Careful with the new .NET overloads of String.Join when moving to .Net 4.0

I was trying to figure out an issue with our product and, in order to understand what was going on, I copied a class from the project into a small sandbox project to see how it would work. Lo and behold, a problem occurred in one of the utility functions that would have made the entire feature unusable. Yet the feature worked fine, except the little detail I was working on. What was going on?

Let me show you the code first (simplified for your pleasure):

Dim al As New ArrayList
al.Add("A")
al.Add("B")
Dim result as String = String.Join(":", al.ToArray(GetType(String))))

What do you think the result will hold?

In our production site the result was "A:B". In my sandbox project the result was "System.String[]". It took me a little to understand what was going on. You see, the sandbox project was .Net 4.0 while the production site still worked with 3.5. New to .Net 4.0 are overloads for the String.Join method, including one that receives a params array of objects. Since ArrayList.ToArray(Type type) returns Array no matter the type boxed inside, this is the overload that is chosen. The list of strings is taken as the first parameter, stringified, and the result is what you saw.

Conclusion: be very careful of the types you send to methods. Even if Visual Basic automatically casts method parameters, you never know for sure which type it will choose to transform into. And if you want to upgrade a VB project from .Net 2.0-3.5 to 4.0, be careful of the new overloads that have appeared.