C# How to use interfaces

Question

This is a relatively straight forward question. But I was wondering what the correct usage is for accessing a method inside a separate project through the use of an interface.

Project: Test.ClassLibrary

Interface:

public interface ITest
{
    string TestMethod();
}

Class:

public class Test : ITest
{
    public string TestMethod()
    {
        return "Test";
    }
}

Project: Test.Web

Controller:

public class HomeController : Controller
{
    private ITest test;
    public ActionResult Index()
    {
        return Content(test.TestMethod());
    }

}

The above returns a NullReferenceException. I'm assuming it's because the controller gets to the interface, and doesn't know where to go next.

What's the best way to fix this? Do I have to reference the Test class in the controller or can I some how get away with only having a reference to ITest?

Answer 1

• You never instantiate ITest test, you only declare it.
• Your Test class doesn't inherit from the interface.

You need to update your class declaration

public class Test : ITest // interface inheritance 
{

And in your controller, instantiate test.

ITest test = new Test();

As you get further along, you'll want to explore techniques for injecting the Test instance into the controller so that you do not have a hard dependency upon it, but just on the interface ITest. A comment mentions IoC, or Inversion of Control, but you should look into various Dependency Inversion techniques techniques (IoC is one of them, dependency injection, etc).