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
    {
        Brush color;
        bool[,] coinc = new bool[9, 9];
        bool[,] start = new bool[9, 9];
        public TextBox[,] boxes()
        {
            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 }};
            return boxes;
        }
        public MainWindow()
        {
            InitializeComponent();
            color = boxes()[1,1].BorderBrush;
            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);
            boxes();
            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]);
                        boxes()[i, j].IsReadOnly = true;
                        start[i, j] = true;
                        ColorChange(i, j, 3);
                    }
                }
            }
        }
        public void kykarekalPetyx(TextBox sender, int hor, int ver)
        {
            if (sender.Text != "")
            {
                int[] COUB = { 0, 1, 2 };
                for (int i = 0; i < 9; i++)
                {
                    if (sender.Text == boxes()[i, hor].Text)
                    {
                        if (boxes()[i, hor] != sender)
                        {
                            coinc[ver, hor] = true;
                            coinc[i, hor] = true;
                            ColorChange(ver, hor, 2);
                            ColorChange(i, hor, 2);
                        }
                    }
                    if (sender.Text == boxes()[ver, i].Text)
                    {
                        if (boxes()[ver, i] != sender)
                        {
                            ColorChange(ver, hor, 2);
                            ColorChange(ver, i, 2);
                            coinc[ver, hor] = true;
                            coinc[ver, i] = true;
                        }
                    }
                }
                //
                double temp1 = ver / 3;
                double temp2 = hor / 3;
                int temp3 = COUB[(int)Math.Floor(temp1)] * 3;
                int temp4 = COUB[(int)Math.Floor(temp2)] * 3;
                for (int i = 0; i < 3; i++)
                {
                    for (int j = 0; j < 3; j++)
                    {
                        if (sender.Text == boxes()[temp3+i, temp4+j].Text)
                        {
                            if (boxes()[temp3 + i, temp4 + j] != sender)
                            {
                                ColorChange(temp3+i, temp4+j, 2);
                                ColorChange(ver, hor, 2);
                                coinc[temp3 + i, temp4 + j] = true;
                                coinc[ver, hor] = true;
                            }
                        }
                    }
                }
            }
        }
        public void ColorChange(int ver, int hor, int G)
        {
            if (G==1)
            {
                boxes()[ver, hor].BorderBrush = color;
            }
            else if(G==2)
            {
                boxes()[ver, hor].BorderBrush = Brushes.Red;
            }
            else
            {
                boxes()[ver, hor].Foreground = Brushes.DimGray;
            }
        }
        private void TextChanged(object sender, TextChangedEventArgs e)
        {
            if ((sender as TextBox).Text != "")
            {
                if ((Convert.ToChar((sender as TextBox).Text) < '1') || (Convert.ToChar((sender as TextBox).Text) > '9'))
                {
                    (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++)
                {
                    ColorChange(i, j, 1);
                    if (coinc[i, j])
                    {
                        kykarekalPetyx(boxes()[i, j], j, i);
                    }
                    if (boxes()[i, j] == (sender as TextBox))
                    {
                        ver = i;
                        hor = j;
                    }
                }
            }
            kykarekalPetyx((sender as TextBox), hor, ver);
        }
    }
}