Question

This function is supposed to set descending order numbers on an IEnumerable<Order>, but it doesn’t work. Can anyone tell me what’s wrong with it?

private void orderNumberSetter(IEnumerable<Order> orders)
{
    var i = 0;            

    Action<Order, int> setOrderNumber = (Order o, int count) =>
    {
        o.orderNumber = i--;
    };

    var orderArray = orders.ToArray();
    for (i = 0; i < orders.Count(); i++)
    {
        var order = orderArray[i];
        setOrderNumber(order, i);
    }            
}

Answer 1

You are re-using i as loop variable and i gets modified in your setOrderNumber lambda – don’t modify i – it’s unclear what you meant to do, maybe the following:

Action<Order, int> setOrderNumber = (Order o, int count) =>
{
    o.orderNumber = count;
};

If the above is the case you could have achieved that much, much easier though, your code seems unnecessarily complex, i.e:

for (i = 0; i < orderArray.Length; i++)
{
    orderArray[i].orderNumber = i;
} 

or even simpler without having to create an array at all:

int orderNum = 0;
foreach(var order in orders)
{
   order.orderNumber = orderNum++;
}

Edit:

To set descending order numbers, you can determine the number of orders first then go backwards from there:

int orderNum = orders.Count();
foreach(var order in orders)
{
   order.orderNumber = orderNum--;
}

Above would produce one based order numbers in descending order. Another approach, more intuitive and probably easier to maintain is to just walk the enumeration in reverse order:

int orderNum = 0;
foreach(var order in orders.Reverse())
{
   order.orderNumber = orderNum++;
}