Question

I have a struct:

struct Order
{
      public string orderNumber;
      public string orderDetail;
}

I then assign some values in Form1 and try to pass them by reference (ref) to Form2:

(Form1)
Order order = new Order();
order.orderNumber = "1234";
order.orderDetail = "Widgets";
Form2 frm2 = new Form2(ref order);

Is it possible to store the values in Form2 so that when Form2 is completed processing the values it will return the updated struct values to Form1?

In this scenario there would be a button that would close the form after validating the data.

Answer 1

One pattern that’s sometimes useful is to define a class something like:

class Holder<T> {public T value;}

Such a class makes it possible to pass and mutate value types with code that requires reference types. Using such an approach, a routine which accepted a structure by reference and was supposed to pop up a modal dialog and fill in the structure from it, could create a Holder<thatStructType>, pass that to the form, and then copy the data from that Holder back to the passed-in reference. While in your particular scenario, it may be better to have the data-holding thing simply be a class, structures have the advantage that one can know that no outstanding references to them exist; if a routine declares a structure and passes it by reference to some outside code, then once that code returns the values in that structure won’t change unless or until the routine writes them itself or passes the structure by reference to some other code. By contrast, if a routine exposes a class reference to outside code, there’s no telling what that code may do with it.

Incidentally, the Holder class is also useful in a number of other scenarios. For example, if one has a Dictionary<String, Holder<Integer>> myDict, one may use Threading.Interlocked.Increment(myDict(myKey).Value)) to perform a thread-safe increment of the indicated item, much more efficiently than would be possible with a Dictionary<String, Integer>.