OOP Advanced: sealed, static, partial

1. sealed: Stop Inheritance Chain

Used on String class. No one can inherit from String.

public sealed class SecurityToken { } // No child allowed
// public class MyToken : SecurityToken {} // Compiler Error

sealed override = child can't override this method further.

2. static Class: Toolbox, Not Blueprint

public static class MathHelper // No objects
{
    public static int Square(int x) => x * x;
    public static double PI = 3.14;
}
// MathHelper m = new MathHelper(); // Error: Cannot create instance
int result = MathHelper.Square(5); // Use directly

3. Method Hiding new vs Override - The #1 Interview Question

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

public class SportsCar : Car { 
    public new void Start() { Console.WriteLine("VROOM"); } // Hides
}

Car c = new SportsCar();
c.Start(); // Output: Car Start - Uses variable type, NOT object type
// This is why we use override for true polymorphism

1. partial Classes: Why Microsoft Loves Them

WinForms splits designer code + your code. Both become one class.

// Car.Designer.cs - Auto generated
public partial class Car { 
    private void InitializeComponent() { }
}

// Car.cs - Your code
public partial class Car { 
    public void Drive() { }
}

2. Static Constructor - Runs Once

public class Car
{
    public static int TotalCars;
    
    static Car() // No parameters, no access modifier
    { 
        TotalCars = 0; // Runs before first object or static call
        Console.WriteLine("Static constructor called once");
    }
}

3. Nested Classes: Class Inside Class

Used when inner class makes no sense outside parent.

public class Car
{
    private class Engine // Only Car can use Engine
    {
        public void Start() { Console.WriteLine("Engine started"); }
    }
    
    public void StartCar() { 
        Engine e = new Engine(); // OK here
        e.Start(); 
    }
}
// Car.Engine e = new Car.Engine(); // Error: Engine is private

4. When To Use What - Interview Cheat Sheet