. New in C#: When to Override or Hide Base Class Methods?

Usage of virtual and new keywords in C#

In object-oriented programming, methods are usually defined in base classes and overridden or redefined in derived classes. Although both the "virtual" and "new" keywords can be used to modify method declarations, they are implemented in different ways.

virtual rewrite

• Declare the method as "virtual" in the base class, indicating that subclasses can override it.
• Allows derived classes to provide different method implementations without breaking the inheritance chain.

new keyword

• Declare a new method in the derived class with the same name as the method in the base class.
• Hide the base class method and create a new implementation in the derived class.
• Breaks the inheritance chain, which means derived class methods have nothing to do with base class methods.

Example

Consider the following code:

<code class="language-csharp">public class Base
{
    public virtual bool DoSomething() { return false; }
}

public class Derived : Base
{
    public override bool DoSomething() { return true; }
}</code>

If we create an instance of Derived and store it in a variable of type Base, the call to DoSomething() will call the overridden method in Derived:

<code class="language-csharp">Base a = new Derived();
a.DoSomething(); // 返回 true</code>

In contrast, if we use the new keyword in Derived, the call to DoSomething() will invoke the new method in Derived, even if the variable is of type Base:

<code class="language-csharp">public class Derived : Base
{
    public new bool DoSomething() { return true; }
}</code>

<code class="language-csharp">Base a = new Derived();
a.DoSomething(); // 返回 true (Derived 中的新方法)</code>

When to use virtual override vs. new

• Use virtual overrides to inherit and extend behavior while maintaining the inheritance chain.
• Use new to create a new, independent implementation, thus breaking the inheritance chain.

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