Limit number of parameters per method?

Question

Assuming the parameters are all the same type, is there a rule of thumb in regards to the number of parameters for a method? Im just wondering where I should draw the line and what my alternatives are (ie interface, array, etc).

Answer 1

If you will be passing an unknown number of arguments, you should use varargs or pass a IEnumerable. Of course, sometimes you pass a fixed number of items of the same type. In the latter case, the fixed number should follow from the method's purpose.

Answer 2

I try to limit it to 4 or so. Some people say less, some say more.

The alternative to tons of parameters would be to create an operation class, i.e. replace:

func(boo, far, lint, pizza, flags);

with

var action = new DoSomethingObject(boo, far, lint);
action.Food = pizza;
action.Go(flags);

This has a few advantages over the function:

• If some parameters are optional, you can expose them as properties (such as pizza above).
• If your function takes that many arguments, chances are it does a lot, and could be broken into smaller functions. A class helps you do this cleanly.

Answer 3

I would say it really depends on your case. Are you doing something to the entire set? Validating all the items or aggregating data, for example? In that case, I would pass in an IEnumerable as a single parameter.

Passing in a lot of parameters can be a good sign of poor separation of concerns (i.e. your method is doing too much), but it sounds like in this case you're passing in a well defined set of items to iterate them in some way. Given the collection initializer syntax in C# 3, I would recommend IEnumerable in pretty much every case over a list of parameters that would be something like Type a, Type b, Type c....

Of course, if your parameters are actually treated differently, then separating them out makes sense, but I would consider what you're doing in that case. A simple case that comes to mind would be building a tree data structure and having a function to build up the children of a node. A poor syntax might be:

Node BuildTree( Node parent, Node child1, Node child2...)

I would probably pursue something more like:

void ConstructChildren( this Node parent, IEnumerable<Node> children)

If you can provide more information about your case, though, and what sort of logic you're performing on the parameters, it would probably be easier to see if it's a good candidate for collapsing or refactoring.

Answer 4

Steve McConnell addresses this in Code Complete, citing research that suggests people can't really handle more than seven chunks of information at a time, making seven a common-sense limit wherever it's practical.

In the concluding paragraph of that section (page 178 in the second edition), he writes:

If you find yourself consistently passing more than a few arguments, the coupling among your routines is too tight ... If you are passing the same data to many different routines, group the routines into a class and treat the frequently used data as class data.

Answer 5

Also, for greater call-site readability, you could use a params parameter list (the varargs mentioned before), so instead of

void ConstructChildren( Node parent, IEnumerable<Node> children)
....
List<Node> children = new List<Node> {child1, child2, child3};
ConstructChildren(parent, children);

I would use

void ConstructChildren( Node parent, params Node[] children)
...
ConstructChildren( parent, child1, child2, child3);

However, the params syntax gets ugly to look at if you use more that 5-6-7 items in the children collection.