The Moth

Here is another question that appears in various guises.
"Event X from control Y gets fired twice" or "I want to stop this event from firing while I am waiting for Z"

For example assume that you do stuff in the event handler of the selectedindexchanged event of a list; you now need to select an item in a list programmatically but do not want to run your event handling in that case (only when it is done by the user). Here is a simple example:

Given a form with a listbox and a button let's assume your event handler looks something like this:

Private Sub ListBox1_SelectedIndexChanged(ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles ListBox1.SelectedIndexChanged
    MessageBox.Show(ListBox1.SelectedItem.ToString())
End Sub

And let's assume as part of a button click you are doing something like this:

Private Sub Button1_Click(ByVal sender As System.Object, _
 ByVal e As System.EventArgs) Handles Button1.Click
    ListBox1.SelectedIndex = 0
End Sub

When you click the button you don't want to see a messagebox.

As you'd expect the solution is simple:
1) Use a boolean flag and check it in your event before processing
or
2) Dynamically remove the event handler

1) Define a boolean in your form code like this:

Private mDontRun As Boolean

Change your button click event handler body to be like this:

mDontRun = True
ListBox1.SelectedIndex = 0
mDontRun = False

And add to the selectedindexchanged event handler the following as the first line:

If mDontRun = True Then Return

2) The alternative is not to declare a boolean, not to change the selectedindexchanged event handler and simply change the button click method body like this:

RemoveHandler ListBox1.SelectedIndexChanged, _
        AddressOf ListBox1_SelectedIndexChanged
ListBox1.SelectedIndex = 0
AddHandler ListBox1.SelectedIndexChanged, _
 AddressOf ListBox1_SelectedIndexChanged

Simple really... and one more entry I can point to from the ng...