What exactly do the inbuilt .NET Interfaces do?  and why would I use them?

I kind of understand what an interface does  -but I am not too sure why I would use a built in Interface  (ie. IClonable, IFormattable, IEnummerable)  are they like some kind of template or is there something fundamental about them?


Below is kind of a second question....

I came across this site... http://articles.directorym.net/_NET_Tip_Take_Advantage_of_Interfaces-a923831.html  ...and it mentioned to take advantage of Interfaces.

In the extract below the "ICollection" is playing a big part -but I'm not sure what?

<extract >
private void LoadListBox(object[] data, ListBox ListBoxControl)
{ for (int i = 0; i < data.Length; i++) ListBoxControl.Items.Add(data[i].ToString()); }

With the LoadListBox method in place, the code to populate the ListBox in the Page_Load method is simplified. string[] TestItems = { "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday" }; LoadListBox(TestItems, ListBox1);

Although this is better than doing all the work in the Page_Load method, you can do much better. This works fine for a ListBox, but what about other list type controls? If you apply interfaces, you can enhance the helper method to support a variety of data sources and list type controls. Instead of the helper method taking an array of data items and a ListBox control, it now takes an ICollection and a ListControl.

private void LoadListControlFromCollection(ICollection data, ListControl ListControlControl)
{ foreach (object item in data) ListControlControl.Items.Add(item.ToString()); }

Usage of the new LoadListControlFromCollection method to load a ListBox with data items remains the same. string[] TestItems = { "Sunday", "Monday", "Tuesday", "Wednesday", "Thursday", "Friday", "Saturday" }; LoadListControlFromCollection(TestItems, ListBox1);

</extract>

many thanks for help in advanceStart Free Trial