ไม่เข้าใจว่า error เพราะอะไรครับ ช่วยหน่อย "not all code paths return a value"

private bool CheckLoginValidation()
        {
            string dbUsername, dbPassword;
            conStr = @"Data Source=" + Database.dbServer + ";Initial Catalog=" + Database.dbName + ";Persist Security Info=True;User ID=" + Database.DecodeFrom64(Database.dbEncodedUsername) + ";Password=" + Database.DecodeFrom64(Database.dbEncodedPassword) + ";";
            conn = new SqlConnection();
            try
            {
                if (conn.State == ConnectionState.Open)
                    conn.Close();          // ถ้าค้างให้ปิดก่อน
                else
                {
                    conn.ConnectionString = conStr;
                    //MessageBox.Show("not ok");
                    conn.Open();          // เปิดการเชื่อมต่อฐานข้อมูล
                }
                string sqlCommandStr = "select UserId, Name1, Name2, UserName, PassWord, Email, PhoneNumber, Misc, IniSettings from tblUsers where UserName='" + txtUsername.Text + "' and PassWord = '" +Database.EncodeTo64(txtPassword.Text) + "';";
                cmd = new SqlCommand(sqlCommandStr, conn);
                SqlDataReader dr = cmd.ExecuteReader();
                dbUsername = dr["UserName"].ToString();
                dbPassword = dr["Password"].ToString();
                if ((dbUsername == txtUsername.Text) && (dbPassword == Database.EncodeTo64(txtPassword.Text)))
                {
                    UserAccount.UserId = dr["UserId"].ToString();
                    UserAccount.Name1 = dr["Name1"].ToString();
                    UserAccount.Name2 = dr["Name2"].ToString();
                    UserAccount.UserName = dr["UserName"].ToString();
                    UserAccount.PassWord = dr["PassWord"].ToString();
                    UserAccount.PhoneNumber = dr["PhoneNumber"].ToString();
                    UserAccount.Email = dr["Email"].ToString();
                    UserAccount.Misc = dr["Misc"].ToString();
                    UserAccount.IniSettings = Convert.ToInt16(dr["IniSettings"]);
                    return true;
                }
                else
                {
                    return false;
                }
            }
            catch (Exception ex)
            {
                MessageBox.Show("Error from CheckLoginValidation()=" + ex.Message);
            }
            finally
            {
                conn.Close();
            }
        }

private void btnLogin_Click(object sender, EventArgs e)
        {
            if (CheckLoginValidation())
            {
                this.DialogResult = DialogResult.OK;
                this.Close();
            }
            else
            {
                MessageBox.Show("ชื่อผู้ใช้หรือรหัสผ่านผิดพลาด","ลงชื่อเข้าใช้",MessageBoxButtons.OK,MessageBoxIcon.Error);
                return;
            }
        }