Thanks to everyone who replied to my question, and especially to Peter for the detailed options. I
think I will use the method I suggested, and see how it goes.
Jon
Jon said:
My main form opens up another form, and from this other form, I'd like to access things in the
main
form. The problem is that although I know the name of the class of the main form (FormMain) I
don't
know the name of the instance of it since it was generated by the VS C# Express 2005 designer. In
program.cs, I notice that there is the line:
Application.Run(new FormMain());
I guess I could replace this with:
FormMain fMain = new FormMain()
Application.Run(fMain);
although since it was generated by VS, I'm a bit hesitant of any knock-on effects.
What you propose is fine.
That, and the rest of this post, assumes you want a specific,
easy-to-access, direct way to get at your form instance. In many cases,
you may find that the Application.OpenForms property is sufficient. You
would have to get the collection and then enumerate that list looking
for the form you want. It's not quite as convenient to _use_ as the
above techniques, but it certainly would be easier to implement
initially, since all of that work has already been done for you (in
other words, the "initial implementation" is zero effort on your part
).
I don't think making a singleton as suggested by the other reply is
strictly necessary, but you might prefer it. Even creating a singleton
class wrapper for your form though, you will need to modify the Program
class as you've suggested, since the call to the Application.Run()
method would need to use the singleton class to retrieve the form instance.
An alternative that wouldn't require changing the Program class (not
that I think there's anything wrong with doing so) would be to create a
singleton-like design in your FormMain class itself. It wouldn't be a
true singleton, because you'd have to provide a public constructor that
the Program class can use. But you could still provide a static
accessor like a singleton has, and even throw an exception in the
constructor if someone tries to instantiate more than one, to enforce
the singleton nature:
class MainForm : Form
{
static MainForm _instance;
static public MainForm Instance
{
get { return _instance; }
}
public MainForm()
{
if (_instance != null)
{
throw new NotSupportedException("Only one instance of
MainForm allowed");
}
_instance = this;
}
}
And if you don't really want the form to be a singleton, for whatever
reason, you could instead change the above as follows:
class MainForm : Form
{
static List<MainForm> _instances = new List<MainForm>();
static public MainForm[] Instances
{
get { return _instances.ToArray(); }
}
public MainForm()
{
_instances.Add(this);
}
protected override void Dispose(bool fDisposing)
{
if (fDisposing)
{
_instances.Remove(this);
}
base.Dispose(fDisposing);
}
}
This lets you get a complete list of the instances. A couple of notes
on the above:
1) I think the Dispose() method is correct, but don't take my word
for it.
I don't have to write Dispose() methods often enough for me
to be sure that's an appropriate way to handle the end of the object's
lifetime.
2) Using the above, it would be critical that you do properly
dispose of unused forms. In particular, if you used the design with
forms that are used as dialogs (they are shown with ShowDialog()), the
form will never be disposed and release unless you do so explicitly,
because the instance list retains a reference to it.
You should be writing correct code that disposes dialog forms anyway, so
#2 shouldn't be an issue. It's just that the design short-cuts the
finalizer fail-safe built into C#, because the form instance will always
be reachable and thus not eligible for garbage collection and finalizing.
Of course, depending on how you use it, that could be a good thing. You
could easily build on top of the above pattern a dialog form cache that
retrieves already-instantiated instances of your dialog forms. One
reason you might do that is if you want previous user entries in the
dialog form to be retained, but don't want to have to write the code to
do that explicitly.
Anyway, have I sufficiently confused the matter yet?
Pete
