C#

public void ValidateText(List<TextBox> textboxes)
{
  Successful = true;    // Assume all are valid until shown otherwise
  foreach (var tb in textboxes)
  {
    if (string.IsNullOrEmpty(tb.Text))  // could also use .IsNullOrWhiteSpace()
    {
      Successful = false;       // this means that at least 1 is failing.
      // collect the failing ones here
    }
  }
}

List<TextBox> textboxes = new List<TextBox>();
textboxes.Add(FirstNTxt);
textboxes.Add(LastNTxt);
textboxes.Add(CourseTxt);
//validation
FormValidation validate = new FormValidation();
validate.ValidateText(textboxes);
if(!validate.Successful)
{
  MessageBox.Show("error");
}
else
{
  //more stuff that i did not paste in.
}

private void CheckTextBox()
       {
           TxtName.Tag = "First Name";
           TxtPhone.Tag = "Phone Number";

           foreach (Control control in this.Controls)
           {
               if (control.GetType().Equals(typeof(TextBox)))
               {
                   if (ValidateText(control.Text))
                   {
                       MessageBox.Show(control.Tag.ToString());
                       MessageBox.Show(control.Name);
                   }

               }
           }
       }

       private bool ValidateText(string text)
       {
           return text.Equals(string.Empty);
       }

public void ValidateText(List textboxes)
{
  Successful = true;    // Assume all are valid until shown otherwise
  foreach (var tb in textboxes)
  {
    if (string.IsNullOrEmpty(tb.Text))  // could also use .IsNullOrWhiteSpace()
    {
      Successful = false;       // this means that at least 1 is failing.
      // collect the failing ones here
    }
  }
}