can we override method if its not defined as a virtual in base class???

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No, you cannot override a non-virtual . The closest thing you can do is hide the method by creating a new method with the same name but this is not advisable as it breaks good design principles.

But even hiding a method won't give you execution time polymorphic dispatch of method calls like a true virtual method call would. Consider this example:

using System;

class Example

{

static void Main()

{

Foo f = new Foo();

f.M();

Foo b = new Bar();

b.M();

}

}

class Foo

{

public void M()

{

Console.WriteLine("Foo.M");

}

}

class Bar : Foo

{

public new void M()

{

Console.WriteLine("Bar.M");

}

}

In this example both calls to the M method print Foo.M. As you can see this approach does allow you to have a new implementation for a method as long as the reference to that object is of the correct derived type but hiding a base method does break polymorphism.

I would recommend that you do not hide base methods in this manner.

I tend to side with those who favor C#'s default behavior that methods are non-virtual by default (as opposed to Java). I would go even further and say that classes should also be sealed by default. Inheritance is hard to design for properly and the fact that there is a method that is not marked to be virtual indicates that the author of that method never intended for the method to be overridden.

Edit: "execution time polymorphic dispatch":

What I mean by this is the default behavior that happens at execution time when you call virtual methods. Let's say for example that in my previous code example, rather than defining a non-virtual method, I did in fact define a virtual method and a true overridden method as well.

If I were to call b.Foo in that case, the CLR would correctly determine the type of object that the b reference points to as Bar and would dispatch the call to M appropriately.