Yazılım | Programlama C# Kayıt formunda hata alıyorum.

        private bool CheckTrChars()
        {
            char[] trChars = { 'ö', 'Ö', 'ü', 'Ü', 'ç', 'Ç', 'İ', 'ı', 'Ğ', 'ğ', 'Ş', 'ş' };
            string[] unkownChars = { "?", ",", ".", "-", "*", "/", "#", "(", ")", "+", "-", "@", "é", ";", ":", "<", ">", "!", "^", "'", "$", "%", "&", "=", "_", "½", "{", "]", "}", "[", "|"  };
            for (int i = 0; i < trChars.Length; i++)
            {
                if (txtUsername.Text.Contains(trChars[i]) || txtPassword.Text.Contains(trChars[i]) || txtUsername.Text.Contains(unkownChars[i]) || string.IsNullOrEmpty(txtUsername.Text) || string.IsNullOrEmpty(txtPassword.Text))
                {
                    MessageBox.Show("Username or Password can not be blank and can not contains Turkish characters or special characters.", "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
                    break;
                }
            }
            return true;
        }

try
            {
                if (CheckTrChars())
                {

                }
                else
                {
                    // open the connection
                    connectDB.Open();
                    // I defined the insert command and wrote the insert query.
                    MySqlCommand ekle = new MySqlCommand("insert into users (username,password) values ('" + txtUsername.Text + "','" + txtPassword.Text + "')", connectDB);
                    // running the query.
                    object sonuc = null;
                    sonuc = ekle.ExecuteNonQuery(); // the query worked, and the returned value passed to the object type variable if the variable is not empty, it is not added if it is empty.
                    if (sonuc != null)
                    {
                        MessageBox.Show("You have successfully registered.", "Information", MessageBoxButtons.OK, MessageBoxIcon.Information);
                    }
                    else
                    {
                        MessageBox.Show("Error while registering.", "Warning", MessageBoxButtons.OK, MessageBoxIcon.Warning);
                        // close the connection
                        connectDB.Close();
                    }
                }
            }
            catch (Exception HataYakala)
            {
                MessageBox.Show("Error: " + HataYakala.Message, "Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }