C# sealed Keyword: Declare A Sealed Class Or Sealed Method

The sealed keyword in C# is used to declare a sealed class or sealed method, preventing further inheritance or method overriding. This tutorial will cover the following topics related to the sealed keyword in C#:

Sealed classes
Sealed methods

Let's begin!

Sealed classes

A sealed class is a class that cannot be inherited. To declare a sealed class, use the sealed keyword:

public sealed class MyBaseClass
{
    // Class members go here
}

Attempting to inherit from a sealed class will result in a compile-time error:

public class MyDerivedClass : MyBaseClass
{
    // This will cause a compile-time error: 'MyDerivedClass': cannot derive from sealed type 'MyBaseClass'
}

Sealed classes are useful when you want to prevent further inheritance for a specific class, ensuring that the class hierarchy remains fixed and cannot be extended.

Sealed methods

A sealed method is a method that cannot be overridden in a derived class. To declare a sealed method, use the sealed keyword with an override keyword:

public class MyBaseClass
{
    public virtual void MyMethod()
    {
        Console.WriteLine("Base class method");
    }
}

public class MyDerivedClass : MyBaseClass
{
    public sealed override void MyMethod()
    {
        Console.WriteLine("Derived class method");
    }
}

In this example, MyDerivedClass overrides the MyMethod method from MyBaseClass and marks it as sealed. This means that any further derived classes cannot override the MyMethod method:

public class MyFurtherDerivedClass : MyDerivedClass
{
    public override void MyMethod()  // This will cause a compile-time error: 'MyFurtherDerivedClass.MyMethod()': cannot override inherited member 'MyDerivedClass.MyMethod()' because it is sealed
    {
        Console.WriteLine("Further derived class method");
    }
}

Sealed methods are useful when you want to allow inheritance for a specific class but prevent a particular method from being overridden in derived classes.

That's it! You've now learned how to use the sealed keyword in C# to declare sealed classes and sealed methods, preventing further inheritance or method overriding. The sealed keyword is useful for ensuring that a class hierarchy remains fixed and specific methods cannot be overridden in derived classes, providing more control over the design of your classes and methods.

How to use sealed classes in C#
Sealed classes prevent further inheritance, making them a final version.
```
sealed class SealedClass
{
    // Class members
}
```

Sealing methods with the sealed keyword in C#

The sealed keyword can be applied to methods to prevent overriding.

class BaseClass
{
    public virtual void MethodToOverride() { }
}

class DerivedClass : BaseClass
{
    public sealed override void MethodToOverride() { }
}

C# sealed keyword and extension methods

Sealed classes can still have extension methods, providing additional functionality.

static class SealedClassExtensions
{
    public static void NewMethod(this SealedClass sealedInstance)
    {
        // Extension method logic
    }
}