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Nice technique for modifying a subset of a List<T>

· 2 min read

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.

* Postscript 2025: the old link explaining this is no dead. My statement is confusing: "In C#, parameters to methods are passed by value..." (source). However, the same page also says "...the value is a reference to a block of memory that stores an instance of the type." So yes, it passes a value, but the value is itself a reference. Therefore the original object is mutable.

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