Setting same SelectedIndex value for multiple combobox

问题

I have a form where i have 23 Comboboxes. Writing SelectedIndex=-1 for every combobox will definitely work but i want to know whether there's any other way of doing it as done in below given example. ?

  For Each ctl As Control In (GroupBox1.Controls)
            If TypeOf ctl Is TextBox Then
                ctl.Text = ""
            End If            
 Next

i tried this,

If TypeOf ctl Is ComboBox Then ctl.selectedIndex=-1 End If

but it doesn't works. Please help me out.

回答1:

In your example your ctl variable is a Control and not a Combobox so it does not have the SelectIndex property - though you could have casted it back with DirectCast(ctl, Combobox).

For Each ctl As Control In (GroupBox1.Controls)
  If TypeOf ctl Is Combobox Then
    DirectCast(ctl, Combobox).SelectedIndex = -1
  End If            
Next

Or create a loop of type specific control for your loop. No need to check type here.

Dim cbs = GroupBox1.Controls.OfType(Of Combobox)()
For Each cb In cbs
 cb.SelectedIndex = -1
Next

来源：https://stackoverflow.com/questions/26441096/setting-same-selectedindex-value-for-multiple-combobox