Complete the giftcard class shown below. a giftcard keeps track of the current balance on the card. a gift card starts with a beginning balance. a giftcard can be used to make a purchase, funds can be added to a giftcard, and the balance can be checked at any time. the balance can never be negative. if the amount added to the card is $100 or more, a bonus of $10 is also added.

public class giftcard
{
// declare instance variables

// precondition: the starting balance is not negative
// postcondition: all instance variables are initialized
public giftcard(double startbal)
{

}

// postcondition: amount is subtracted from balance
// if there are insufficient funds, the balance is set to zero and a message
// is displayed to indicate how much of the transaction is still owed
public void spendfunds(double amount)
{

}
// postcondition: amount is added to balance, if the amount is at least 100 dollars,
// 10 dollars is added
public void addfunds(double amount)
{

}

// postcondition: the current balance is returned
public double checkbalance()
{

}
}

I solved this program using C# and Visual Studio. The output of this program is given attached image. I initialized the start balance with 300$.

the code of this program is given in explanation section

Explanation:

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

namespace GiftCard

{

   class Program

   {

       static void Main(string[] args)

       {

           Program a = new Program();

            //Console.WriteLine("Please enter the starting balance");// you can uncomment this statement if you enter startbalance at run time

            //double startingBalance = Convert.ToDouble(Console.ReadLine());//initializing starting balance at run time

           double startingBalance = 300; //let suppose we enter 300

           a.GiftCard(startingBalance);//balance is set to starting blance value i.e. enter by user

           Console.ReadLine();// here we pause, and check variables values from GiftCard constructor

           a.spendFunds(100);// here we assume to spend 100$

           Console.ReadLine();//here we check the spendFund function and about its values

          a.addFunds(600);// here we enter fund 600$

           Console.ReadLine();

           a.checkBalance(); //here we are checking balance;    

       }

       // declare instance variables

       // precondition: the starting balance is not negative

       // postcondition: all instance variables are initialized

       private double balance=0;// keep the balance

       private double purchase;//how much you spend or purchasing value

       public void GiftCard(double startBal)

       {

           balance = startBal;

           if (balance < 0)

           {

               Console.WriteLine("Balance is negative, Please enter positive balance");

               balance = 0;

           }

           else

           {

               balance = startBal;

               Console.WriteLine(balance);

           }

       }

       // postcondition: amount is subtracted from balance

       // if there are insufficient funds, the balance is set to zero and a message

       // is displayed to indicate how much of the transaction is still owed

       public void spendFunds(double amount)

       {

             purchase = balance - amount;// suppose you spend 300 that mean you have purchased of 300

           if (purchase <0) //if your spending is more than your balance

           {

               double owed = amount - balance;// how much amount you need more for this transaction

               balance = 0;

               Console.WriteLine("The transaction is still owed "+owed+"$");

           }

           else if(purchase==0) // if after doing purchase, your balance is equal to zero

           {

               Console.WriteLine("you have no balance after this transaction");

           }

           else

           {

               balance = balance - amount;// if your purchase is less than the balance

               Console.WriteLine("your balance after spending " + amount +"$ is " + balance+"$");

           }

       }

       // postcondition: amount is added to balance, if the amount is at least 100 dollars,

       // 10 dollars is added

       public void addFunds(double amount)

       {

           if (amount >= 100)// if you add fund more than 100$

           {

               balance = balance + amount + 10; //then add the amount + 10$ as bonus into already existing balance

               Console.WriteLine("balance is after adding funds i.e " + amount +"$  is "+ balance+"$");

           }

           else// if added fund is less than 100$

           {

               balance = balance + amount;

           }

       }

// postcondition: the current balance is returned

       public double checkBalance()//check balance anytime

       {

           Console.WriteLine("balance is "+balance+"$");

           Console.ReadLine();

           return balance;

       }

   }

}

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hope this , mark me brainliest.