5.11 Sealed Classes & Methods

Let’s go through sealed classes and methods in C# — a key feature for controlling inheritance and overriding.

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📌 1. What Is a sealed Class?

• A sealed class cannot be inherited.

• Useful when you want to prevent other classes from deriving from it.

• Often used for security, immutability, or performance reasons.

Example: Sealed Class

sealed class Logger
{
    public void Log(string message)
    {
        Console.WriteLine(message);
    }
}
 
// Trying to inherit from Logger will cause a compile-time error
// class FileLogger : Logger {} // ❌ Error

• The compiler will prevent any class from inheriting a sealed class.

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📌 2. What Is a sealed Method?

• A sealed method prevents further overriding in derived classes.

• Only makes sense when overriding a virtual method in a class hierarchy.

Syntax Example

class Base
{
    public virtual void Display()
    {
        Console.WriteLine("Base display");
    }
}
 
class Derived : Base
{
    public sealed override void Display()
    {
        Console.WriteLine("Derived display");
    }
}
 
class MoreDerived : Derived
{
    // public override void Display() {} // ❌ Error: cannot override sealed method
}

• sealed override stops further overriding beyond the current class.

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📌 3. Key Points

Feature	Notes
sealed class	Cannot be inherited
sealed method	Cannot be overridden in derived classes
Use case	Security, immutability, controlling inheritance
Relationship	A method can only be sealed if it overrides a virtual method

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📌 4. Analogy

• Sealed class → like a locked box: no one can make a copy of it.

• Sealed method → like a final lock on a drawer: derived classes can’t change what’s inside, even if they inherit the box.

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✅ Summary:

• Use sealed classes to prevent inheritance.

• Use sealed methods to stop further overriding in a hierarchy.

• Helps enforce design constraints and maintain control over class behavior.

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If you want, I can make a diagram showing class hierarchy with sealed classes and sealed methods — it makes the concept visually clear.

Do you want me to make that diagram?