Does anyone have a good analogy for dependency injection?

Analogy? I'll give it a whack... Your CD Player stereo is useless without a CD with music on it... (It's dependent on the CD). If they built CD Players with the CD already in it, it would get boring very quickly...

So they build them so you can "inject" the CD, (on which it is dependent) into the player. That way you can inject a different one each time, and get "different" behavior (music) dependent on which one you inject.

The only requirement is that the CD must be compatible with the interface defined by the player. (You can't play a blue-ray disk in a 1992 CD player.)

Think of it as a realisation of the "Inversion of Control" pattern. I guess, your problem is, you are so used to it, you don't realize it's that simple.

Let's start at the beginning.

In the early days programs followed a given path through the code. The order of the called functions was given by the programmer.

In interactive programs, e.g. mostly ANY program, you can not say, which function is called at what time. Just look at a GUI or website. You can not say, at what time what button or link is clicked. So the "control" of what's happening is no longer at the program, it's at an outer source. The "control" has been inverted. The function is no longer "acting" it is instead "listening". Think of the hollywood principle: "Don't call us, we call you". A listener is a good example for a realisation of this pattern.

IoC is realized by functions or "methods" in the "object oriented world" of today.

"Dependency Injection" now means the same, but not for "methods", which do something, but for "objects", which hold data.

The data is no longer part of the object holding it. It is "injected" into the object at runtime. To stay in hollywood, think of a film star, playing golf to talk about the business, but to keep in shape, she hungers herself down, minimizing her muscle weight and therefore she is only able to carry one club at a time.

So, on the golf course her game would heavily depend on the one club, she is carrying.

Lucky for her, there are caddies, carrying a whole lot of clubs at one time, and also having the knowledge what club to use at what time. Now she is independent of her limited possibility to carry golf clubs. "Don't think about a concrete club to wear, we know them all and give you the right one at the right time".

The film star is the object and the golf clubs are the members of the object. That's dependency injection.

The best analogy I can think of is that of hiring a mechanic.

Without dependency injection, you hire a mechanic and the mechanic brings his own tools. He may have lousy tools, he may have great tools, he may be using a pipe wrench when he should be using a socket. You don't know, and may not care, so long as he gets the work done.

With dependency injection, you hire a mechanic and you provide him with the tools that you want him to do his work with. You get to choose what you consider to be the best or most appropriate tools for the work you are hiring him to do.