Develop Reference

Why isn't this causing an infinite loop of events?

I have a simple application that reverses any text typed to it in another textbox. The catch is, you can modify either textbox and the changes will be (literally) reflected in the other.
I wrote this code, believing for it to cause problems.
private void realText_TextChanged(object sender, EventArgs e)
{
mirrorText.Text = mirror(realText.Text);
}
private void mirrorText_TextChanged(object sender, EventArgs e)
{
realText.Text = mirror(mirrorText.Text);
}
private string mirror(string text)
{
return new string(text.Reverse().ToArray()).Replace("\n\r", "\r\n");
}
I then tried it out, believing that it would cause an infinite loop (realText changes mirrorText, another event happens, mirrorText changes realText, etc). However, nothing except the intended behavior happened.
I'm of course happy about this, I could just leave it here. Or could I?
I'm quite sure the TextChanged event is supposed to be fired whenever Text is changed. Is this intended behavior of some error protection in the events, or was I just lucky? Can this code misbehave on another computer, with other build settings, etc? It can be easily fixed:
private void realText_TextChanged(object sender, EventArgs e)
{
if (realText.Focused)
{
mirrorText.Text = Mirror(realText.Text);
}
}
I'll probably do it anyway to be safe, but is it required to check this? (I'm not even going to ask if it's recommended.)

Per the comments, and as already answered, the TextChanged event is not getting raised when you set the Text property to the value it already has.
It's not clear whether this is something you can safely rely upon. It is a sensible optimisation, and I would be very surprised if future versions of .NET Framework drop it, but I cannot speak for older versions, nor for third-party implementations (Mono).
To be absolutely safe, I would not use the Focused check you put in your question. I would do exactly what the Text setter does now.
private void realText_TextChanged(object sender, EventArgs e)
{
var newMirrorText = Mirror(realText.Text);
if (mirrorText.Text != newMirrorText)
mirrorText.Text = newMirrorText;
}
This has the same advantage of preventing infinite recursion, but plays more nicely with other code you may put in your form that changes the text as a result of some other event.

The reason it doesn't cause a loop is that it checks whether the Text property actually changed, i.e. if the new value does not equal the old value. In your case the mirror function happens to reverse itself, which leads to the same text after two passes.

It's pretty easy to check.
First, replace both textbox controls with
class T : TextBox
{
public override string Text
{
get
{
return base.Text;
}
set
{
base.Text = value;
}
}
}
Second, set the breakpoint on setter. Add these expressions to the Watch window:
Name
Text
value
Third, launch the app, copy '123' from somewhere and paste it to the first textbox. Here it goes:
1st break:
Name: "mirrorText"
Text: ""
value: "321"
2nd break:
Name: "realText"
Text: "123"
value: "123"
3rd... whoops, it does not breaks anymore. To detect why we had to go deeper. Look at referencesource: text box setter does nothing unusual, but TextBoxBase's one looks interesting:
set {
if (value != base.Text) { // Gotcha!
base.Text = value;
if (IsHandleCreated) {
// clear the modified flag
SendMessage(NativeMethods.EM_SETMODIFY, 0, 0);
}
}
}
So, as hvd already answered, the reason is the textbox does not raise TextChanged if old and new values are the same. I don't think the behavior will change, at least for winforms. But if you want more robust solution, here it is:
private void RunOnce(ref bool flag, Action callback)
{
if (!flag)
{
try
{
flag = true;
callback();
}
finally
{
flag = false;
}
}
}
private bool inMirror;
private void realText_TextChanged(object sender, EventArgs e)
{
RunOnce(ref inMirror, () =>
{
mirrorText.Text = mirror(realText.Text);
});
}
private void mirrorText_TextChanged(object sender, EventArgs e)
{
RunOnce(ref inMirror, () =>
{
realText.Text = mirror(mirrorText.Text);
});
}
private string mirror(string text)
{
return new string(text.Reverse().ToArray()).Replace("\n\r", "\r\n");
}
P.S. mirror() will fail on surrogate pairs. Here're some solutions.

If textbox has a Text, and we try to change it with the same Text, the TextChange event is not raising because new text is same as the previous.
In your code, the realText_TextChanged event reverses the text and changes the mirrorText with it.
The mirrorText_TextChanged event reverses the text and try to change the realText.
The realText has already this text and does not raises the realText_TextChanged event.

Determine who fired an event

Background:
In my winforms form, I have a Checked ListView and a "master" checkbox called checkBoxAll.
The behaviour of the master is as follows:
If the master is checked or unchecked, all ListViewItems must change accordingly.
If the user unchecks a ListViewItem, the master must change accordingly.
If the user checks a ListViewItem, and all other ListViewItems are checked aswell, the master must change accordingly.
I have written the following code to mimic this behaviour:
private bool byProgram = false; //Flag to determine the caller of the code. True for program, false for user.
private void checkBoxAll_CheckedChanged(object sender, EventArgs e)
{
//Check if the user raised this event.
if (!byProgram)
{
//Event was raised by user!
//If checkBoxAll is checked, all listviewitems must be checked too and vice versa.
//Check if there are any items to (un)check.
if (myListView.Items.Count > 0)
{
byProgram = true; //Raise flag.
//(Un)check every item.
foreach (ListViewItem lvi in myListView.Items)
{
lvi.Checked = checkBoxAll.Checked;
}
byProgram = false; //Lower flag.
}
}
}
private void myListView_ItemChecked(object sender, ItemCheckedEventArgs e)
{
//Get the appropiate ListView that raised this event
var listView = sender as ListView;
//Check if the user raised this event.
if (!byProgram)
{
//Event was raised by user!
//If all items are checked, set checkBoxAll checked, else: uncheck him!
bool allChecked = true; //This boolean will be used to set the value of checkBoxAll
//This event was raised by an ListViewItem so we don't have to check if any exist.
//Check all items untill one is not checked.
foreach (ListViewItem lvi in listView.Items)
{
allChecked = lvi.Checked;
if (!allChecked) break;
}
byProgram = true; //Raise flag.
//Set the checkBoxAll according to the value determined for allChecked.
checkBoxAll.Checked = allChecked;
byProgram = false; //Lower flag.
}
}
In this example, I use a flag (byProgram) to make sure an event was caused by the user or not, thereby preventing an infinite loop (one event can fire another, which can fire the first one again etc. etc.). IMHO, this is a hacky solution.
I searched around but I couldn't find a MSDN documented method to determine if an User Control Event was directly fired thanks to the user. Which strikes me as odd (again, IMHO).
I know that the FormClosingEventArgs has a field which we can use to determine if the user is closing the form or not. But as far as I know, that is the only EventArg that provides this kind of functionality...
So in summary:
Is there a way (other than my example) to determine if an event was fired directly by the user?
Please note: I don't mean the sender of an event! It won't matter if I code someCheckBox.Checked = true; or manually set someCheckBox, the sender of the event will always be someCheckBox. I want to find out if it is possible to determine whether it was through the user (click) or by the program (.Checked = true).
Aaand also: 30% of the time it took to write this question was to formulate the question and the title correctly. Still not sure if it is a 100% clear so please edit if you think you can do better :)

No, there's no practical way to determine whether the change came from GUI or was done by program (in fact, you could analyze the callstack - but that's not recommended because it's very slow and error-prone).
BTW, there's one other thing you could do instead of setting byProgram. You could remove and add the event handler prior or after, respectively, change your controls:
checkBoxAll.CheckedChanged -= checkBoxAll_CheckedChanged;
// do something
checkBoxAll.CheckedChanged += checkBoxAll_CheckedChanged;

Instead of using the changed event, you could use the clicked event to cascade the change through to the relevant controls. This would be in response to a user click, and not the value being changed programatically.

This is something I come across quite a lot and what I tend to try do is not split it between user interaction vs program interaction - I use more generic code i.e. the UI is being updated and doesn't require any events to be handled. I usually package this up through BeginUpdate/EndUpdate methods e.g.
private int updates = 0;
public bool Updating { get { return updates > 0; } }
public void BeginUpdate()
{
updates++;
}
public void EndUpdate()
{
updates--;
}
public void IndividualCheckBoxChanged(...)
{
if (!Updating)
{
// run code
}
}
public void CheckAllChanged(...)
{
BeginUpdate();
try
{
// run code
}
finally
{
EndUpdate();
}
}

What is the best way to mask an UI Event?

I am working with a DataGridView, and I use the CellValueChanged event.
I dont want this event to be triggered when I change a cell value by the code. However, I want it to be triggered when the user edits it.
That's why I enclose my cell value change operations with the following code :
void changeCellOperation()
{
dgv.CellValueChanged -= new DataGridViewCellEventHandler(dgv_CellValueChanged);
...
cell.Value = myNewCellValue
...
dgv.CellValueChanged += new DataGridViewCellEventHandler(dgv_CellValueChanged);
}
I ended to have several differents functions where my DataGridView cells are updated this way.
Because these functions are called from different places and can be nested, I cannot afford to keep this code as is to avoid event unwanted event reactivation.
So I ended up this way :
int valueChangedEventMask = 0;
void changeCellOperation()
{
valueChangedEventMask++;
...
cell.Value = myNewCellValue
...
valueChangedEventMask--;
}
void dgv_CellValueChanged(object sender, DataGridViewCellEventArgs e)
{
if (valueChangedEventMask > 0)
return
...
}
This works fine. Also when the calls are nested, including inside the event itself.
But the CellValueChanged event is now fired too many times for no reasons.
Because I often have to cope with this pattern, I am looking for a solution that can be applicable generally for Events in UIs, not only the DataGridView.
So my question is:
What is the best tip to mask UI Events correctly and avoid unnecessary Events fires ?

CellValueChanged is not an UI event, but a property changed event. That means you can not use it to distinguish user input from programmatic change. You can always use subscriber/unsucscribe or flag+/- or BeginEdit/EndEdit-similar technique, but maybe you have to find another (better) approach. To example, in case of checkbox you can use Click event instead of Changed, because (surprise!) it will tell you when the user click it and otherwise safely change value of Checked programmatically.
In case of DataGridView easiest would be to use Changed with some flag (which will be set when edit begins and reset when ends - see, CellBeginEdit/CellEndEdit ).

You could use CellEndEdit instead of CellValueChange. I don't know what your method dgv_CellValueChanged does, just be careful that CellEndEdit is fired every time you exit the edit mode for the cell, even if its value has not been changed. This means that you have to keep trace of the current values of your cells if you don't want the method to be executed when the value doesn't change.
I would avoid events related with the mouse such as CellClick because your users could use just the keyboard.
Anyway I usually avoid this kind of problems by separating the logic from the user interface, i.e. I write a separate class which is bound to the form. Take a look at MVVM (you can implement your own version in WinForms if you want) or the good old MVC.

I ended up mixing both solutions in a very simple one. I use a counter and I only hook/unhook the events I want to mask.
EventMask valueChangedEventMask;
// In the class constructor
valueChangedEventMask = new EventMask(
() => { dgv.CellValueChanged += new DataGridViewCellEventHandler(dgv_CellValueChanged); },
() => { dgv.CellValueChanged -= new DataGridViewCellEventHandler(dgv_CellValueChanged); }
);
// The value change operation I want to hide from the event
void changeCellOperation()
{
valueChangedEventMask.Push();
...
cell.Value = myNewCellValue
...
valueChangedEventMask.Pop();
}
// The class
public class EventMask
{
Action hook;
Action unHook;
int count = 0;
public EventMask(Action hook, Action unHook)
{
this.hook = hook;
this.unHook = unHook;
}
public void Push()
{
count++;
if (count == 1)
unHook();
}
public void Pop()
{
count--;
if (count == 0)
hook();
}
}

how to handle events from variable number of sources?

I have such class (sorry about posible mistakes, i'm writing it right here.) Class is simplified for this example, it must be more complex of course.
class SP500Index {
SP500Index(List<OrderBook> stocks) {
foreach (var stock in stocks) {
stock.StockUpdated += stockUpdated; // how to handle?
}
}
}
So I have a lot of sources and I need to handle StockUpdated event from them. In handler I need to know index of stock in stocks list which raised the event. How to do that?
upd for perfomance reasons I don't want "sender look-up" instead I want index. Lookup is not trivial operation and likely involves Hashcode calculation Equals method call etc. Imagine how ofthen SP500 index changes...

This is not provided automatically.
But the StockUpdated event should look like
void StockUpdated (object sender, MyEventArgs e)
and you can cast senderto a stock and look it up in the original list. If you still need the index.
void stockUpdated (object sender, MyEventArgs e)
{
OrderBook stock = (OrderBook) sender;
....
}

It is good practice to use a signature of the form:
public delegate void CustomEventHandler(object sender, CustomEventArgs a);
In your event handler you can use sender to find out which object raised the event.
If you don't have a sender parameter then I don't think there is any (reasonable) way to find out which object raised the event.
Related
How to: Publish Events that Conform to .NET Framework Guidelines (C# Programming Guide)

If your benchmark shows that you need the index when the event occurs you can add the index as a property to OrderBook and when you add an element to the list, set this property. This value will be available to the event handler.
This will work assuming that you keep the OrderBook objects in a single List and do not do any rearranging of this original list. If you have multiple lists with each object stored on only one of those lists, then you could add an Owner property that references the list that it is stored on.
For example...
When you build List<OrderBook>:
...
OrderBook book = CreateOrderBook(...);
list.Add(book);
book.ListIndex = list.Count - 1;
// Assign Owner here if that is needed.
...
Or better, use a helper for managing the list that cares for the book keeping of the index update:
public class OrderBookManager
{
private List<OrderBook> list = new List<OrderBook>();
public void Add(OrderBook book)
{
list.Add(book);
book.ListIndex = list.Count - 1;
// Assign Owner here if that is needed.
}
// Make this read-only if you want to ensure the manager controls all updates to the list (better design) but use it this way for higher performance.
public List<OrderBook> List { get { return list; } }
}
The updated OrderBook:
public class OrderBook
{
...
public int ListIndex { get; set; }
}
And then an sample event handler using this index:
public void StockUpdated(object sender, MyEventArgs eventArgs)
{
OrderBook book = (OrderBook) sender;
//Here use book.ListIndex to access the original list element.
}
What is your reason for having that index? Everything you need should be in the object. If you use this to manipulate the original list (such as removing this item from the list) then you have the problem of recomputing the saved index of all the previously stored objects.
If you are maintaining a parallel list with other objects, then you perhaps should consider a different design.

You can define in delegate of that event sending source (which is basically the suggested guideline by Microsoft. Having object sender, in other words, like a first parameter of the delegate's signature).
After make (say) a cast and determine in some if/else the real object type.

C# WinForms: Waiting for a button press inside an infinite loop

I'm making a simple Guess-The-Number game with a GUI. I need to wait on a loop waiting for the user to input a number in a text box and press "OK". How do I wait for an event inside a loop?
Note: I don't want message boxes. This is done in the main window, hence the need to wait for input.
EDIT: I should have explained myself better. I know that there's a loop inside the GUI. What I want is another loop inside a method. Maybe there's a better way to do this. I could code stuff inside the button's event handler, now that I think about it. Although I'd need global variables. Whataver, I'll think about it, but I hope my question is clearer now.
EDIT 2: Sorry that my question wasn't clear and the edit didn't do much help. First of all, the code is too big to be posted here. I'd probably have to post a screenshot of the GUI, so it wouldn't be of much use. Basically, I have two fields, "Max number" and "Number of allowed guesses". The user enters these two and clicks "Play". A new panel becomes available, with a text box and a "Guess" button. The user enters a guess, and the program checks to see if it's correct.
The purpose of the second infinite loop is to avoid global variables. See, each time the user clicks "Play", the game has to generate a new random number as the correct guess. If everything is done inside a method, no problem. But if the "Guess" button's event handler is called multiple times, the number has to be stored as an instance variable of the Form. Sure, it's not big deal, but I think the number should be a property of the method directing the current game, not of the Form.
I'd also have to keep track of the remaining number of guesses outside of the method. Again, it's no big deal. I just want to avoid globals if I can.
Again, I'm sorry that my question wasn't too clear. I'm kind of tired, and I didn't feel like writing too much. If this still isn't clear, then don't bother. I'll think of something.

C# automatically loops infinitely waiting for events until your form is closed. You just need to respond to the button click event.

Jason Down's suggestion is wise, create a new GuessingGame class and add it to your project. I know you're worried about "global variables" (which everyone is taught in school never to use unless you absolutely have to), but think about your design specifications for a minute.
But if the "Guess" button's event handler is called multiple times, the number has to be stored as an instance variable of the Form. Sure, it's not big deal, but I think the number should be a property of the method directing the current game, not of the Form.
As an alternative, store an instance of your GuessingGame class in the form. This is not a global variable! You said so yourself, the point of the game is keep track of the guesses and generate new numbers to guess every time "Play" is clicked. If you store an instance of the game in the form then open another form (e.g. a Help or About box), then the game's instance would not be available (thus, not global).
The GuessingGame object is going to look something like:
public class GuessingGame
{
private static Random _RNG = new Random();
private bool _GameRunning;
private bool _GameWon;
private int _Number;
private int _GuessesRemaining;
public int GuessesRemaining
{
get { return _GuessesRemaining; }
}
public bool GameEnded
{
get { return !_GameRunning; }
}
public bool GameWon
{
get { return _GameWon; }
}
public GuessingGame()
{
_GameRunning = false;
_GameWon = false;
}
public void StartNewGame(int numberOfGuesses, int max)
{
if (max <= 0)
throw new ArgumentOutOfRangeException("max", "Must be > 0");
if (max == int.MaxValue)
_Number = _RNG.Next();
else
_Number = _RNG.Next(0, max + 1);
_GuessesRemaining = numberOfGuesses;
_GameRunning = true;
}
public bool MakeGuess(int guess)
{
if (_GameRunning)
{
_GuessesRemaining--;
if (_GuessesRemaining <= 0)
{
_GameRunning = false;
_GameWon = false;
return false;
}
if (guess == _Number)
{
_GameWon = true;
return true;
}
else
{
return false;
}
}
else
{
throw new Exception("The game is not running. Call StartNewGame() before making a guess.");
}
}
}
This way, all the data related to the game is encapsulated within the class. Hooking up the events is easy in the codebehind of the form:
GuessingGame game = new GuessingGame();
private void btnPlay_Click(object sender, EventArgs e)
{
int numberOfGuesses = Convert.ToInt32(txtNumberOfGuesses.Text);
int max = Convert.ToInt32(txtMax.Text);
game.StartNewGame(numberOfGuesses, max);
}
private void btnGuess_Click(object sender, EventArgs e)
{
int guess = Convert.ToInt32(txtGuess.Text);
bool correct = game.MakeGuess(guess);
if (correct)
lblWin.Visible = true;
if (game.GameEnded)
{
// disable guess button, show loss label
}
}

You should probably look for a book to actually learn windows programming.
The very basics:
1) There is already an infinite loop deep down in the windows code somewhere. Any windows program is constantly looping and scanning for input.
2) Once input is found, this loop fires off an Event.
3) Your mission, should you choose to accept it, is to write event handlers to handle those events.

you are most likely doing it wrong as it has already been pointed out, but you can use this
Application.DoEvents();
to process events when you are on an actual loop
to do it the right way
- don't use a loop
- use an edit box for the input, then a button
- implement the button onclick event

Yes, and What if I am waiting for Speech events, it could happen anytime event when a function is running, I need to handle that without recursively call a function