An example of the keywords virtual and sealed

example1.cs

 public class ClassA  
    {  
        public virtual void Method1() { Console.WriteLine("ClassA"); }  
    }  
 
    public class ClassB : ClassA  
    {  
        public override sealed void Method1() { Console.WriteLine("ClassB"); }  
        public void Method2() { Console.WriteLine("ClassB"); }  
    }  
 
    public class ClassC : ClassB  
    {  
        public override void Method1() { Console.WriteLine("ClassC"); }  
        public override void Method2() { Console.WriteLine("ClassC"); }  
    } 

example2.cs

 public class ClassA  
    {  
        public virtual void Method1() { Console.WriteLine("ClassA"); }  
    }  
 
    public class ClassB : ClassA  
    {  
        public override sealed void Method1() { Console.WriteLine("ClassB"); }  
        public void Method2() { Console.WriteLine("ClassB"); }  
    }  
 
    public class ClassC : ClassB  
    {  
        new public void Method1() { Console.WriteLine("ClassC"); }  
        new public void Method2() { Console.WriteLine("ClassC"); }  
    } 

example3.cs

        public void Run()  
        {  
            ClassC classC = new ClassC();  
            ClassB classB = classC;  
            ClassA classA = classC;  
 
            classC.Method1();  
            classB.Method1();  
            classA.Method1();  
 
            classC.Method2();  
            classB.Method2();  
              
        } 

example4.cs

 public class ClassA  
    {  
        public virtual void Method1() { Console.WriteLine("ClassA"); }  
    }  
 
    public class ClassB : ClassA  
    {  
        new public void Method1() { Console.WriteLine("ClassB"); }  
        public void Method2() { Console.WriteLine("ClassB"); }  
    }  
 
    public class ClassC : ClassB  
    {  
        new public void Method1() { Console.WriteLine("ClassC"); }  
        new public void Method2() { Console.WriteLine("ClassC"); }  
    } 

gistfile1.txt

ClassC
ClassB
ClassB
ClassC
ClassB

output1.txt

Now, when you try to compile this, you'll get not one, but two compiler errors:

'MyNamespace.ClassC.Method1()': cannot override inherited member 'GetPropertiesTest.ClassB.Method1()' because it is sealed
'MyNamespace.ClassC.Method2()': cannot override inherited member 'GetPropertiesTest.ClassB.Method2()' because it is not marked virtual, abstract, or override

Either way, you can't override the methods.  However, you CAN implement a new Method1 and Method2:

output2.txt

This compiles just fine.  When using the "new" keyword, you are essentially saying "These methods have nothing to do with the methods that are inherited.  I just really want to implement methods that use the same name." 

So here's how they would execute.  Let's use this method

output4.txt

We'll get the following output:

ClassC
ClassB
ClassA
ClassC
ClassB

Hopefully this will clear some stuff up.
David Morton - http://blog.davemorton.net/

Taken from https://social.msdn.microsoft.com/Forums/vstudio/en-US/58888c86-47bc-4f70-a01f-c5441a7f77da/nonoverridable-method?forum=csharpgeneral