How to set tooltips to combobox items @ VB.NET ?

Question

Hi,

I have a combobox with a given width. It may occur that one row of the data is partially hidden (the combobox might be too narrow). I'd like to show the whole line by using a tooltip or right-click context menu.

Currently I can't find how to 'catch' the row that I'm currently holding on or passing over by mouse. Please tell me.

Thanks in advance!

Answer 1

Have you tried to increase the DropDownWidth property so that everything is visible?

Edit: To find the ideal width based on the items in the list:

var maxLength = 0;
// using the ComboBox to get a Graphics object:
using (var g = Graphics.FromHwnd(comboBox2.Handle)) {
  foreach (var item in comboBox2.Items.Cast<string>()) {
    var itemLength = g.MeasureString(item, comboBox2.Font);
    maxLength = Math.Max((int) itemLength.Width, maxLength);
  }
}
if (comboBox2.Items.Count > comboBox2.MaxDropDownItems) {
  // correction when ScrollBar is displayed
  maxLength += 15;
}
comboBox2.DropDownWidth = maxLength;

I put this code in the DropDown event of the ComboBox for testing. Maybe you can find a better place for it, like after populating the ComboBox...

Answer 2

Your're right, there really isn't a "Item.OnMouseOver", but I suppose you could (off the top of my head, so I've likely forgotten something)...

• inherit from ComboBox,
• override OnDrawItem (you may need to turn change .DrawMode to "Owner Drawn").
  • you will know which item is hovered in the OnDrawItem event/override from the EventArgs.
• Set the tooltip on the control at that point.
• optionally set a timer to manually show the tooltip if the above doesn't work automatically.

Answer 3

Going in the same direction that Julien went, here is a generic extension method that will resize the drop down area regardless of how the combobox is filled (manually with strings or via data binding).

<Extension()> _
Public Sub AutosizeDropDownWidth(ByVal combobox As ComboBox)
    Dim longestItem As Integer = 0
    Using g = Graphics.FromHwnd(combobox.Handle)
        Dim itemsAsText = (From item In combobox.Items _
                           Select combobox.GetItemText(item))
        longestItem = CInt(itemsAsText.Max(Function(text) g.MeasureString(text, combobox.Font).Width))
    End Using

    ' Account for scrollbar
    If (combobox.Items.Count > combobox.MaxDropDownItems) Then longestItem += 15

    ' Resize as needed (but never smaller than the default width)
    combobox.DropDownWidth = Math.Max(longestItem, combobox.Width)
End Sub

To use it then you can simply do the following...

Private Sub MyCombobox_DropDown(ByVal sender As Object, ByVal e As System.EventArgs) Handles MyCombobox.DropDown
    MyCombobox.AutosizeDropDownWidth()
End Sub

Note, I didn't test corner cases like an empty combobox in this code example.