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sealed Keyword

• The sealed modifier can be applied to classes, instance methods and properties.
• A sealed class cannot be inherited.
• A sealed method overrides a method in a base class, but itself cannot be overridden further in any derived class.
• When applied to a method or property, the sealed modifier must always be used with override.

Use the sealed modifier in a class declaration to prevent inheritance of the class, as in this example:

sealed class SealedClass 
{
    public int x; 
    public int y;
}

• It is an error to use a sealed class as a base class or to use the abstract modifier with a sealed class.
• Structs are implicitly sealed; therefore, they cannot be inherited.

Example

// cs_sealed_keyword.cs
using System;
sealed class SealedClass
{
    public int x;
    public int y;
}

class MainClass
{
    static void Main()
    {
        SealedClass sc = new SealedClass();
        sc.x = 110;
        sc.y = 150;
        Console.WriteLine("x = {0}, y = {1}", sc.x, sc.y);
    }
}

Output :

x = 110, y = 150

In the preceding example, if you attempt to inherit from the sealed class by using a statement like this:

class MyDerivedC: SealedClass {} // Error

you will get the error message:

'MyDerivedC' cannot inherit from sealed class 'SealedClass'.

Reference

sealed (C# Reference)