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Winforms: How to dispatch properly collection changes when bound to UI Controls?

I'm facing some issues when I'm trying to deal with IBindingList inherited collections that are modified by a non-UI Thread and are at the same time bound to the DataSource Property of some Winforms Controls (e.g. an old fashion DataGridView).
My point is that Invoke / Begin are not suitable in my solution, cause the collections are not supposed to be really aware of the controls they are bound to.
Is there any other elegant solution than passing the UI Dispatcher or the appropriate SynchronizationContext reference to the collection which is going to be modified later on?
I was thinking about how one can detect whether that the current Thread is the UI accessing to the collection? If could know that,
therefore I could select to dispatch or not the collection modifications to the UI Thread. This solution still does not prevent to pass the reference of the UI SynchronizationContext or Dispatcher, though.
It would be nice to have real static properties which could give access, instead we have Dispatcher.CurrentDispatcher and WinformsSynchronizationContext that actually cannot be accessed from another thread... don't really get the point of the design if we still need to pass the reference from the UI Thread then. But I guess there is some reasons behind the scenes.
I may misunderstood and twist the definitions of some terms, so feel free to correct me if that's the case. I do not want to pretend that I'm expert or whatsoever.
The fact that delegating systematically everything to the UI seems a bit too much and might somehow overload the message pump cause in this scenario it would be better to create the collection right beforehand within the UI scope but then it would loose the interest of delegating the modifications among several threads.
I know that IBindingList is going to reflect through ListChanged (which is subscribed via the Control bound to a IBindingList inherited collection) the collection changes.
Basically those changes are going to be triggered by the non-UI Thread and that basically the very bottom line. Although, I'm bit frustrated by the lack of elegant solutions.
By the way, I am not willing to use the async/await and neither some special events of the BackgroundWorker that automatically Marhsall to the UI Thread when they are triggered. Maybe I should have a look at http://referencesource.microsoft.com/
Please find below the results of my investigations about the UI Marshalling:
public static class Program
{
[STAThread]
public static void Main(params String[] arguments)
{
var bindingList = new BindingList<Record>();
Action<Object> action = (argument) =>
{
var synchronizationContext = argument as SynchronizationContext;
var dispatcher = argument as Dispatcher;
if ((synchronizationContext == null) && (dispatcher == null))
{
throw new ArgumentException(#"argument");
}
else
{
var random = new Random();
while (true)
{
Action<Object> sendOrPostCallbackAction = (state) =>
{
var record = new Record();
bindingList.Add(record);
};
var sendOrPostCallback = new SendOrPostCallback(sendOrPostCallbackAction);
// WindowsFormsSynchronizationContext.Current.Send(sendOrPostCallback, new Object());
// *** NullReferenceException
// *** Object reference not set to an instance of an object...
// *** Thought this could be found not matter where.
// *** Cause the winforms application is already started at that point in the code.
// Dispatcher.CurrentDispatcher.Invoke(sendOrPostCallback, (Object)null);
// *** InvalidOperationException
// *** Cross-thread operation not valid: Control '' accessed from a thread other than the thread it was created on.
if (dispatcher != null)
{
// Only Works if the synchronizationContext is the one used by the UI Thread...
dispatcher.Invoke(sendOrPostCallback, (Object)null);
}
else
{
// Only Works if the synchronizationContext is the one used by the UI Thread...
synchronizationContext.Send(sendOrPostCallback, (Object)null);
}
Thread.Sleep(100);
}
}
};
Application.EnableVisualStyles();
Application.SetCompatibleTextRenderingDefault(false);
var mainForm = new MainForm
{
DataGridView = { DataSource = bindingList },
};
mainForm.ButtonPassSynchronizationContext.Click += (sender, args) =>
{
mainForm.ButtonPassSynchronizationContext.Enabled = false;
mainForm.ButtonPassDispatcher.Enabled = false;
// Of course here the synchronization context is the one used by the UI Thread...
// WindowsFormsSynchronizationContext.Current should be equal here.
var areTheSameSyncrhonizationContexts = SynchronizationContext.Current == WindowsFormsSynchronizationContext.Current;
Debug.WriteLineIf(areTheSameSyncrhonizationContexts, #"The sychronization contexts are the same.");
Task.Factory.StartNew(action, SynchronizationContext.Current);
};
mainForm.ButtonPassDispatcher.Click += (sender, args) =>
{
mainForm.ButtonPassSynchronizationContext.Enabled = false;
mainForm.ButtonPassDispatcher.Enabled = false;
// The dispatcher is indeed not null, since it is the one used by the UI Thread...
// Dispatcher.CurrentDispatcher should be not null here.
var isDispatcherNotNull = Dispatcher.CurrentDispatcher != null;
Debug.WriteLineIf(isDispatcherNotNull, #"The Current Dispatcher is not null.");
Task.Factory.StartNew(action, Dispatcher.CurrentDispatcher);
};
Application.Run(mainForm);
}
public class Record
{
public Record()
{
var random = new Random();
this._dummyProperty = random.Next(0, 100);
}
private readonly Int32 _dummyProperty;
public Int32 DummyProperty
{
get { return this._dummyProperty; }
}
}
public class MainForm : Form
{
public MainForm()
{
this.InitializeComponent();
}
public Button ButtonPassSynchronizationContext { get; private set; }
public Button ButtonPassDispatcher { get; private set; }
public DataGridView DataGridView { get; private set; }
private void InitializeComponent()
{
this.Text = #"Main Form";
this.StartPosition = FormStartPosition.CenterScreen;
this.DataGridView = new DataGridView
{
Dock = DockStyle.Fill,
AllowUserToAddRows = false,
AllowUserToDeleteRows = false,
DefaultCellStyle = { Alignment = DataGridViewContentAlignment.MiddleCenter },
AlternatingRowsDefaultCellStyle = { BackColor = Color.LightGoldenrodYellow },
};
this.ButtonPassSynchronizationContext = new Button()
{
Dock = DockStyle.Bottom,
Text = #"Pass SynchronizationContext",
};
this.ButtonPassDispatcher = new Button()
{
Dock = DockStyle.Bottom,
Text = #"Pass Dispatcher",
};
this.Controls.Add(this.DataGridView);
this.Controls.Add(this.ButtonPassSynchronizationContext);
this.Controls.Add(this.ButtonPassDispatcher);
}
}
}

Avoid UI Lockups when doing lots of PropertyUpdates

I am trying to discover the best way of avoiding UI-Lockups when you are doing a lot of updates to the UI at once.
The basic premise is that on start up my tool runs a perforce FSTAT on the within a background worker. This generates a very large list of files and their information. Once this is completed, in its RunWorkerCompleted function, I then propagate this information to the UI inside of a TreeView.
This however, involves lots of property updates! Depending on the number of files that its propagating to. It can be 5000+ files. This completely locks up the UI for about 3-5 seconds.
I was wondering if I can asynchronously update the UI, such that I say, propagate 10-20 files at once & Still let the UI thread continue to update so that its still responsive.
Thank you.

If you are updating information inside of the TreeView using property bindings you could set your Binding.IsAsync flag to true. If you aren't updating the values using bindings then that might be something to look into.
Binding.IsAsync Property
Another option would be to update all your properties but, to not call the PropertyChanged event for the property (Assuming you are using INotifyPropertyChanged to update your bindings) until all your data has been changed and then call the PropertyChanged event for each of your properties on a Task so, it is still Async but even with 5000+ binding updates it should not take 3-5 seconds.

lots of suggestions you made finally got me to a good answer. Here is my code below. Basically we can use ReportProgress to allow the UI to update-while-running. Then adjust for how often we want this to happen. Here is my solution below.
The key is that PropegateMetaData is called for every N number of items (I specified 25). Then the list is emptied.
This will call report progress for every 25 items, then continue on. And eventually pass the rest to WorkerCompleted.
public static void Refresh(List<string> refreshPaths)
{
BackgroundWorker worker = new BackgroundWorker();
worker.WorkerReportsProgress = true;
worker.DoWork += delegate(object s, DoWorkEventArgs args)
{
List<string> filesPath = null;
if (refreshPaths == null)
{
filesPath = DatabaseViewModel.Instance.Records.Select(record => record.Filepath).ToList();
}
else
{
filesPath = new List<string>(refreshPaths);
}
if (m_Repository != null && filesPath.Count > 0)
{
IList<FileSpec> lfs = new List<FileSpec>();
int index = 0;
foreach (DataRecord rec in DatabaseViewModel.Instance.Records)
{
lfs.Add(new FileSpec(new LocalPath(rec.Filepath), null));
index++;
if (index > MaxFilesIteration)
{
GetFileMetaDataCmdOptions opts = new GetFileMetaDataCmdOptions(GetFileMetadataCmdFlags.AllRevisions, null, null, 0, null, null, null);
worker.ReportProgress(0, m_Repository.GetFileMetaData(lfs, null));
lfs.Clear();
index = 0;
}
}
args.Result = m_Repository.GetFileMetaData(lfs, null); //pass the remaining results across
}
};
worker.ProgressChanged += (sender, args) => PropegateMetaData(args.UserState as IList<FileMetaData>);
worker.RunWorkerCompleted += (sender, args) => PropegateMetaData(args.Result as IList<FileMetaData>);
worker.RunWorkerAsync();
}
private static void PropegateMetaData(IList<FileMetaData> fileList)
{
IList<FileMetaData> fileState = fileList as IList<FileMetaData>;
if (fileState != null)
{
foreach (FileMetaData fmd in fileState)
{
DataRecord currentRecord = DatabaseViewModel.Instance.GetRecordByFilepath(fmd.LocalPath.Path);
if (currentRecord != null)
{
switch (fmd.Action)
{
case FileAction.Add:
currentRecord.P4Status = P4FileState.Added;
break;
case FileAction.Edit:
currentRecord.P4Status = P4FileState.Edit;
break;
case FileAction.MoveAdd:
currentRecord.P4Status = P4FileState.MoveAdd;
break;
default:
currentRecord.P4Status = P4FileState.None;
break;
}
}
}
}
}

Cross-thread operation not valid. Multiple solutions fail to work,

Before reading, I want everyone reading this to know that I have tried multiple delegate/Cross-Threading/Invoking Solutions from all over stack overflow.
With that said, this is what my program is supposed to do:
Worker Thread 1 is called to start Async Operation.
If it detects a line that has a typical PRIVMSG header along with the word subscribed!
Create a new MetroTaskWindow with a TaskWindowControl and Add it to the queue
Worker Thread 2 is called after worker thread 1
Worker Thread 2 checks every 5 seconds if queue contains something
If it does, show it and get rid of it
Here is the associated Code If you need more, let me know to the above requirements:
Worker Thread 1 Segment
string line = "";
while (!backgroundWorker1.CancellationPending)
{
try
{
line = reader.ReadLine();
}
catch { }
if (line != null && !line.Contains("JOIN"))
{
try
{
if (line.Contains("PING") && !line.Contains("PRIVMSG"))
{
writer.Write(line.Replace("PING", "PONG"));
Trace.WriteLine(line.Replace("PING", "PONG"));
}
else if (line.Split(new char[] { ' ' })[0].Equals(":twitchnotify!twitchnotify#twitchnotify.tmi.twitch.tv") ||
line.Split(new char[] { ' ' })[0].Equals(":stds_catchemall!stds_catchemall#stds_catchemall.tmi.twitch.tv") && line.Contains("subscribed!"))
{
total += 1;
checkNotifications();
}
}
catch
{
continue;
}
}
if (!String.IsNullOrEmpty(line))
Trace.WriteLine(line);
}
}
private void checkNotifications()
{
List<Achievement> tempQueue = new List<Achievement>();
foreach (Achievement a in achievements) {
//I know i could shorten this, but i need it left like this...
if (a.AfterSub)
tempQueue.Add(a);
if (total - a.Goal == start)
tempQueue.Add(a);
if (total == a.Goal)
tempQueue.Add(a);
}
foreach (Achievement a in Sort(tempQueue))
{
MetroTaskWindow m = new MetroTaskWindow(a, this, a.Type.ToString(), new TaskWindowControl(a.Name, a.Message, a), 4, r, ((ScreenRegion)r).getGS());
queue.Add(m);
}
}
Worker Thread 2
private void CheckAvailable_DoWork(object sender, DoWorkEventArgs e)
{
while (true)
{
Thread.Sleep(5250);
BeginInvoke((MethodInvoker)delegate
{
if (queue.Count > 0)
{
queue[0].Show(); // <---- Error Occurs Here
//Cross-thread operation not valid: Control 'TaskWindowControl' accessed from a thread other than the thread it was created on.
queue.RemoveAt(0);
}
});
}
}
Metro Task Window
public MetroTaskWindow(Achievement a, IWin32Window parent, string title, Control userControl, int secToClose, MetroForm r, Form gs)
{
controlContainer = new MetroPanel();
Controls.Add(controlContainer);
controlContainer.Controls.Add(userControl);
userControl.Dock = DockStyle.Fill;
closeTime = secToClose * 500;
this.a = a;
form = r;
chroma = gs;
p = (Form1)parent;
this.Text = title;
this.Resizable = false;
this.Movable = true;
this.StartPosition = FormStartPosition.Manual;
if (parent != null && parent is IMetroForm)
{
this.Theme = ((IMetroForm)parent).Theme;
this.Style = ((IMetroForm)parent).Style;
this.StyleManager = ((IMetroForm)parent).StyleManager.Clone(this) as MetroStyleManager;
this.ShadowType = MetroFormShadowType.None;
}
switch (a.Type)
{
case PopupType.Achievement:
Text = "Achievement!";
break;
case PopupType.Milestone:
Text = "Milestone!";
break;
case PopupType.Notification:
Text = "Notification";
break;
}
}
TaskWindowControl
public partial class TaskWindowControl : UserControl
{
public TaskWindowControl(string name, string info, Achievement a)
{
InitializeComponent();
metroLabel1.Text = name;
metroTextBox1.Text = info;
metroTextBox1.Select(0, 0);
try
{
Trace.WriteLine(Directory.GetCurrentDirectory() + "\\" + a.Picture);
pictureBox1.Image = Image.FromFile(Directory.GetCurrentDirectory() + "\\" + a.Picture);
}
catch
{
MessageBox.Show("There was an error loading the image for this achievement.");
}
}
}
And as I stated above, there are a LOT of duplicates, none of which have helped my answer. I also don't know much about the Delegate/Invoking process which is why I need some extra help.
Update #1
Anywhere I had queue.Add(m); is now replaced with queue.Enqueue(a);
And my update method (Without timer so far) is just:
public void DisplayDialog()
{
Achievement a = null;
queue.TryDequeue(out a);
MetroTaskWindow m = new MetroTaskWindow(a, this, a.Type.ToString(), new TaskWindowControl(a.Name, a.Message, a), 4, r, ((ScreenRegion)r).getGS());
m.Show();
}
Something that I found is that when I changed the code to this, the Thread that does the animations and things on my MetroTaskWindow doesn't get activated. The Windows Stays in with a windows loading circle and it never goes away. Any ideas? I'm using the OnActivated event, so it SHOULD fire when i .Show();
Edit #2
What I ended up doing to get my above error to work, was to switch all of my code to a new Windows Form Timer. This polls every 5 seconds, and keeps the MetroTaskWindow from hanging due to the while loop.
Form1.Designer.cs
private System.Windows.Forms.Timer timer2;
timer2 = new System.Windows.Forms.Timer(this.components);
timer2.Interval = 5250;
timer2.Tick += new System.EventHandler(this.timer2_Tick);
Form1.cs
private void timer2_Tick(object sender, EventArgs e)
{
if (queue.Count > 0)
{
DisplayDialog();
}
}

The error makes sense. The queue contains MetroTaskWindow instances created on a worker thread, not on the UI thread. You call checkNotifications in the background worker's entry point method, which runs on a separate thread.
What I recommend you do is:
Store in the queue only metadata about the windows. Create a new class with name, message and anything MetroTaskWindow needs to be created. Add instances of this class to the queue in checkNotifications. By the looks of it you might be able to use the Achievement class directly and push that to the queue.
Create the windows and show them in CheckAvailable_DoWork, where you now call queue[0].Show();.
After you solve this you should post a new question about how to refactor your code into async/await and get rid of that ugly background worker.
A quick refactoring idea
I would remove the second background worker altogether and use a timer instead that polls the queue every X seconds (or 5250ms, if you prefer).
To make this work you need to change the queue's type which is now actually a List<T> to a ConcurrentQueue<T>. This will allow you to push stuff from the worker, and pop from the UI thread (in the timer callback).
This way you can remove the second background worker and that while(true).

LinkedList modified, Thread crashing the program

My problem is a synchronization problem with a thread and the user simultaneously accessing and modifying a LinkedList.
I’m making a program in C# that will display some messages in a panel. I’m getting an error called “The collection was modified after the enumerator was instantiated.”, that is because I’m adding or removing messages while a thread is accessing the LinkedList.
I have read some solutions but I am still unable to make them work. I’m using an Enumerator for the thread work in my LinkedList. I tried to make some locks in my code so the thread would not iterate the list while I remove or add an element. I also tried to lock the thread for the operations on my list. But all my attempts failed.
Here is some code of my project. This one is for adding a message:
public void addMsg(MsgForDisplay msg) {
Label lbl = new Label();
lbl.Text = (msg.getMsgText() + " -");
lbl.ForeColor = color;
lbl.Font = textFont;
lbl.BackColor = backg;
lbl.Visible = true;
lbl.AutoSize = true;
lbl.Location = new Point(width(), 0);
//lock(labels) { tried to lock here but failed
labels.AddLast(lbl);
lastLb = lbl;
this.Controls.Add(lbl);
this.Refresh();
//}
}
Removing a message:
public void removeMsg(string msg) {
string remove = msg + " -";
Label lbRemove = null;
//lock(labels) { also tried to lock here
var it = labels.GetEnumerator();
while(it.MoveNext()) {
Label label = it.Current;
if (label.Text.Equals(remove)) {
lbRemove = label;
}
}
labels.Remove(lbRemove);
this.Controls.Remove(lbRemove);
this.Refresh();
//}
}
And there is the problem, in my thread:
public void run() {
while (true) {
// lock (labels) { also tried to lock here
var it = labels.GetEnumerator();
while (it.MoveNext()) { // the crash occurs here
Label lb = it.Current;
if (lb.Location.X + lb.Width < 0) {
this.Invoke(new MethodInvoker(() => { this.Controls.Remove(lb); }));
if (labels.Count > 1)
this.Invoke(new MethodInvoker(() => { lb.Location = new Point(lastLb.Right, 0); }));
else
this.Invoke(new MethodInvoker(() => { lb.Location = new Point(2000, 0); }));
lastLb = lb;
this.Invoke(new MethodInvoker(() => { this.Controls.Add(lb); }));
this.Invoke(new MethodInvoker(() => { this.Refresh(); }));
}
if (leftLb != null)
if (leftLb.Location.X + leftLb.Width - lb.Location.X < -20)
this.Invoke(new MethodInvoker(() => { lb.Location = new Point(leftLb.Right, 0); }));
else
this.Invoke(new MethodInvoker(() => { lb.Location = new Point(lb.Location.X - 3, lb.Location.Y); }));
leftLb = lb;
}
System.Threading.Thread.Sleep(30);
// }
}
}
As you can see I’m using an GetEnumerator of my labels, what in Java should be the Iterator. With this I shouldn’t be able to iterate the list without problem when the user add or remove messages?
Is there a way to synchronize the accesses to the list?
EDIT: I have tried the ConcurrentBag and ConcurrentDictionary but without any improvement to the project as you can see in the comments…
Please before you post an answer read the comments bellow to make sure that you know what is going on.
EDIT: Tried to add a mutex to my code for addMsg and removeMsg but still crashing. If I use the mutex in the thread it will be slowed down.

I created a Timer in step of the thread and that solved the crashing problem. Here is the code if you want to see it.
private System.Windows.Forms.Timer myTimer = new System.Windows.Forms.Timer();
private void startThread() {
myTimer.Tick += new EventHandler(timerEvent);
myTimer.Interval = 30;
myTimer.Start();
}
private void timerEvent(object sender, EventArgs e) {
var it = labels.GetEnumerator();
while (it.MoveNext()) {
Label lb = it.Current;
// Label lb = labels.ElementAt(b);
if (lb.Location.X + lb.Width < 0) {
this.Controls.Remove(lb);
if (labels.Count > 1)
lb.Location = new Point(lastLb.Right, 0);
else
lb.Location = new Point(2000, 0);
lastLb = lb;
this.Controls.Add(lb);
this.Refresh();
}
if (leftLb != null)
if (leftLb.Location.X + leftLb.Width - lb.Location.X < -20)
lb.Location = new Point(leftLb.Right, 0);
else
lb.Location = new Point(lb.Location.X - 3, lb.Location.Y);
leftLb = lb;
}
}

The source of your problem is that while you are iterating over the list of labels You call either Remove or Add functions which modifies this list whis is not allowed while iterating over it. Instead of this
var it = labels.GetEnumerator();
while (it.MoveNext()) // the crash occurs here
I suggest something like that:
for(int i = 0; i < labels.Count; i++)
{
labels.remove(labels[i]); //this is valid of course the count of the list will change
//Here you can add or remove elements from the labels
}
Or you can try first to collect the removable items into a temporal list and later remove it from the original.

As others have already stated, the problem is you are modifying the collection while enumerating over it.
Now, the easiest workaround is obviously not to enumerate over the same collection that is being modified. And how do you do that? Simple, you just clone the collection, and iterate over it:
lock (labels)
{
var clone = new LinkedList<Label>(labels);
it = labels.GetEnumerator();
}
Now you can enumerate over it safely, without worrying about inconsistencies.
A few notes:
I am using a lock, because the cloning also must enumerate over your collection, and while it does it in a very short time, it is still required for synchronization. Off course, you need to uncomment the locks you've already added to addMsg and removeMsg.
The reason that locking your whole loop didn't work, is that when you call Invoke, you are essentially returning control to the thread that owns the object (the main GUI thread in this case). The problem is, that this thread is already stuck on handling whatever event caused addMsg or removeMsg to be called, leading to a deadlock.
You should also note that cloning a collection every 30 ms, isn't exactly efficient, and shouldn't be used in a production code, but given that this probably just an exercise, it should suffice. In real life, you should probably use a separate collection for the changes you are about to do (adding or removing labels), change this collection in addMsg and removeMsg, and then merge the changes to labels inside your thread, but outside of the iteration over the labels.
Not directly related to your question, but still: you should use a foreach loop instead of directly creating an enumerator object in C#.

As stated before, changing any collection while enumerating it, results in an exception in .Net. You can avoid this by using for or while loops.
However I don't see the point in using a Linked List in this scenario. It should be way simpler and more performant to use a ConcurrentDictionary and just add or remove the requested item. There is also a ObservableConcurrentDictionary available, although not part of the Framework. It is very stable, in my experience.
http://www.codeproject.com/Articles/208361/Concurrent-Observable-Collection-Dictionary-and-So

Closing or Hiding forms causes a cross thread error

I am baffled by this simple task i do over and over again.
I have an array of child forms. The array is initiated in another form's constructor:
frmChildren = new ChildGUI[20];
When the user requests to see a child form, i do this:
if (frmChildren[nb] == null)
{
frmChildren[nb] = new ChildGUI();
frmChildren[nb].MdiParent = this.MdiParent;
}
frmChildren[nb].Show();
So far this works. In the background i can download new content for these forms. When a download is finished i fire a ChildChange event. Here is where it stops working.
I simply want to close/hide any forms open then regenerate a new set of -frmChildren = new ChildGUI[20];- here is one of many trials:
for (int i = 0; i < frmChildren.Length;i++ )
{
if (frmChildren[i] != null)
{
//frmChildren[i].BeginInvoke(new EventHandler(delegate
//{
frmChildren[i].Close();
//}));
}
}
frmChildren= new ChildGUI[20];
I get a Cross Thread exception on the .Close(). Notice i've already tried doing an invoke, but doing so bypasses the !=null for some reason. I think it may have something to do with the garbage collector. Anybody have an input?

The problem is that your anonymous method is capturing i - so by the time it's actually invoked in the UI thread, you've got a different value of i, which may be null. Try this:
for (int i = 0; i < frmChildren.Length; i++)
{
ChildGUI control = frmChildren[i];
if (control != null)
{
control.BeginInvoke(new EventHandler(delegate
{
control.Close();
}));
}
}
frmChildren = new ChildGUI[20];
See Eric Lippert's blog post for why introducing a new variable within the loop fixes the problem.
EDIT: If you want to use a foreach loop, it would look like this:
foreach (ChildGUI control in frmChildren)
{
// Create a "new" variable to be captured
ChildGUI copy = control;
if (copy != null)
{
copy.BeginInvoke(new EventHandler(delegate
{
copy.Close();
}));
}
}
frmChildren = new ChildGUI[20];
Just as an aside, you can use the fact that you just want to call a void method to make the code slightly simpler. As this no longer uses an anonymous method, you can make do away with the "inner" variable:
foreach (ChildGUI control in frmChildren)
{
if (control != null)
{
control.BeginInvoke(new MethodInvoker(control.Close));
}
}
frmChildren = new ChildGUI[20];