In which scenario,i should declare a method as virtual in c#

In polymorphism we have method overloading and method overriding.

Method overloading is the process of creating new method with same name but different signature.

Method overriding is the process of creating new method with same name and same signature but in a derived class.

In method overriding you have to declare a method with virtual keyword in base class which indicates that it is going to be overridden. In derived class the overriding method should use override keyword.

class Class1
{
    public virtual int GetNumber()
    {
        return 1;
    }
}

class Class2 : Class1
{
    public override int GetNumber()
    {
        int i = base.GetNumber();
        return i + 1;
    }
}

private void button1_Click(object sender, EventArgs e)
{
    Class1 class1 = new Class1();
    Class2 class2 = new Class2();

    MessageBox.Show("GetNumber() from Class1: " + class1.GetNumber());
    MessageBox.Show("GetNumber() from Class2: " + class2.GetNumber());
}