Three examples of how to get a dependency, FooProvider, into a class that needs it.

DI

// Bad!
// The FooProvider is instantiated in the constructor. Very hard to test.
public class OldSchool
{
    private FooProvider _fooProvider;

    public OldSchool()
    {
        _fooProvider = new FooProvider();
    }
}

// Better!
// A concrete instance of FooProvider is given by TheAppServiceLocator,
//  but we still have to know to set up the TheAppServiceLocator beforehand.
public class SerivceLocator
{
    private IFooProvider _fooProvider;

    public SerivceLocator()
    {
        _fooProvider = TheAppServiceLocator.Get<IFooProvider>();
    }
}

// Best!
// An instance of FooProvider is injected into the constructor.
// All dependencies are resolved in the ctor(), and IFooProvider can be easily mocked for testing.
public class DependencyInjection
{
    private IFooProvider _fooProvider;

    public DependencyInjection(IFooProvider fooProvider)
    {
        _fooProvider = fooProvider;
    }
}

public interface IFooProvider
{
}

public class FooProvider : IFooProvider
{
  ...
}