How to load an Excel Addin using Interop

Question

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

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 2

It seems that you have to get the correct Excel process to work with. Use this class to open the Excel document:

 class ExcelInteropService
{
    private const string EXCEL_CLASS_NAME = "EXCEL7";

    private const uint DW_OBJECTID = 0xFFFFFFF0;

    private static Guid rrid = new Guid("{00020400-0000-0000-C000-000000000046}");

    public delegate bool EnumChildCallback(int hwnd, ref int lParam);

    [DllImport("Oleacc.dll")]
    public static extern int AccessibleObjectFromWindow(int hwnd, uint dwObjectID, byte[] riid, ref Window ptr);

    [DllImport("User32.dll")]
    public static extern bool EnumChildWindows(int hWndParent, EnumChildCallback lpEnumFunc, ref int lParam);

    [DllImport("User32.dll")]
    public static extern int GetClassName(int hWnd, StringBuilder lpClassName, int nMaxCount);

    public static Application GetExcelInterop(int? processId = null)
    {
        var p = processId.HasValue ? Process.GetProcessById(processId.Value) : Process.Start("excel.exe");
        try
        {
            Thread.Sleep(5000);
            return new ExcelInteropService().SearchExcelInterop(p);
        }
        catch (Exception)
        {
            Debug.Assert(p != null, "p != null");
            return GetExcelInterop(p.Id);
        }
    }

    private bool EnumChildFunc(int hwndChild, ref int lParam)
    {
        var buf = new StringBuilder(128);
        GetClassName(hwndChild, buf, 128);
        if (buf.ToString() == EXCEL_CLASS_NAME) { lParam = hwndChild; return false; }
        return true;
    }

    private Application SearchExcelInterop(Process p)
    {
        Window ptr = null;
        int hwnd = 0;

        int hWndParent = (int)p.MainWindowHandle;
        if (hWndParent == 0) throw new Exception();

        EnumChildWindows(hWndParent, EnumChildFunc, ref hwnd);
        if (hwnd == 0) throw new Exception();

        int hr = AccessibleObjectFromWindow(hwnd, DW_OBJECTID, rrid.ToByteArray(), ref ptr);
        if (hr < 0) throw new Exception();

        return ptr.Application;
    }
}

Use the class in your application like this:

static void Main(string[] args)
{
    Microsoft.Office.Interop.Excel.Application oExcel = ExcelInteropService.GetExcelInterop();
    foreach (AddIn addIn in oExcel.AddIns)
        {
            addIn.Installed = true;
        }
}