What is a sealed class in C#?

A sealed class in C# is a class that cannot be inherited by other classes. When you declare a class as sealed, it prevents any other class from deriving from it. The sealed keyword can also be applied to methods to prevent them from being overridden in further derived classes.

Sealed classes are useful when you want to restrict inheritance for security, performance, or design reasons. Common examples include the string class and many value types in .NET Framework.

Syntax

Following is the syntax for declaring a sealed class −

public sealed class ClassName {
    // class members
}

Following is the syntax for a sealed method −

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

public class DerivedClass : BaseClass {
    public sealed override void Method() {
        // implementation
    }
}

Key Rules of Sealed Classes

• A sealed class cannot be inherited by any other class.

• A sealed method must be an override method in a derived class.

• You can create instances of sealed classes directly.

• Sealed classes can inherit from other classes but cannot be inherited themselves.

Using Sealed Classes

Example

using System;

public sealed class Result {
    public string Display() {
        return "Passed";
    }
    
    public void ShowStatus(string studentName) {
        Console.WriteLine(studentName + " has " + Display());
    }
}

class Program {
    static void Main(string[] args) {
        Result ob = new Result();
        string str = ob.Display();
        Console.WriteLine(str);
        
        ob.ShowStatus("John");
    }
}

The output of the above code is −

Passed
John has Passed

Using Sealed Methods

Example

using System;

public class Vehicle {
    public virtual void Start() {
        Console.WriteLine("Vehicle starting...");
    }
}

public class Car : Vehicle {
    public sealed override void Start() {
        Console.WriteLine("Car engine started");
    }
}

public class SportsCar : Car {
    // Cannot override Start() method here - it's sealed
    public void Accelerate() {
        Console.WriteLine("Sports car accelerating fast!");
    }
}

class Program {
    static void Main(string[] args) {
        SportsCar myCar = new SportsCar();
        myCar.Start();
        myCar.Accelerate();
    }
}

The output of the above code is −

Car engine started
Sports car accelerating fast!

Sealed vs Abstract Classes

Conclusion

Sealed classes in C# prevent inheritance, making them useful for creating final implementations that cannot be extended. Use sealed classes when you want to restrict further derivation for security, performance, or design integrity reasons.