issues with text box value changes

I have a TextBox, The situation is that when a user leaves it empty, OK button is disabled. If user types in a value of duration based on days, the button gets enabled.

The problem is: let's say the user clicks on text box, types in : 100, and he realizes that he made a mistake, he erase that value and tries to start filling some other textboxes and come back again afterwards, at this level at the time user clicks on other part of form, the value becoems 0 autoamtically and Ok button is Enabled Now.

How can I avoid this?

开发者_如何学PythonPrivate Sub txtMembershipDuration_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles txtMembershipDuration.TextChanged
If txtMembershipDuration.Text = "" Then
  btnOK.Enabled = False
End If  End Sub

Also, how can I control that auttomatic 0?

Use a TryParse on your check, and make sure it's greater than 0, instead of checking if it's "". Link: TryParse

You could check for "0" if it's not a valid value

Private Sub txtMembershipDuration_TextChanged(ByVal sender As Object, ByVal e As EventArgs) Handles txtMembershipDuration.TextChanged
    If txtMembershipDuration.Text = "" OrElse
       txtMembershipDuration.Text = "0" Then
        btnOK.Enabled = False
    End If
End Sub