Sealed Classes in C#

What are Sealed Classes in C#?

A sealed class in C# is a class that cannot be inherited. It is used to restrict further extension of the class to maintain security and prevent unintended modifications.

Key Features of Sealed Classes

• Prevents inheritance by derived classes.
• Improves performance by allowing compiler optimizations.
• Ensures security by preventing modification of behavior.
• Cannot act as a base class for other classes.

Example of a Sealed Class

The following example demonstrates how a sealed class is declared and used.

Example: Declaring and Using a Sealed Class

sealed class Calculator
{
    public int Add(int a, int b)
    {
        return a + b;
    }
}

// This will cause an error
// class AdvancedCalculator : Calculator { } // Error: Cannot inherit from sealed class

// Usage
Calculator calc = new Calculator();
Console.WriteLine(calc.Add(5, 10)); // Output: 15
        

In this example, the Calculator class is marked as sealed, preventing it from being inherited.

Sealed Methods

In C#, you can use the sealed keyword on methods inside a base class to prevent them from being overridden in derived classes.

Example: Sealing a Method

public class BaseClass
{
    public virtual void Show()
    {
        Console.WriteLine("Base class method.");
    }
}

public class DerivedClass : BaseClass
{
    public sealed override void Show()
    {
        Console.WriteLine("Sealed method in derived class.");
    }
}

public class SubDerivedClass : DerivedClass
{
    // Cannot override Show() because it is sealed in DerivedClass
    // public override void Show() { } // Error
}

// Usage
DerivedClass obj = new DerivedClass();
obj.Show(); // Output: Sealed method in derived class.
        

Here, the method Show() is sealed in DerivedClass, preventing further overrides in SubDerivedClass.

When to Use Sealed Classes?

• When you want to prevent inheritance to maintain class integrity.
• To improve performance by allowing compiler optimizations.
• When a class is designed to be self-contained without needing extension.

Sealed Classes vs. Abstract Classes

Both sealed and abstract classes control inheritance, but they serve different purposes.