Tic Tac Toe - Detecting win, lose, or draw

Question

This is a tic tac toe generator. Computer vs. Computer only, a little different than the usual Player vs. Computer. I have most of my code written for this, but the issue I am having is sometimes when I generate the game, the whole board fills up and there will be a line of X's and a line of O's and it will come up as a tie. Sometimes there is two lines of X's or two lines of O's generated, and the game doesn't stop after the first line with 3 in a row...any insights? Thank you.

namespace TicTacToe
{
    public partial class Form1 : Form
    {
        private Random rn = new Random();

        const int SIZE = 9;
        char[] cell = new char[SIZE];
        char firstPlayer = ' ', secondPlayer = ' ';

        private void button1_Click(object sender, EventArgs e)
        {
            //Clear the labels and starting values

            for (int i = 0; i < SIZE; i++)
            {
                cell[i] = ' ';
            }
            label10.Text = "";

            //Pick X or O to go first
            switch (rn.Next(2))
            {
                case 0: firstPlayer = 'O'; secondPlayer = 'X'; break;
                case 1: firstPlayer = 'X'; secondPlayer = 'O'; break;
            }

            //Get five non-repeating numbers from 0 to 8
            int[] positions = new int[5];
            positions[0] = rn.Next(9);
            for (int i = 1; i < 5; i++)
            {
                int temp = rn.Next(9);
                for (int j = 0; j < i; j++)
                {
                    if (temp == positions[j])
                    {
                        i--;
                        break;
                    }
                    else
                    {
                        positions[i] = temp;
                    }
                }
            }

            //Set each position found to have first players letter
            for (int i = 0; i < 5; i++)
            {
                cell[positions[i]] = firstPlayer;
            }

            for (int i = 0; i < SIZE; i++)
            {
                if (cell[i] != firstPlayer)
                {
                    cell[i] = secondPlayer;
                }
            }
            //Place cell values into the labels
            label1.Text = cell[0].ToString();
            label2.Text = cell[1].ToString();
            label3.Text = cell[2].ToString();
            label4.Text = cell[3].ToString();
            label5.Text = cell[4].ToString();
            label6.Text = cell[5].ToString();
            label7.Text = cell[6].ToString();
            label8.Text = cell[7].ToString();
            label9.Text = cell[8].ToString();

            //Check for a winner
            switch(checkWinner())
            {
                case 'T' : label10.Text = "It's a tie!"; break;
                case 'O' : label10.Text = "O Wins!"; break;
                case 'X' : label10.Text = "X Wins!"; break;
                default: label10.Text = "This will never appear"; break;
            }
        }

        private char checkWinner()
        {
            //return either 'T' for tie, 'O' for O wins, and 'X' for X wins
            char winner = ' ';
            int winning_line = 0;
            //check for a row win
            if(cell[0].Equals(cell[1]) && cell[0].Equals(cell[2]))
            {
                winning_line++;
                winner = cell[0];
            }
            if (cell[3].Equals(cell[4]) && cell[3].Equals(cell[5]))
            {
                winning_line++;
                winner = cell[3];
            }
            if (cell[6].Equals(cell[7]) && cell[6].Equals(cell[8]))
            {
                winning_line++;
                winner = cell[6];
            }

            //check for column wins
            if (cell[0].Equals(cell[3]) && cell[0].Equals(cell[6]))
            {
                winning_line++;
                winner = cell[0];
            }
            if (cell[1].Equals(cell[4]) && cell[1].Equals(cell[7]))
            {
                winning_line++;
                winner = cell[1];
            }
            if (cell[2].Equals(cell[5]) && cell[2].Equals(cell[8]))
            {
                winning_line++;
                winner = cell[2];
            }
            //check for diagonal winner
            if (cell[0].Equals(cell[4]) && cell[0].Equals(cell[8]))
            {
                winning_line++;
                winner = cell[0];
            }
            if (cell[2].Equals(cell[4]) && cell[2].Equals(cell[8]))
            {
                winning_line++;
                winner = cell[2];
            }

            if (winning_line == 0 || winning_line > 1)
            winner = 'T';

            return winner;
        }

        public int i { get; set; }
    }
}

Answer 1

if (winning_line == 0 || winning_line > 1)

If there are two lines, it will r port a tie. If you want to stop when a line is made, you needto check for a winner after each move, not after the entire board has been filled.