C# How to use interfaces

• 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).

The class needs to read:

public class Test : ITest

in its declaration.

First off, you need to have your Test class inherit/implement ITest.

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

Then, in your controller class, you need to initialize test -- whether directly, or in the constructor.

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

Although in many cases, you should prefer to create the ITest outside of the constructor and pass it in or something.