What are Dependency Injections and why are they useful?

Question

I have heard of ninject and I have to build a lot of apps that have to integrate with other tools.

SO i was wondering what this was and how it is used.

Answer 1

What are dependency Injections :- I will try to help you understand this with examples

Without DI Container --

LinqValueCalculator lc = new LinqValueCalculator();
ShoppingCart sc = new ShoppingCart(lc);
decimal total = sc.CalculateStockValue();
Console.WriteLine("Total: {0:c}", total);

With DI Container -- (Ninject is used in this example)

IKernel ninjectKernel = new StandardKernel();
ninjectKernel.Bind<IValueCalculator>().To<LinqValueCalculator>();

IValueCalculator calcImpl = ninjectKernel.Get<IValueCalculator>();
ShoppingCart cart = new ShoppingCart(calcImpl);
Console.WriteLine("Total: {0:c}", cart.CalculateStockValue());

How often do I use a DI Container?

Very often. Close to always.

What requirements make me do so?

In ASP.NET MVC I always use a container, because when one uses Constructor Injection in Controllers one breaks the default convention of having default constructors. This means that a custom IControllerFactory is required, and while it's possible to write and maintain one by hand, it's more work. Using a DI Container that supports convention-based configuration, one can use Constructor Injection in a convention-based manner and less maintenance is required.

Can we do without them?

Yes, but at the cost of more maintenance of infrastructure code: http://stackoverflow.com/questions/5667801/arguments-against-inversion-of-control-containers/5668093#5668093