C# code

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace WpfApplication6
{
    /// <summary>
    /// Interaction logic for MainWindow.xaml
    /// </summary>
    public partial class MainWindow : Window
    {
        private void StopTextBoxClick(object sender, MouseButtonEventArgs e)
        {
            TextBox text = sender as TextBox;
            if (text != null && !text.IsKeyboardFocusWithin)
            {
                e.Handled = true;
                text.Focus();
            }
        }
        public MainWindow()
        {
            InitializeComponent();
            string[] lines = System.IO.File.ReadAllLines(Environment.CurrentDirectory + @"\words.txt");
            char[,] table = new char[9, 9];
            Random rndA = new Random();
            int rnd = rndA.Next(0, lines.Length/10);
            TextBox[,] boxes = { { textBox1, textBox2, textBox3, textBox4, textBox5, textBox6, textBox7, textBox8, textBox9 } ,
                                { textBox10, textBox11, textBox12, textBox13, textBox14, textBox15, textBox16, textBox17, textBox18 },
                                { textBox19, textBox20, textBox21, textBox22, textBox23, textBox24, textBox25, textBox26, textBox27 },
                                { textBox28, textBox29, textBox30, textBox31, textBox32, textBox33, textBox34, textBox35, textBox36 },
                                { textBox37, textBox38, textBox39, textBox40, textBox41, textBox42, textBox43, textBox44, textBox45 },
                                { textBox46, textBox47, textBox48, textBox49, textBox50, textBox51, textBox52, textBox53, textBox54 },
                                { textBox55, textBox56, textBox57, textBox58, textBox59, textBox60, textBox61, textBox62, textBox63 },
                                { textBox64, textBox65, textBox66, textBox67, textBox68, textBox69, textBox70, textBox71, textBox72 },
                                { textBox73, textBox74, textBox75, textBox76, textBox77, textBox78, textBox79, textBox80, textBox81 }};
            for (int i = 0; i<9; i++)
            {
                char[] home = lines[rnd * 10 + i].ToCharArray();
                for (int j = 0; j<9; j++)
                {
                    table[i, j] = home[j];
                    if(Convert.ToString(home[j]) != " ") boxes[i, j].Text = Convert.ToString(home[j]);
                }
            }
        }

        private void TextChanged(object sender, TextChangedEventArgs e)
        {
            TextBox[,] boxes = { { textBox1, textBox2, textBox3, textBox4, textBox5, textBox6, textBox7, textBox8, textBox9 } ,
                                { textBox10, textBox11, textBox12, textBox13, textBox14, textBox15, textBox16, textBox17, textBox18 },
                                { textBox19, textBox20, textBox21, textBox22, textBox23, textBox24, textBox25, textBox26, textBox27 },
                                { textBox28, textBox29, textBox30, textBox31, textBox32, textBox33, textBox34, textBox35, textBox36 },
                                { textBox37, textBox38, textBox39, textBox40, textBox41, textBox42, textBox43, textBox44, textBox45 },
                                { textBox46, textBox47, textBox48, textBox49, textBox50, textBox51, textBox52, textBox53, textBox54 },
                                { textBox55, textBox56, textBox57, textBox58, textBox59, textBox60, textBox61, textBox62, textBox63 },
                                { textBox64, textBox65, textBox66, textBox67, textBox68, textBox69, textBox70, textBox71, textBox72 },
                                { textBox73, textBox74, textBox75, textBox76, textBox77, textBox78, textBox79, textBox80, textBox81 }};
            // ТЫ СКАЗАЛ СДЕЛАТЬ ЭТО
            for (int i = 0; i < 9; i++)
            {
                for (int j = 0; j < 9; j++)
                {
                    boxes[i, j].Background = Brushes.White;
                }
            }
            //
            try { int G = Convert.ToInt32((sender as TextBox).Text);  }
            catch { (sender as TextBox).Text = ""; return; }
            if((sender as TextBox).Text == "0"){ (sender as TextBox).Text = ""; return;}
            int hor = 0;
            int ver = 0;
            for (int i = 0; i < 9; i++)
            {
                for (int j = 0; j < 9; j++)
                {
                    if (boxes[i, j] == (sender as TextBox))
                    {
                        ver = i;
                        hor = j;
                    }
                }
            }
            for (int i = 0; i < 9; i++)
            {
                if ((sender as TextBox).Text == "")
                {
                    (sender as TextBox).Background = Brushes.White;

                    break;
                }
                if ((sender as TextBox).Text == boxes[i , hor].Text)
                {
                    if (boxes[i, hor] != (sender as TextBox))
                    {
                        boxes[i, hor].Background = Brushes.Red;
                        (sender as TextBox).Background = Brushes.Red;
                    }
                }
                if ((sender as TextBox).Text == boxes[ver, i].Text)
                {
                    if(boxes[ver, i] != (sender as TextBox))
                    {
                        boxes[ver, i].Background = Brushes.Red;
                        (sender as TextBox).Background = Brushes.Red;
                    }
                }
            }
        }
    }
}