Namespace not recognized even its added

im making an AR application using unity and vuforia as my graduation project, so im not so experienced in unity and C#.

everything is going smooth till i reached DB, now im trying to connect MySql DB to Unity and found this code some where in internet.

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 using System.Windows;
 using MySql.Data.MySqlClient;

 public class DBConnect : MonoBehaviour
 {

 private MySqlConnection connection;
 private string server;
 private string database;
 private string uid;
 private string password;

 //Constructor
 public DBConnect()
 {
     Initialize();
 }


 //Initialize values
 private void Initialize()
 {
     server = "localhost";
     database = "3datlas_db";
     uid = "root";
     password = "root";
     string connectionString;
     connectionString = "SERVER=" + server + ";" + "DATABASE=" +
     database + ";" + "UID=" + uid + ";" + "PASSWORD=" + password + ";";

     connection = new MySqlConnection(connectionString);
 }

 //open connection to database
 private bool OpenConnection()
 {


     try
     {
         connection.Open();
         return true;
     }
     catch (MySqlException ex)
     {
         //When handling errors, you can your application's response based 
         //on the error number.
         //The two most common error numbers when connecting are as follows:
         //0: Cannot connect to server.
         //1045: Invalid user name and/or password.
         switch (ex.Number)
         {
             case 0:
              //   MessageBox.Show("Cannot connect to server.  Contact administrator");
                 break;

             case 1045:
           //      MessageBox.Show("Invalid username/password, please try again");
                 break;
         }
         return false;
     }
 }

 //Close connection
 private bool CloseConnection()
 {
     try
     {
         connection.Close();
         return true;
     }
     catch (MySqlException ex)
     {
         //MessageBox.Show(ex.Message);
         return false;
     }
 }

 //Insert statement
 public void Insert()
 {

 string query = "INSERT INTO tableinfo (name, age) VALUES('John Smith', '33')";

     //open connection
     if (this.OpenConnection() == true)
     {
         //create command and assign the query and connection from the constructor
         MySqlCommand cmd = new MySqlCommand(query, connection);

         //Execute command
         cmd.ExecuteNonQuery();

         //close connection
         this.CloseConnection();
     }
 }

 //Update statement
 public void Update()
 {
     string query = "UPDATE tableinfo SET name='Joe', age='22' WHERE name='John Smith'";

     //Open connection
     if (this.OpenConnection() == true)
     {
         //create mysql command
         MySqlCommand cmd = new MySqlCommand();
         //Assign the query using CommandText
         cmd.CommandText = query;
         //Assign the connection using Connection
         cmd.Connection = connection;

         //Execute query
         cmd.ExecuteNonQuery();

         //close connection
         this.CloseConnection();
     }
 }

 //Delete statement
 public void Delete()
 {
     string query = "DELETE FROM tableinfo WHERE name='John Smith'";

     if (this.OpenConnection() == true)
     {
         MySqlCommand cmd = new MySqlCommand(query, connection);
         cmd.ExecuteNonQuery();
         this.CloseConnection();
     }
 }

 //Select statement
 public List<string>[] Select()
 {
     string query = "SELECT * FROM tableinfo";

     //Create a list to store the result
     List<string>[] list = new List<string>[3];
     list[0] = new List<string>();
     list[1] = new List<string>();
     list[2] = new List<string>();

     //Open connection
     if (this.OpenConnection() == true)
     {
         //Create Command
         MySqlCommand cmd = new MySqlCommand(query, connection);
         //Create a data reader and Execute the command
         MySqlDataReader dataReader = cmd.ExecuteReader();

         //Read the data and store them in the list
         while (dataReader.Read())
         {
             list[0].Add(dataReader["id"] + "");
             list[1].Add(dataReader["name"] + "");
             list[2].Add(dataReader["age"] + "");
         }

         //close Data Reader
         dataReader.Close();

         //close Connection
         this.CloseConnection();

         //return list to be displayed
         return list;
     }
     else
     {
         return list;
     }
 }






 }

which is working till i found few errors: 1- Assets/DBconnection.cs(6,7): error CS0246: The type or namespace name MySql' could not be found. Are you missing an assembly reference? 2- Assets/DBconnection.cs(11,13): error CS0246: The type or namespace name MySqlConnection' could not be found. Are you missing an assembly reference? 3- Assets/DBconnection.cs(5,14): error CS0234: The type or namespace name Windows' does not exist in the namespace System'. Are you missing an assembly reference?

i have already added the necessary references but the problem is still there. i tried to search the internet but all of them saying to add references or to copy the necessary Dll files to assets folder in unity which i tried but still didn't work.

help plz :(