how to validate multiple comboboxes containing years?

Firstly I personally dislike cross-validation between controls on the validating events of each of the controls - it's a little bit too easy to get into a right mess where you have to enter valid data before you can even exit the form - be careful of this (My personal preference is cross-validation on a specific event on say a button push - "Ok" or "Submit")

Having said that ... something like this should work...

A generic validation function

private bool InvalidYear(object sender)
       {
           // cast the sender object to a ComboBox so that we can access it's properties
           ComboBox cb = (ComboBox)sender;

           /* Find out the number of this combo box  - Note there are loads of ways of doing this but
            * I've assumed they're called comboBox1, comboBox2, comboBox3, comboBox4
            * and I'm just removing the text "combBox" from the name
            * I'm also not assuming that their tabindexes are consecutive
            * and remember the user can select them in any order
           */
           int cbNum = int.Parse(cb.Name.Replace("comboBox", ""));

           // Locate the comboBoxes before and after the one we're validating
           // Taking care at either end of the list of comboBoxes...
           ComboBox prev = null, post = null;
           if (cbNum > 1)
               prev = (ComboBox)this.Controls.Find("comboBox" + (cbNum - 1).ToString(), true)[0];

           if (cbNum < 4)
               post = (ComboBox)this.Controls.Find("comboBox" + (cbNum + 1).ToString(), true)[0];

           bool errorFound = false;

           // don't validate against a checkbox the user hasn't picked yet!
           if (prev != null && prev.Text != "")
           {
               if (cb.Text.CompareTo(prev.Text) <= 0)  // this one must be greater than previous
                   errorFound = true;
           }
           if (post != null && post.Text != "")
           {
               if (cb.Text.CompareTo(post.Text) >= 0) // this one must be less than the one after
                   errorFound = true;
           }


           if (errorFound) MessageBox.Show("Invalid year");

           return errorFound;
       }

And then you can call this from each of the comboBoxes like this ...

private void comboBox1_Validating(object sender, CancelEventArgs e)
 {
     if (InvalidYear(sender))
         e.Cancel = true;
 }