c＃-根据每个列表项的布尔值添加或减去一个值

Question

Well, I will explain what I'm trying to do.
I have a List for my purchasers and also a listbox. When I add a purchaser to the listbox, he is also added to the List called Purchasers. In this List, I have a value for the name of the purchaser and a bool to say if he paid or not. See my List and the class for it:

public class Purchaser
{
     public string Name { get; set; }
     public bool Paid { get; set; }
}

List<Purchaser> Purchasers = new List<Purchaser>();

Now I will explain what I'm planning to do: all the purchasers must pay $5. I have a checkbox called box_Paid and when I check or uncheck it, the variable paid is turned in true or false for the actual selected purchaser in my listbox.
I have a limit in the listbox for the purchasers: 14 in total, and a value in one label called label_AmountLeft that is 70$.
70/14 = 5$. Each purchaser needs to pay 5$. What I need now is, when I turn the variable for determinate purchaser using the box_Paid.CheckedChanged event, the label with the value 70 decreases 5 units. Example, I've checked my checkbox when purchaser Tom is selected in the listbox. The Tom's variable paid value now will be true and the value of the label_AmountLeft will now be 65. I check now for Dylan, the value goes to 60. If I uncheck the textbox for Tom, the values goes to 65, because Dylan's variable paid is true . If I uncheck the checkbox for both purchasers, the variables will be false and the label_AmountLeft value will return to 70.
I was trying using:

double a, b = 5, rest;
private void box_Paid_CheckedChanged(object sender, EventArgs e)
{
    if (box_Paid.Checked == true)
    {
        Purchaser p = Purchasers[listDOF.SelectedIndex];
        p.Paid = true;
        a = System.Convert.ToDouble(label_AmountLeft.Text);
        rest = a - b;
        label_AmountLeft.Text = System.Convert.ToString(rest);
    }
    else
    {
        Purchaser p = Purchasers[listDOF.SelectedIndex];
        p.Paid = false;
        a = System.Convert.ToDouble(label_AmountLeft.Text);
        rest = a + b;
        label_AmountLeft.Text = System.Convert.ToString(rest);
    }
}

But this do not worked. I think this issue is hard to understand, but I hope someone is able to help me. Thanks in advance!

Answer 1

If you were updating your list properly, then the sum could simply be calculated using:

var sum = Purchasers.Where(p => p.Paid).Count() * 5;

Or, you could have a separate field for the sum to avoid parsing the text box value each time:

// oh, and btw, don't use double when working with money
private decimal _sum;
private void box_Paid_CheckedChanged(object sender, EventArgs e)
{
    if (box_Paid.Checked == true)
    {
        _sum += 5;
    }
    else
    {
        _sum -= 5;
    }

    label_AmountLeft.Text = _sum.ToString();
}

But the weird thing is that you're not updating items from your original list at all, but instead creating new Purchaser instances each time (which are collected as soon as you return from the event handler).

So, I am guessing the code should look something like:

private void box_Paid_CheckedChanged(object sender, EventArgs e)
{ 
    // get the currently selected item
    var selectedPurchaser = Purchasers[listDOF.SelectedIndex];

    // set its state
    selectedPurchaser.Paid = box_Paid.Checked;

    // calculate the sum and display it
    UpdateSum();
}

private void UpdateSum()
{
    // calculate the paid sum
    var paidSum = Purchasers.Where(x => x.Paid).Count() * 5;

    // get the amount left
    var amountLeft = 70 - paidSum;

    // update the text box (using whatever formatting you prefer)
    label_AmountLeft.Text = string.Format("${0:0.00}", amountLeft);
}