Now that we have an interface implemented we can take a look at how we can actually use this interface. One nice thing we can do now when writing a method which will just be calling our interface's methods is that we can have it take the interface instead of the class. Observe the following example.

Listing 4: Using an Interface as a Parameter

public void GreetEveryone(IHelloWorlder hw)
{
    // Perform whatever logic you wish here
    hw.SayHello();
}
 
public void DoSomething()
{
    MyHelloWorld helloWorld = new MyHelloWorld();
    GreetEveryone(helloWorld);
}

If we were to write another class implementing the IHelloWorlder we could alter the DoSomethingMethod and change it to use the new class. Since we just specified the interface in the GreetEveryone method we will not have to change anything in there. A good example of when this type of scaling might occur can be seen with the IDataReader interface.

As an example, pretend that we have a web site and that the web site uses XML files to store all of its data. We use an XmlDataReader to read all of the data from the XML files. Since we have used IDataReader in the program instead of always specifying XmlDataReader everywhere, we can easily change to using a database later. We would be able to change to using a SqlDataReader for example. We will have less code to change because we are using the IDataReader interface and both classes implement it so a lot of the interaction remains the same. If we were to then decide we wanted a different type of database we could easily switch to another IDataReader-implementing class and continue working.