C# incrementing static variables upon instantiation

Question

I have a bankAccount object I'd like to increment using the constructor. The objective is to have it increment with every new object the class instantiates.

Note: I've overriden the ToString() to display the accountType and accountNumber;

Here is my code:

public class SavingsAccount
{
    private static int accountNumber = 1000;
    private bool active;
    private decimal balance;

    public SavingsAccount(bool active, decimal balance, string accountType)
    {
        accountNumber++;
        this.active = active;
        this.balance = balance;
        this.accountType = accountType;
    }
}

Why is it that when I plug this in main like so:

class Program
{
    static void Main(string[] args)
    {
        SavingsAccount potato = new SavingsAccount(true, 100.0m, "Savings");
        SavingsAccount magician = new SavingsAccount(true, 200.0m, "Savings");
        Console.WriteLine(potato.ToString());
        Console.WriteLine(magician.ToString());
    }
}

The output I get does not increment it individually i.e.

savings 1001
savings 1002

but instead I get:

savings 1002
savings 1002

How do I make it to be the former and not the latter?

Solution 1

Because a static variable is shared among all instances of the class. What you want is a static variable to keep the global count and a non-static variable to save the current count at the time of instantiation. Change your code above to:

public class SavingsAccount
{
    private static int accountNumber = 1000;
    private bool active;
    private decimal balance;
    private int myAccountNumber;

    public SavingsAccount(bool active, decimal balance, string accountType)
    {
        myAccountNumber = ++accountNumber;
        this.active = active;
        this.balance = balance;
        this.accountType = accountType;
    }
}

class Program
{
    static void Main(string[] args)
    {
        SavingsAccount potato = new SavingsAccount(true, 100.0m, "Savings");
        SavingsAccount magician = new SavingsAccount(true, 200.0m, "Savings");
        Console.WriteLine(potato.ToString());
        Console.WriteLine(magician.ToString());
    }
}

And then in your ToString() overload you should print myAccountNumber instead of the static variable.

Solution 2

Because it is a static variable. It is shared by all instances of the class. You need to save off the incremented value to an instance variable.

public class SavingsAccount
{
    private static int accountNumberCounter = 1000;
    private int accountNumber;
    private bool active;
    private decimal balance;

    public BankAccount(bool active, decimal balance, string accountType)
    {
        accountNumberCounter++;
        this.accountNumber = accountNumberCounter;

        this.active = active;
        this.balance = balance;
        this.accountType = accountType;
    }

    public string ToString() 
    {
        return String.Format("{0} {1}", accountType, accountNumber);
    }
}