Nice technique for modifying a subset of a List
March 4, 2009
One of my team members sent in the following piece of code, which is clearly intended to update the OrderNumber field for all objects in a List<T> of objects that match a particular productId. I took one look at it and thought "you can't do that!". But then I let the automated test run to see what happens... lo and behold, it worked. And well it should, once I thought about it.
list = cardQueue.FindAll(delegate(MyObject obj1)
{
if (obj1.ProductId == processedProductId)
{
obj1.OrderNumber = orderNumber;
}
return true;
});
It had never occurred to me that you can modify an object inside of a Predicate<T> method. But now that I look at it, why not? After all, in C#, objects are passed around by reference, not value – when the method tests obj1 to see if its ProductId value is the one we are searching for, then that is the "real" object, not just a copy. Thus the real object can be modified.