Toward less injection

Here’s a habit that may help to reduce the complexity of your dependency injection tree. Ask yourself: how much of an injected dependency are you actually using?

The conversation goes like this…

I need to inject an HttpContext

Great. Why? Because we need to read a cookie. So really….

Ah, OK I only need to inject an HttpRequest

Better! But seriously, are you only looking at cookies?

Right, I only ever touch Request.Cookies

Now we are getting somewhere! You only need an HttpCookieCollection. That’s a pretty simple data type.

If that’s all you need, you don’t need to inject it at all — just pass it as an argument to the method that needs it.

Further, you likely don’t need to mock it (in the mocking-framework sense) for testing. Just new one up and add some test values. Nice!

You could push it further and say, OK, which of CookieCollection’s methods am I actually using? Is there an smaller type that I could accept, maybe an IEnumerable<HttpCookie>? How about just the HttpCookie, no collection at all? A string, even?

The smaller the interface, the easier the mock — though there may be a trade-off in asking the caller to do more work.

Now: does your service have state, or more precisely, do its methods depend on the service’s state? If not, the method can — and likely should — be static. One less dependency on injection, one hopes.

Published September 18, 2017