Using Interface variables

Question

I\'m still trying to get a better understanding of Interfaces. I know about what they are and how to implement them in classes.

What I don\'t understand is when you

Answer 1

I was in same position and took me few days to figure out why do we have to use interface variable.

 IDepartments rep = new DepartmentsImpl();

why not

 DepartmentsImpl rep = new DepartmentsImpl();

Imagine If a class implements two interfaces that contain a member with the same signature, then implementing that member on the class will cause both interfaces to use that member as their implementation.

class Test 
{
static void Main()
{
    SampleClass sc = new SampleClass();
    IControl ctrl = (IControl)sc;
    ISurface srfc = (ISurface)sc;

    // The following lines all call the same method.
    sc.Paint();
    ctrl.Paint();
    srfc.Paint();
}

}

interface IControl
{
  void Paint();
}
 interface ISurface
{
  void Paint();
}
 class SampleClass : IControl, ISurface
 {
   // Both ISurface.Paint and IControl.Paint call this method. 
 public void Paint()
 {
    Console.WriteLine("Paint method in SampleClass");
 }

}

 // Output:
 // Paint method in SampleClass
// Paint method in SampleClass
// Paint method in SampleClass

If the two interface members do not perform the same function, however, this can lead to an incorrect implementation of one or both of the interfaces.

public class SampleClass : IControl, ISurface
{
    void IControl.Paint()
    {
        System.Console.WriteLine("IControl.Paint");
    }
    void ISurface.Paint()
    {
        System.Console.WriteLine("ISurface.Paint");
    }
}

The class member IControl.Paint is only available through the IControl interface, and ISurface.Paint is only available through ISurface. Both method implementations are separate, and neither is available directly on the class. For example:

   IControl c = new SampleClass();
   ISurface s = new SampleClass();
   s.Paint();

Please do correct me if i am wrong as i am still learning this Interface concept.