Question

I have the below piece of code which Prefixs a string to the start of each member of a string array. ie. [“a”,”b”,”c”] prefixed with “z” becomes [“za”,”zb”,”zc”].

private string[] Prefix(string[] a, string b) {
   for(int i = 0;i < a.Length;i++) {
     a[i] = b + a[i];
   }
  return a;
}

The function works fine (although if theres a better way to do this, I’m happy to hear it), but I’m having issues when passing parameters.

string[] s1 = new string[] {"a","b"};
string[] s2 = Prefix(s1,"z");

Now as far as I can tell, I’m passing s1 by Value. But when the Prefix function has finished, s2 and s1 have the same value of [“za,”zb”], or s1 has been passed by reference. I was certain you had to explicitly declare this behaviour in c#, and am very confused.

Answer 1

As others have said, the reference is passed by value. That means your s1 reference is copied to a, but they both still refer to the same object in memory. What I would to do fix your code is write it like this:

private IEnumerable<string> Prefix(IEnumerable<string> a, string b) {
   return a.Select(s => b + s);
}

.

string[] s1 = new string[] {"a","b"};
string[] s2 = Prefix(s1,"z").ToArray();

This not only fixes your problem, but also allows you to work with Lists and other string collections in addition to simple arrays.