There are umpteen ways to determine whether or not a TextBox in a group of TextBoxes is empty. Usually whilst doing Form validation, you need to ensure all data has been filled in. A quick and easy way, with the help of LINQ, is as follows:
var emptyTextBox = Controls.OfType().FirstOrDefault(t => string.IsNullOrEmpty(t.Text));
if (emptyTextBox != null)
//Add code here if all textboxes were NOT filled in
//Add code here if all textboxes were filled in
You create an object that will host the result of the LINQ query. The LINQ query looks through all the controls on the form, determines if the current control is a TextBox, then checks to see if the TextBox is empty or not. If there is a TexBox that is not empty it will exit the query and the object will host the TextBox that is empty. You then simply do an If check. If no controls have been returned from LINQ, every TextBox has been filled in and your validation passes.