How to load an Excel Addin using Interop

Question

Hi,

I have an addin which I want to invoke through Excel interop from a C# winforms application.

I can't get the addin etc. to load unless I uninstall and resinstall it each time (this is apparantly something to do with Excel not loading addins when you use interop - btw, can't get their example to work in C#). Unfortunately this is slow and annoying to the user so I need to streamline it.

I want to have one instance of Excel but load an already installed addin without forcing this install/reinstall problem.

I've searched and searched but everything I find on google gives the solution to install/reinstall. Is there any other way? The add-in is installed, I just want excel to load it.

This is what I am doing at the moment (taken from google'd advice):

                // loop over the add-ins and if you find it uninstall it.
                foreach (AddIn addIn in excel.AddIns)
                    if (addIn.Name.Contains("My Addin"))
                        addin.Installed = false;

                // install the addin
                var addin = excel.AddIns.Add("my_addin.xll", false);
                addin.Installed = true;

Answer 1

but only if I uninstall and resinstall it each time.

That doesn't seem right. You should be able to install it and have it stay there. How are you installing it?

Answer 2

After a while I found the answer hidden in strange places in the MS help: and this blog post.

That isn't all the info you need though. Things to note: you must have at least one workbook open or otherwise Excel barfs. Here's some rudementry code to get started:

var excel = new Application();
var workbook = excel.workbooks.Add(Type.Missing);
excel.RegisterXLL(pathToXll);
excel.ShowExcel();

If you want you can close the temporary workbook (if you've run some macros etc.) and remember to tidy everything up with plenty of calls to Marshal.ReleaseComObject!

Answer 3

Thanks for the post. This saved some precious time!