How to use a delegate to get a reference to an item in a list

Question

Currently I have a list containing structs, and I use a delegate to find a struct in the list as such:

stTest item = myList.Find(delegate(stTest test) 
{
   return ( string.Compare(test.StringA, somestring, true) == 0 );
});

This has been working perfectly for getting an item, but it seems that it returns a copy of the item it found and not the actual item, so if I modify fields in the item (which I do), the updates don't take effect on the item in the list.

Is there any way to get the delegate to return a reference to the item in the list?

Answer 1

If you are trying to mutate the object in-place, you can "fake it" with a slightly nicer syntax by employing an extension method like this:

public delegate void Mutator<T>(ref T arg) where T : struct;

public static void FindAndMutate<T>(
    this IList<T> self,
    Predicate<T> predicate,
    Mutator<T> mutator) where T : struct
{
    if (self == null) { throw new ArgumentNullException("self"); }
    if (predicate == null) { throw new ArgumentNullException("predicate"); }
    if (mutator == null) { throw new ArgumentNullException("mutator"); }

    for (int i = 0; i < self.Count; ++i) {
        var value = self[i];

        if (predicate(value)) {
            mutator(ref value);

            self[i] = value;
        }
    }
}

The usage would look something like this:

struct AStruct
{
    public string String;
    public int Int;
}

var list = new[] {
    new AStruct() { String = "hello", Int = 0},
    new AStruct() { String = "world", Int = 1}
};

list.FindAndMutate(i => i.Int == 0, (ref AStruct i) => i.String = "goodbye");

This is obviously not a perfect solution, and does involve copying of the struct inside of the extension method.

Really, if you need reference semantics you should be using a reference type instead of a value type.

Answer 2

item=myList.FirstOrDefault(entry=> 
     string.Compare(entry.StringA, someString, true)==0);

Note that struct is not a reference type, you cannot do that, unless you want to box it into a reference type, but that will bring additional overhead.

If you really want to use reference, consider to repalce your struct with class.

Answer 3

如果将列表成员从struct更改为class ，它将起作用，因为始终会复制结构。