using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            // Get Empty Controls
            var controls = groupBox1.Controls
                .OfType<TextBox>()
                .Where(t => string.IsNullOrEmpty(t.Text));

            // Display an error
            if(controls.Count() > 0)
            {
                // ERROR!
            }
        }
    }
}