Some Real Life examples - also, another way/reason to use interfaces.
In my own code I have an Engine which processes code to produce reports in Excel. In this engine, i had to write the code two different ways, one using the Microsoft Excel Interop, the other using the Open Office Interop. Rather than duplicate my entire engine to work two different ways, or write a lot of if statements in all the actual interop functions, I implemented an interface. Then I declared two classes, each one implementing the interface, but one uses Excel and the other uses open office. Then, in my code, I simple reference the interface and its functions and use a single if statement at the very beginning of the function to tell the interface which class to implement.
public class ExcelEngineInterop : ISSinterface
{ ... }
public class OOEngineInterop : ISSinterface
{ ... }
//cant use two variables with the same name, so use 1 interface reference instead
ISSinterface ssInt;
if(ExcelFlag)
ssInt = new ExcelEngineInterop();
else
ssInt = new OOEngineInterop();
//two VERY different functions between Excel and OpenOffice.
ssInt.OpenApp();
ssInt.OpenFile(fileName);
//etc etc and so on
This is the other reason to use an interface. When you need one block of code to act two (or more) different ways depending on some external flag.
Another Example.
There is a top level form with lots of custom user controls under it. The user fires a form level event, like a button click, but depending on which user controls are active and what settings are on them at the time the click happens, the controls themselves need to do something different. Rather than writing what could be a rediculously large number of if statements to make sure each one acts correctly from the top level, just implement an interface on each control, then do something like this:
public ButtonClick(object sender, EventArgs e)
{
//note: I dont know which of my classes currentrightcontrol belongs to at the moment.
// it could be any one of 22 different controls. It must be cast to something
// in order to call the ButtonClick method (its actual type is generic "UserControl"
IMyRunControl ctrl = CurrentRightControl as IMyRunControl;
ctrl.FormButtonClicked();
}