Question

I have created an instance of a generic collection in C#, and need to pass one of the members of a struct in this collection by reference to a method. Can I use the generic collection’s indexer to select which object’s member I want to modify in the method? I seem to get an error (“Cannot modify the return value of ‘expression’ because it is not a variable”) but what I have is similar to this:

Deque<Card> deck_of_cards = new Deque<Card>();  // standard deck of 52 playing cards (structs)

ModifyRank( ref deck_of_cards[4].rank, 8);  // Changes the rank (field, int) of the 5th card to 8

I’m converting C++ code which is using std::deque and global methods, and I want to preserve as much as I can in terms of syntax. Does anyone know an elegant solution to this problem?

Answer 1

It’s not possible to pass by reference a field of a struct got through an indexer. To modify the content of a struct in such a collection requires separate get and set operations because an indexer can’t return a value by reference (at least not in C# – it might be possible with unverifiable MSIL).

The most straightforward solution to the specific problem would be:

Deque<Card> deck_of_cards = new Deque<Card>();  // standard deck of 52 playing cards
var tmp = deck_of_cards[4];
ModifyRank( ref tmp.rank, 8);  // Changes the rank (int) of the 5th card to 8
deck_of_cards[4] = tmp;