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)
        {
            bool hasEmpty = false;

            var controls = groupBox1.Controls.OfType<TextBox>();
            foreach(Control c in controls)
            {
                if(string.IsNullOrEmpty(c.Text))
                {
                    hasEmpty = true;
                }
            }

            if(hasEmpty)
            {
                // ERROR!
            }
        }
    }
}