How do you prevent a class from overriding in .NET?

You can prevent a class from being overridden. This is possible by using a keyword. In C# you will be using the keyword sealed to prevent overriding and in VB.NET you will be using the keyword NotInheritable to achieve the same. Here is an example to demonstrate both of them:

Prevention of Class Overriding in C# Using sealed Keyword:
Usage of sealed keyword is demonstrated in the example shown below:
public sealed class sealedClass {
public void displayMsg() {
Console.WriteLine(“In Sealed Class…”);
}
}
class derivedClass : sealedClass {
public displayMsg() {
Console.WriteLine(“In Derived Class…”);
}

}
class testClass {
public static void Main() {
derivedClass obj = new derivedClass();
}
}

In this example, you have defined the class sealedClass as sealed. You try to inherit sealedClass using the derivedClass and define/override the method displayMsg of sealedClass. You have also defined the class testClass. Then you have created an instance of derivedClass inside Main method of testClass. Now try compiling this code, you will end up in the following error:
“derivedClass cannot inherit from sealed class sealedClass.”

In this example, you have used sealed keyword on a class and ensured that the class cannot be overridden or inherited. C# also provides a provision to prevent overriding of a specific method in the class by specifying the sealed keyword along with the method signature instead of specifying sealed keyword along with the class name.

Prevention of Class Overriding in VB.NET Using NotInheritable Keyword:
Consider the following example in which you have a sealed class called sampleClass and you are going to access this class and its method by creating an instance of it:

Module sampleModule
sub Main()
Dim sampleObj As New SampleClass()
sampleObj.displayMsg()
End Sub
End Module

Public NotInheritable Class sampleClass
Public Sub displayMsg()
Console.WriteLine(“Inside displayMsg of sampleClass”)
End Sub
End Class

This example will work fine and it will print the following output in the console:
Inside displayMsg of sampleClass

Problem arises when you try to inherit this class sampleClass which is marked as NotInheritable. Consider that the following class is added into your program:
Public class derivedClass
Inherits sampleClass
Public Overrides Sub displayMsg()
Console.WriteLine(“Inside displayMsg of derivedClass”)
End Sub
End Class

Now your program will not compile and you will end up in the following error:
derivedClass cannot inherit from class sampleClass because sampleClass is declared NotInheritable