What do programmers mean when they say, “Code against an interface, not an object.”?

Question

I\'ve started the very long and arduous quest to learn and apply TDD to my workflow. I\'m under the impression that TDD fits in very well with IoC principle

Answer 1

The big advantage of programming against interfaces when performing unit testing is that it allows you to isolate a piece of code from any dependencies you want to test separately or simulate during the testing.

An example I've mentioned here before somewhere is the use of an interface to access configuration values. Rather than looking directly at ConfigurationManager you can provide one or more interfaces that let you access config values. Normally you would supply an implementation that reads from the config file but for testing you can use one that just returns test values or throws exceptions or whatever.

Consider also your data access layer. Having your business logic tightly coupled to a particular data access implementation makes it hard to test without having a whole database handy with the data you need. If your data access is hidden behind interfaces you can supply just the data you need for the test.

Using interfaces increases the "surface area" available for testing allowing for finer grained tests that really do test individual units of your code.