Chance of breaking existing code by adding optional parameter to VB.NET function? - com

Question

Is there a chance that existing code in a large project might bomb if I add a new optional parameter to a function that's used everywhere? I know I can overload the function instead and minimize the risk, but really.. what's the risk if I insist on going with an optional parameter?
Here's an example:
Public Function GetContent(ByVal URL As String, ByVal ID As String, Optional ByRef PageTitle As String = "") As String
Try
Dim web As New HtmlWeb()
Dim doc As HtmlDocument = web.Load(URL)
ID = "//div[#id='" & ID & "']"
Dim ContentNode As HtmlNode = doc.DocumentNode.SelectSingleNode(ID)
' The two lines below are the mere extent of what's new inside this function, besides the new Optional ByRef parameter in its signature
Dim PageTitleNode As HtmlNode = doc.DocumentNode.SelectSingleNode("//title")
If Not PageTitleNode Is Nothing Then PageTitle = PageTitleNode.InnerHtml
Return ContentNode.InnerHtml
Catch ex As Exception
Return "<h4> Bad. Very bad. </h4>"
End Try
End Function
PS: I'd like to comment on my question after the fact, having read others' responses below and having done some additional research myself. Originally, I didn't want to question the validity of the approach of using an optional parameter. That was something VB.NET was allowing me to do and I felt I had every right to use--besides that it was simply very convenient! But my original question had more to do with whether there may be gaps in how optional parameters are implemented, from compilation down to execution--gaps that I should consider as I design my code. I was unaware of the historical significance of the optional parameter approach in relation to the overload approach. I've learned now that it's not that there are gaps or flaws in the optional parameter approach; rather, it was a solution designed for a different and older set of concerns that was simply overridden with the advent of the Common Language Runtime. I'm using VS2013. Sure, everything compiled fine with the optional parameter approach and seemed to run fine but I wanted to confirm I wasn't potentially breaking something else by adding an optional parameter--especially since someone looked at my code and suggested I should overload the function instead. I wanted to prove why I shouldn't keep my optional parameter method. James Thorpe answered that question for me now, I think. But as Tim Schmelter asked, is there a benefit for doing it this way (optional parameters) as opposed to the overload approach? To me now the overload approach seems the best and only way, and that is because I'm using a newer set of technologies that the optional parameter approach--which was implemented for Microsoft's older Component Object Model, or COM--simply wasn't designed to address (see page 83 of the book, "Microsoft Visual C# 2013 Step By Step" by John Sharp). Particularly now, if there are external modules expecting to find the old function signature (i.e., the function parameter layout that existed before I added the new optional parameter), they'll break unless I recompile them too! That's a hindrance for me. But overloading handles this software development problem much better without need for recompilation, something only now supported by the newer Common Languange Runtime, or CLR. I suppose the optional parameter support in VB.NET is more of a historical holdover now from the old COM days--and not the very best solution for my specific requirements. I've also just learned that, "The Common Language Specification, which defines the subset of the CLR that all languages should support, explicitly disallows a reliance on optional parameters. This means they are not a candidate for use in the Base Class Library and will probably never been seen in any of the other libraries shipped as part of the .NET Framework." (from the online article, "Optional Parameters Are Gaining Ground in .NET", by Jonathan Allen). Although the rules are laxer for us regular developers that consume Microsoft technologies, I think there's something to be said for their internal decision not to rely on optional parameters. I just wanted to post and share that with you in case like me you've also come here wondering!

Answer 1

Within a single project? No, it should be fine. However, in the comments you said:
Let's say there were other projects calling it (there is a possibility). Would it break those if I didn't rebuild them?
Optional parameters are actually baked in at compile time, so if you have your original method signature:
Public Function GetContent(ByVal URL As String, ByVal ID As String)
And someone is calling it thusly:
GetContent(someUrl, someId)
It will be compiled into their assembly as-is. With your new optional parameter, anything calling it as above without passing in the parameter would actually get compiled as:
GetContent(someUrl, someId, "")
Note how the default value of the optional parameter has automatically been brought in. If you're rebuilding everything, it's all good. However, in those projects that are referencing this one that aren't rebuilt, they will have the original two-parameter call. Your GetContent method now requires 3 parameters at runtime - you'll get a runtime error as it can't find an overload of the function that still takes 2 parameters.

Answer 2

Here's how you can do it without breaking code...
Public Function GetContent(ByVal URL As String, ByVal ID As String, ByRef PageTitle As String = "") As String
' the rest of your function here
End Function
Public Function GetContent(ByVal URL As String, ByVal ID As String) As String
Return GetContent(URL, ID, "")
End Function
That way you have a version of the function with 2 parameters for the existing code and one with 3.
If you want to encourage programmers to switch to the 3 parameter version, then you can mark the 2 parameter version like this:
<Obsolete("Use the version that takes pageTitle as a 3rd parameter">
Public Function GetContent(ByVal URL As String, ByVal ID As String) As String
Return GetContent(URL, ID, "")
End Function
That will give you a compiler warning if you try to call the 2-parameter version.