I have not used .NET extensibility, but I have done a lot with Java. Using the addin does not require any IDE. For Java and I assume .NET is similar, there is a wizard that creates a basic template class for you in the IDE. You don't have to actually use it though. You can write the code yourself even in notepad if you want. You will need a compiler though. The express editions are probably fine. Once you have a compiled version of your custom addin, you can install it on any machine and it will not required development sources. The addin will show up on the QTP splash screen just like all the others.
I'm thinking that I didn't explain myself enough in my first post.
A client of mine is attempting to install the .NET Extensibility Add-In from his QTP10 install DVD. During the install process, an error message appears telling him that Visual Studio 2003/2005/2008 isn't installed, and that the installation of the add-in will be canceled. While he does have Visual Studio for C#, C++, and VB installed, they are all Express editions.
At this point, I've already assumed that he needs to install a full version of Visual Studio, I was just hoping to get some confirmation.
The install from the QTP disk is an addin to Visual Studio IDE itself, so that probably requires the full VS to be installed. What I am saying is that the VS addin itself is not required to code .NET extensibility. It only has a little wizard to help you create a template class. TBH, I found the wizard to confuse the process so its probably better to not use it anyway.
We are trying to use the .NET extensibility add-in but we have Visual Studio 2010 Ultimate edition and it is looking for older versions. Does anyone know if the extensibility add in will work with VS 2010? Frustrating HP ! It's 2012 and you don't support VS 2010 yet.