Sealed Classes And Methods In C#

The sealed modifier is used to prevent derivation from a class. An error occurs if a sealed class is specified as the base class of another class. A sealed class cannot also be an abstract class.
The sealed modifier is primarily used to prevent unintended derivation, but it also enables certain run-time optimizations. In particular, because a sealed class is known to never have any derived classes, it is possible to transform virtual function member invocations on sealed class instances into non-virtual invocations.
In C# structs are implicitly sealed; therefore, they cannot be inherited.
using System;
sealed class MyClass
{
public int x;
public int y;
}

class MainClass
{
public static void Main()
{
MyClass mC = new MyClass();
mC.x = 110;
mC.y = 150;
Console.WriteLine("x = {0}, y = {1}", mC.x, mC.y);
}
}
In the preceding example, if you attempt to inherit from the sealed class by using a statement like this:
class MyDerivedC: MyClass {} // Error
You will get the error message:
'MyDerivedC' cannot inherit from sealed class 'MyBaseC'.
In C# a method can't be declared as sealed. However when we override a method in a derived class, we can declare the overrided method as sealed as shown below. By declaring it as sealed, we can avoid further overriding of this method.
using System;
class MyClass1
{
public int x;
public int y;

public virtual void Method()
{
Console.WriteLine("virtual method");
}
}

class MyClass : MyClass1
{
public override sealed void Method()
{
Console.WriteLine("sealed method");
}
}

class MainClass
{
public static void Main()
{
MyClass1 mC = new MyClass();
mC.x = 110;
mC.y = 150;
Console.WriteLine("x = {0}, y = {1}", mC.x, mC.y);
mC.Method();
}
}