How many parameters are too many? [closed]

Routines can have parameters, that's no news. You can define as many parameters as you may need, but too many of them will make your routine difficult to understand and maintain.

Of course, you could use a structured variable as a workaround: putting all those variables in a single struct and passing it to the routine. In fact, using structures to simplify parameter lists is one of the techniques described by Steve McConnell in Code Complete. But as he says:

Careful programmers avoid bundling data any more than is logically necessary.

So if your routine has too many parameters or you use a struct to disguise a big parameter list, you're probably doing something wrong. That is, you're not keeping coupling loose.

My question is, when can I consider a parameter list too big? I think that more than 5 parameters, are too many. What do you think?

When is something considered so obscene as to be something that can be regulated despite the 1st Amendment guarantee to free speech? According to Justice Potter Stewart, "I know it when I see it." The same holds here.

I hate making hard and fast rules like this because the answer changes not only depending on the size and scope of your project, but I think it changes even down to the module level. Depending on what your method is doing, or what the class is supposed to represent, it's quite possible that 2 arguments is too many and is a symptom of too much coupling.

I would suggest that by asking the question in the first place, and qualifying your question as much as you did, that you really know all of this. The best solution here is not to rely on a hard and fast number, but instead look towards design reviews and code reviews among your peers to identify areas where you have low cohesion and tight coupling.

Never be afraid to show your colleagues your work. If you are afraid to, that's probably the bigger sign that something is wrong with your code, and that you already know it.

A function can only have too many parameters if some of the parameters are redundant. If all the parameters are used, the function must have the correct number of parameters. Take this often used function:

HWND CreateWindowEx
(
  DWORD dwExStyle,
  LPCTSTR lpClassName,
  LPCTSTR lpWindowName,
  DWORD dwStyle,
  int x,
  int y,
  int nWidth,
  int nHeight,
  HWND hWndParent,
  HMENU hMenu,
  HINSTANCE hInstance,
  LPVOID lpParam
);

That's 12 parameters (9 if you bundle the x,y,w and h as a rectangle) and there's also the parameters derived from the class name as well. How would you reduce this? Would you want to reduce the number more to the point?

Don't let the number of parameters bother you, just make sure it's logical and well documented and let intellisense^* help you.

^* Other coding assistants are available!

In Clean Code, Robert C. Martin devoted four pages to the subject. Here's the gist:

The ideal number of arguments for a function is zero (niladic). Next comes one (monadic), followed closely by two (dyadic). Three arguments (triadic) should be avoided where possible. More than three (polyadic) requires very special justification -- and then shouldn't be used anyway.

Some code I've worked with in the past used global variables just to avoid passing too many parameters around.

Please don't do that!

(Usually.)