where to use Method overiding

When you want to add different or additional functionality with the same method call. A good example would be ToString(). When you can a collection of objects that you want to display in a listbox (or other list container), you can simply override the Object.ToString() method to return the desired text, like so:

public class MyObject
{
    public string DisplayName { get; set; }

    public MyObject(string name)
    {
        DisplayName = name;
    }

    public override string ToString()
    {
        return DisplayName;
    }
}

Usage:

List<MyObject> myObjects = new List<MyObject>;

// populate the collection, and then do this:
listbox1.Items.AddRange(MyObjects.ToArray());

Since the Items collection in the listbox is an object (as opposed to a string), it will call the object's ToString method to get displayed text. Normally, ToString would return the class name of the object (in the example shown above, that would have been "MyObject". However, by overriding ToString(), the listbox will show the contents of the DisplayName property instead.