The value change in the integer array in one index works in one case, and in another it doesn’t, why?

Question

   private void btnRodaLabirinto_Click(object sender, EventArgs e)
    {
        char[,] Lab = new char[10, 10];
        int[,] Lab2 = new int[10, 10];
        int I = 0, J = 0;
        Random NumerosRandomicos = new Random();

        //abaixo, setando labirinto com 0 e 1
        for (I = 0; I < 10; I++)
        {
            for (J = 0; J < 10; J++)
            {
                Lab2[I, J] = NumerosRandomicos.Next(0, 2);
                if (I == 9 && J == 9) Lab2[I, J] = 3;
                if (I == 0 && J == 0) Lab2[I, J] = 3;
            }
        }

        //Printando na Texbox
        for (I = 0; I < 10; I++)
        {
            for (J = 0; J < 10; J++)
            {
                txtLabirinto.Text = txtLabirinto.Text +
                    Convert.ToString(Lab2[I, J]).ToString();
            }
            txtLabirinto.AppendText("\r\n");
        }

    }

I’ve done away with it, like this:

   I = 0;
   J = 0;
   Lab2[I, J] = 3;

But unsuccessfully too, someone can help me?