Using Recursion in C#

Question

Are there any general rules when using recursion on how to avoid stackoverflows?

Answer 1

If you're finding yourself generating that many stack frames, you might want to consider unrolling your recursion into a loop.

Especially if you are doing multiple levels of recursion (A->B->C->A->B...) you might find that you can extract one of those levels into a loop and save yourself some memory.

Answer 2

The default stack size for a thread is 1 MB, if you're running under the default CLR. However, other hosts may change that. For example the ASP host changes the default to 256 KB. This means that you may have code that runs perfectly well under VS, but breaks when you deploy it to the real hosting environment.

Fortunately you can specify a stack size, when you create a new thread by using the correct constructor. In my experience it is rarely necessary, but I have seen one case where this was the solution.

You can edit the PE header of the binary itself to change the default size. This is useful if you want to change the size for the main thread. Otherwise I would recommend using the appropriate constructor when creating threads.

Answer 3

I wrote a short article about this here. Basically, I pass an optional parameter called, depth, adding 1 to it each time I go deeper into it. Within the recursive method I check the depth for a value. If it is greater than the value I set, I throw an exception. The value (threshold) would be dependent on your applications needs.

Answer 4

How many times you will be able to recurse will depend on:

• The stack size (which is usually 1MB IIRC, but the binary can be hand-edited; I wouldn't recommend doing so)
• How much stack each level of the recursion uses (a method with 10 uncaptured Guid local variables will be take more stack than a method which doesn't have any local variables, for example)
• The JIT you're using - sometimes the JIT will use tail recursion, other times it won't. The rules are complicated and I can't remember them. (There's a blog post by David Broman back from 2007, and an MSDN page from the same author/date, but they may be out of date by now.)

How to avoid stack overflows? Don't recurse too far :) If you can't be reasonably sure that your recursion will terminate without going very far (I'd be worried at "more than 10" although that's very safe) then rewrite it to avoid recursion.

Answer 5

Remember, if you have to ask about system limits, then you are probably doing something horribly wrong.

So, if you think you might get a stack overflow in normal operation then you need to think of a different approach to the problem.

It's not difficult to convert a recursive function into an iterative one, especially as C# has the Generic::Stack collection. Using the Stack type moves the memory used into the program's heap instead of the stack. This gives you the full address range to store the recursive data. If that isn't enough, it's not too difficult to page the data to disk. But I'd seriously consider other solutions if you get to this stage.

Answer 6

The normal limit, if not much is left on the stack between successive calls, is around 15000-25000 levels deep. 25% of that if you are on IIS 6+.

Most recursive algorhitms can be expressed iteratively.

There are various way to increase allocated stack space, but I'll rather let you find an iterative version first. :)