Question

I’ve seen some developers use the out keyword on parameter lists of void functions. I’m quite unclear on what the pros and cons are of code below:

List<string> listOfResult;    

public void public void (out listOfResult)
{
    //bla bla
}

versus

public List<string> c(out listOfResult)
{
    List<string>  list= new List<string>();
    //bla bla

    return list;
}

Are these two code snippets perfectly valid or is there any catch around the out keyword?

Answer 1

out keyword is handy when you need to return more than one value from function. Nice example is TryXXX methods, which return status of operation instead of throwing exceptions:

public bool TryParse(string str, out int value);

But I don’t see any reason to use single out parameter with void methods… Simply return that value from your method. It will be much easier to use. Compare:

List<string> list;
GetList(out list); // confusing method name

With

List<string> list = GetList(); // nice name, one line of code

If getting of list could throw exceptions, then you can create method like this:

List<string> list;
if (TryGetList(out list)) // better than exception handling
{
   // list was filled successfully
}