Dropdownlists in Microsoft Outlook VSTO Add-In not responding

Question

The Outlook add-in I build worked before, but now some of the users have notified me that the dropdownlists does not respond anymore. The add-in is an adjoining window below a new compose message. The Add-in window uses a System.Windows.Forms.WebBrowser to access a page on a website I created with all the dropdownlists.

I have uninstalled recent Microsoft updates but it did nothing.
It does the same if 'n run it in debug mode.
Same happens when I run the website on localhost.
The checkboxes works and can be checked it is just the dropdownlists which are unable to open, but if I press the down key I can access the lists items but the dropdown window does not show.

I presume it must be some change to Outlook and how it interacts with the WebBrowser control? It is the only thing that could have changed.

--EDIT-- Here is my code:

Dropdownlist asp.net code on website

<asp:DropDownList ID="ddlDiscipline" runat="server" AutoPostBack="True" ForeColor="Black" OnSelectedIndexChanged="ddlDiscipline_SelectedIndexChanged" Width="136px" TabIndex="6" Visible="True">
</asp:DropDownList>

C# Code part

protected void ddlDiscipline_SelectedIndexChanged(object sender, EventArgs e)
    {
        try
        {
            foreach (ListItem item in ddlDiscipline.Items)
            {
                if (item.Selected)
                {
                    if (txtDiscipline.Text == "")
                    {
                        txtDiscipline.Text = item.Text;
                    }
                    else
                    {
                        txtDiscipline.Text = txtDiscipline.Text + "," + item.Text;
                    }
                }
            }
            ddlDiscipline.SelectedIndex = 0;
        }
        catch { }
    }

What happens is that the Outlook Add-In uses the WebBrowser control to access this webpage. If the dropdownlist text changed event fires the selected item is added to a textbox.

This dropdownlists just stopped from opening suddenly without any changes to the code. There is no reference used to the dropdownlist in the VSTO program. It only needs the user to select it.

The dropdownlist responds in Internet Explorer:

It does not open in the Add-In window in Outlook:

Could you post some code so we can see more clearly what you mean? — Alexander Ryan Baggett

Eugene Astafiev Eugene Astafiev · Accepted Answer · 2017-06-04T05:09:20

Do you get the same results with a windows form application?

Try to use any .net web browser control based on the Chromium engine. Based on my experience only the DotNetBrowser control works without issues for COM add-ins (including VSTO based). You may also consider open-source .net browsers such as Awesomium and CefSharp.

Dropdownlists in Microsoft Outlook VSTO Add-In not responding

1 Answers