Why i shoud use dependency injection?

Class without any dependency injection: class MyRepository { MyDatabase database = new MyDatabase(); public IEnumerable<MyEvent> GetEventsAfter(DateTime date) { return this.database.GetQueryable<MyEvent>() .Where(e => e.DateLogged > date); } } The problem here is testing whether GetEventsAfter works. You need access to an actual database with data set up specifically for testing. That introduces overhead in actually accessing the database (you want these tests to be fast - they're checking logic, not how quickly you can access data). It also has the problem of what happens if someone changes/deletes the test data? Will your tests start to fail even though they're actually correct? With some dependency injection: class MyRepository { IMyDatabase database; public MyRepository(IMyDatabase database) { this.database = database; } public IEnumerable<MyEvent> GetEventsAfter(DateTime date) { return this.database.GetQueryable<MyEvent>() .Where(e => e.DateLogged > date); } } Here you can pass a class that mocks the IMyDatabase interface to return whatever data you need for a particular test to pass. In the live system the value passed in as IMyDatabase would be the concrete class that actually provides access to the underlying database.

Dependency injection makes it easy to change your code for different customers, thus allowing you to create flexible code. It even enables you to change behavior at runtime. Why wouldn't you use it?