I have come code here that writes, modifies, and deletes records from an SQL database using a Car class. The code runs fine but it produces warnings. How do I take this code and its various functions and implement unit tests for this particular piece of code (particularly in VS Code on a Mac)? The code is written in C# and the SQL database has 9 different columns which are the attributes of the Car class
using System;
using Xunit;
using System.Data;
using Microsoft.Data.Sqlite;
using System.Threading.Tasks;
namespace sqlite_app
{
public class Car{
    public string _id;
    public string _year;
    public string _make;
    public string _model;
    public string _mileage;
    public string _is_in_service;
    public string _rental_rate;
    public string _color;
    public string _location;
    public Car(string id, string year, string make, string model, string mileage, string is_in_service, string rental_rate, string color, string location){
            _id=id;
            _year=year;
            _make=make;
            _model=model;
            _mileage=mileage;
            _is_in_service=is_in_service;
            _rental_rate=rental_rate;
            _color=color;
            _location=location;
    }
    public class car_record{
        public static void Main(String[] args){
         
            bool is_true=false;
            string choice="0";
            while(is_true==false){
                System.Console.WriteLine("What would you like to do?");
                System.Console.WriteLine("Enter the following numbers for each choice:");
                System.Console.WriteLine("1. Enter a record");
                System.Console.WriteLine("2. delete a record");
                System.Console.WriteLine("3. modify a record");
                choice = Console.ReadLine();
                if(choice=="1"||choice =="2" ||choice=="3"){
                    is_true=true;
                }
                else{
                    Console.WriteLine("Not a valid choice. Enter 1, 2 or 3");
                }
            }
            if(choice=="1"){
                string id, year, make, model, mileage, is_in_service, rental_rate, color, location;
                System.Console.WriteLine("Assign the car an id number");
                id=Console.ReadLine();
                System.Console.WriteLine("What is the car's year?");
                year=Console.ReadLine();
                System.Console.WriteLine("What is the make?");
                make=Console.ReadLine();
                System.Console.WriteLine("What is the model?");
                model=Console.ReadLine();
                System.Console.WriteLine("What is the current mileage");
                mileage=Console.ReadLine();
                System.Console.WriteLine("Is the car currently in service (type yes or no)?");
                is_in_service=Console.ReadLine();
                System.Console.WriteLine("What is the car's per day rental rate");
                rental_rate=Console.ReadLine();
                System.Console.WriteLine("What is the car's color");
                color=Console.ReadLine();
                System.Console.WriteLine("What location is the car at?");
                location=Console.ReadLine();
                Car car1 = new Car(id, year, make, model, mileage, is_in_service, rental_rate, color, location);
                
                insertdata(car1);
            }
            if(choice=="2"){
                string id_str;
                System.Console.WriteLine("What is the record id?");
                id_str=Console.ReadLine();
                int flag =0;
                deleteRecord(id_str,flag);
            }
            if(choice=="3"){
                string id_string;
                Console.WriteLine("What is the id of the record you would like to update?");
                id_string=Console.ReadLine();
                modifyRecord(id_string);
            }  
        }
    
        public static void insertdata(Car car1)
        {
           var connectionStringBuilder = new SqliteConnectionStringBuilder();
            //Use DB in project directory.  If it does not exist, create it:
            connectionStringBuilder.DataSource = "carRecords.db";
            using (var connection = new SqliteConnection(connectionStringBuilder.ConnectionString))
            {
                connection.Open();
                //Create a table (drop if already exists first):
              
                //Seed some data:
                using (var transaction = connection.BeginTransaction())
                {
                    var insertCmd = connection.CreateCommand();
                    insertCmd.CommandText = "INSERT INTO carRecords VALUES ('" + car1._id+ "','" + car1._year + "','" + car1._make + "','" + car1._model + "','" +car1._mileage+"','"+ car1._is_in_service +"','"+car1._rental_rate+"','"+car1._color+"','"+car1._location+"')";
                    insertCmd.ExecuteNonQuery();
                    transaction.Commit();
                }
                Console.WriteLine("Record Entered");
            }
        }
        
        public static int deleteRecord(string id_num, int flag)
        {
            var connectionStringBuilder = new SqliteConnectionStringBuilder();
            //Use DB in project directory.  If it does not exist, create it:
            connectionStringBuilder.DataSource = "carRecords.db";
            using (var connection = new SqliteConnection(connectionStringBuilder.ConnectionString))
            {
                connection.Open();
                //Create a table (drop if already exists first):
                //Seed some data:
                using (var transaction = connection.BeginTransaction())
                {
                    var insertCmd = connection.CreateCommand();
                    insertCmd.CommandText = "DELETE FROM carRecords WHERE id="+id_num+"";
                    insertCmd.ExecuteNonQuery();
                    transaction.Commit();
                }
                Console.WriteLine("Record Deleted");
                flag=1;
                return flag;
            }
            return flag;
        }
        private static void modifyRecord(string id_string)
        {
            string choose = "0";
            bool iss_true=false;
            while(iss_true==false){
                System.Console.WriteLine("What would you like to do?");
                System.Console.WriteLine("Enter the following numbers for each choice:");
                System.Console.WriteLine("1. Update a car's mileage");
                System.Console.WriteLine("2. Update a car's rental cost per day.");
                System.Console.WriteLine("3. Whether a car is avaiable or not.");
                System.Console.WriteLine("4. Update a car's location.");
                choose = Console.ReadLine();
                if(choose=="1"||choose =="2" ||choose=="3" || choose=="4"){
                    iss_true=true;
                }
                else{
                    Console.WriteLine("Not a valid choice. Enter 1, 2 or 3");
                }
            }
             var connectionStringBuilder = new SqliteConnectionStringBuilder();
            //Use DB in project directory.  If it does not exist, create it:
            connectionStringBuilder.DataSource = "carRecords.db";
            if(choose=="1"){
                Console.WriteLine("What is the new mileage");
                string Mileage=Console.ReadLine();
                using (var connection = new SqliteConnection(connectionStringBuilder.ConnectionString))
                {
                connection.Open();
                //Create a table (drop if already exists first):
                //Seed some data:
                    using (var transaction = connection.BeginTransaction())
                    {
                        var insertCmd = connection.CreateCommand();
                        insertCmd.CommandText = "UPDATE carRecords SET mileage="+Mileage+" WHERE id ="+id_string+"";
                        insertCmd.ExecuteNonQuery();
                        transaction.Commit();
                    }
                }
            }
            if(choose=="2"){
                Console.WriteLine("What is the new cost for rental");
                string Cost=Console.ReadLine();
                using (var connection = new SqliteConnection(connectionStringBuilder.ConnectionString))
                {
                connection.Open();
                //Create a table (drop if already exists first):
                //Seed some data:
                    using (var transaction = connection.BeginTransaction())
                    {
                        var insertCmd = connection.CreateCommand();
                        insertCmd.CommandText = "UPDATE carRecords SET cost="+Cost+" WHERE id ="+id_string+"";
                        insertCmd.ExecuteNonQuery();
                        transaction.Commit();
                    }
                }
            }
            if(choose=="3"){
                Console.WriteLine("is the car avaiable yes or no");
                string is_in_service=Console.ReadLine();
                using (var connection = new SqliteConnection(connectionStringBuilder.ConnectionString))
                {
                connection.Open();
                //Create a table (drop if already exists first):
                //Seed some data:
                    using (var transaction = connection.BeginTransaction())
                    {
                        var insertCmd = connection.CreateCommand();
                        insertCmd.CommandText = "UPDATE carRecords SET avaiable='"+is_in_service+"' WHERE id ='"+id_string+"'";
                        insertCmd.ExecuteNonQuery();
                        transaction.Commit();
                    }
                }
            }
            if(choose=="4"){
                Console.WriteLine("Enter the new location of the car");
                string location=Console.ReadLine();
                using (var connection = new SqliteConnection(connectionStringBuilder.ConnectionString))
                {
                connection.Open();
                //Create a table (drop if already exists first):
                //Seed some data:
                    using (var transaction = connection.BeginTransaction())
                    {
                        var insertCmd = connection.CreateCommand();
                        insertCmd.CommandText = "UPDATE carRecords SET location='"+location+"' WHERE id ='"+id_string+"'";
                        insertCmd.ExecuteNonQuery();
                        transaction.Commit();
                    }
                }
            }
            
            
            Console.WriteLine("Record Updated");
        
        
        }    
    }
}
}    
 
     
    