Re: How do I make a thread safe call to a listview. - C# Discussion Boards

How to show ListView items of First TreeNode?

gerbenschmidt31-Oct-07 5:07

31-Oct-07 5:07

Hi,

In a TreeView/ListView Form a TreeNodeMouseClickEvent on a TreeView is showing the ListView items.
When loading the Form, it also must return the ListView items of the First Node (rootNode).
But the ListView items are only returned after selecting a Node.

Has anyone an idea how to load the the Listview Items of the First TreeView Node(rootNode)?

Re: How to show ListView items of First TreeNode?

led mike31-Oct-07 5:22

led mike

31-Oct-07 5:22

You mean like using the Forms OnLoad event?

Re: How to show ListView items of First TreeNode?

gerbenschmidt31-Oct-07 5:39

gerbenschmidt

31-Oct-07 5:39

Thanx man....you brought me on an idea and it worked...

How to enter a record to database using TextBox control

Sinan Akyuz31-Oct-07 4:33

Sinan Akyuz

31-Oct-07 4:33

Hello,

I am currently working on a windows app project that intend to add records to a ms sql server 2000 database. It consists only a single simple form which have a TextBox, a save button, a cancel button. Now, the task is simple how can I enter new record to the database by typing record data into the texbox and hitting the save button to save it?

thanks in advance.

PS: I don't want to use any special toolbox control or component. I just want to use my simple form controls with only add record ability.

Re: How to enter a record to database using TextBox control

Colin Angus Mackay31-Oct-07 4:45

Colin Angus Mackay

31-Oct-07 4:45

sharpseer wrote:
Now, the task is simple how can I enter new record to the database by typing record data into the texbox and hitting the save button to save it?

In code create a SqlConnection object, and a SqlCommand object. In the command object create your INSERT statement, or refer to the relevant stored procedure. Add the parameters to pass the data to the command.

Ensuring your connection is open, use ExecuteNonQuery() on the command. Finally closed the connection.

Upcoming FREE developer events:
* Glasgow: SQL Server Managed Objects AND Reporting Services ...

My website

How do I make a thread safe call to a listview.

cirkit131-Oct-07 4:01

cirkit1

31-Oct-07 4:01

I get an Invalid Operation Exception error because the program is trying to make a cross-thread call to a listview object that was created by the main thread.

The basic operation of the program is that items can be added, removed, modified,and cleared for the listview component on the panel. If I want, I can start a timer. Meanwhile a 'listener' sits in another thread waiting for the time to elapse. On this elapsed timer event, it enters the OnTimerDoThis() event-handler. Within the event-handler, on the foreach line, the 'Invalid Operation Exception' error occurs only after an item is added to the listview, or if an item exists already. I have put a comment at the end of the line -> '//ERROR HAPPENS HERE' so that using Find will take you to that line. The purpose of the foreach is to search the listview items and indicate if it matches to a string constant.

I was able to create a delegate for a radio component and able to handle the same exception error, but need to do the same for the listview. Question is how do I get around the error?

FYI. I am not using the background watcher.

protected void OnTimerDoThis(Object source, ElapsedEventArgs e)
{
// immediatly disable timer after timeout interval reached
DisableTimer();

MessageBox.Show("Inside OnTimerDoThis asnyc event\n\nTimer Disabled\n\n" +
"Press OK to re-enable Timer");

foreach (ListViewItem item in lvData.Items) //ERROR HAPPENS HERE
{
//if (sWindowTitle.StartsWith(item.Text))
if (matchString.StartsWith(item.Text))
{
MessageBox.Show("Inside match string and asnyc event\n\nTimer Disabled\n\n" +
"Press OK to re-enable Timer");
}
}

EnableTimer();
}
Regards....

Re: How do I make a thread safe call to a listview.

ejuanpp31-Oct-07 4:09

ejuanpp

31-Oct-07 4:09

I am not sure I understood everything but ... aren't you trying to iterate through listview items with a thread while eventually modifying the same collection with another one (the main one) ?

Re: How do I make a thread safe call to a listview.

cirkit131-Oct-07 4:18

cirkit1

31-Oct-07 4:18

Yes. The main thread allows the user to add, remove, modify, clear, the listview.

I have a button the allows the user to Start of Stop the timer. If I start the timer, and the timer has elapsed, it will search the items in the listview for a match in the async thread event-handler OnTimerDoThis().

This is mainly for me to understand why the error occurs and how to handle the error.

I plan on using something like this in the future with better functionality, but before going BIG, I need to understand the smaller stuff.

If you want I can forward the file for your review

thanks for the quick reply

Re: How do I make a thread safe call to a listview.

led mike31-Oct-07 4:54

led mike

31-Oct-07 4:54

cirkit1 wrote:
Yes.

I strongly urge you to read some material about multi-threading and synchronization. I recommend the book Advanced Windows by Richter. I have also seen people recommend a web site tutorial on threading but I don't know the URL. I am sure you can find it using google. I think there are articles here on CodeProject as well.

Re: How do I make a thread safe call to a listview.

ejuanpp31-Oct-07 5:07

ejuanpp

31-Oct-07 5:07

Is it http://www.albahari.com/threading/ ?

Re: How do I make a thread safe call to a listview.

cirkit131-Oct-07 5:10

cirkit1

31-Oct-07 5:10

I have searched a multitude of sites and so far nothing specific to listview and my particular problem, hence my posting. There is a lot out there though, I agree.

As an FYI, I was able to handle the same exception error regarding a radio control where I needed to change the boolean state of radio component from an async process, specifically when called from event-handler OnTimerDoThis(). I did this by following some examples for a textbox from MSDN. The following is portions of code to do this:

// create a delegate within the class
delegate void SetRadioCallback(bool Status);

private void SetRadio(bool status)
{
// InvokeRequired required compares the thread ID of the
// calling thread to the thread ID of the creating thread.
// If these threads are different, it returns true.
if (this.rdoTimerStatus.InvokeRequired)
{
// calling and creating thread are different
SetRadioCallback r = new SetRadioCallback(SetRadio);
this.Invoke(r, new object[] { status });
}
else
{
this.rdoTimerStatus.Checked = status;
}
}

// EnableTimer() is called from the main thread and async thread
// at different times, never at the same time
protected void EnableTimer()
{
m_Timer.Enabled = true;

// call SetRadio to determine which thread is calling and update
this.SetRadio(true);
}

Now, how do I do the same for a listview? I am not really changing anything in the listview. I just want it to search in the listview items for a match on the time elapsed event.

Re: How do I make a thread safe call to a listview.

led mike31-Oct-07 5:21

led mike

31-Oct-07 5:21

cirkit1 wrote:
I have searched a multitude of sites and so far nothing specific to listview and my particular problem

That's why I posted what I did. In my opinion you don't want to try to learn about mult threading issues by being "specific" to using a listview. You seem resistant to my suggestion so I am not going to argue with you.

Have you found this MSDN Article[^]

Re: How do I make a thread safe call to a listview.

cirkit131-Oct-07 5:26

cirkit1

31-Oct-07 5:26

Don't mean sound resistant. Just providing background.

I will look at the info provided. Both sources are great. Much appreciated, really.

I will play some more.

Re: How do I make a thread safe call to a listview.

ejuanpp31-Oct-07 5:23

ejuanpp

31-Oct-07 5:23

Have a look at:

http://msdn2.microsoft.com/en-us/library/system.collections.ienumerator(VS.80).aspx

In particular:

An enumerator remains valid as long as the collection remains unchanged. If changes are made to the collection, such as adding, modifying, or deleting elements, the enumerator is irrecoverably invalidated and the next call to MoveNext or Reset throws an InvalidOperationException

And:

http://www.yoda.arachsys.com/csharp/threads/winforms.shtml

Regards

Re: How do I make a thread safe call to a listview.

Luc Pattyn31-Oct-07 5:35

Luc Pattyn

31-Oct-07 5:35

Hi,

cirkit1 wrote:
I am not really changing anything in the listview.

whatever you want to do to a Control (set a property, get a property, call a method),
you must do so from the thread that created the Control in the first place.
With the exception of a few: Control.InvokeRequired, Control.Invoke(), etc.

So yes, you need the same "if InvokeRequired Invoke self" construct even to read a
ListView item.

BTW: Now you may choose to do this for each and every single Control access, or you may move
a higher-level method to the Control's thread of course, that would take more GUI time
(i.e. freeze your GUI longer), but all-in-all be better performance wise.

Smile | :)

Luc Pattyn [Forum Guidelines] [My Articles]

this months tips:
- use PRE tags to preserve formatting when showing multi-line code snippets
- before you ask a question here, search CodeProject, then Google

Beginner, querying random number of nodes in LinQ

welldone10131-Oct-07 3:21

welldone101

31-Oct-07 3:21

I'm a programming beginner so please be as harsh as you like but keep in mine I'm a complete "newb". I'm starting with C# and this is my first program.. a windows form app "cookbook" for my wife.

I'm outputting one element of an XML document to a windows form using LinQ in C# (2008 ve beta) and I'm having trouble with there being a random number of elements. For example here is the document.xml outline:

<br />
<cookbook><br />
  <recipe><br />
    <name /><br />
    <description /><br />
    <instructions /><br />
    <ingredients><br />
      <ingredient1><br />
        <amount /><br />
        <measurement /><br />
        <item /><br />
        </ingredient1><br />
      [...]random number of ingredient nodes<br />
      <ingredientX /><br />
    </ingredients><br />
  </recipe><br />
</cookbook><br />

So each "recipe" will have a different amount of ingredients and I won't know how many that is until I start querying the document. They will be named etc.

I've built the code to display everything but the ingredients, displayed below. I think I will have to next a foreach loop inside my current foreach loop to do this but the syntax I've tried has all been deadends. Does anybody have any experience with this in Linq?

<br />
XDocument cookbook = XDocument.Load(GlobalLogin.filename); //Load doc<br />
<br />
var display = from item in cookbook.Descendants("Recipe")  //Select XElements<br />
              where item.Element("Name") != null && item.Element("Name").Value == name<br />
              select new<br />
              {<br />
              Name = item.Element("Name").Value,<br />
              Description = item.Element("Description").Value,<br />
              Instructions = item.Element("Instructions").Value<br />
              };                                        //Nested "from" loop here?<br />
<br />
foreach (var item in display)                           //Display on form<br />
{<br />
 Label recipename = new Label();                        //Name + Description<br />
 recipename.Text = item.Name + ": " + item.Description;<br />
 recipename.Location = new Point(10, 10);<br />
 Label instructions = new Label();                      //Instructions<br />
 instructions.Text = item.Instructions;<br />
 instructions.Location = new Point(10, 35);<br />
 this.Controls.Add(recipename);                         //Add to form<br />
 this.Controls.Add(instructions);<br />
}

I put nice little spaces in there to stagger it but it is ignoring them.. how do I get that to show up on this message board? Is there a way?

Re: Beginner, querying random number of nodes in LinQ

Christian Wikander31-Oct-07 4:17

Christian Wikander

31-Oct-07 4:17

Do you create the XML structure? If so, don't add the number after ingredient. It's quite possible to have several elements with the same name. This will probably simplify things a bit for you. You just look for all elements with a certain name and loop through them.

Re: Beginner, querying random number of nodes in LinQ

welldone10131-Oct-07 14:47

welldone101

31-Oct-07 14:47

Wow didn't know you could do that, heh. That makes things A LOT easier, thanks!

How do I find username of logged on user from a service

dave@dmwilson.net31-Oct-07 3:08

dave@dmwilson.net

31-Oct-07 3:08

I have written a c# console application that updates active directory. To have the proper permissions to update active directory it runs under NT/Authority. I need to be able to find the user who logged on to update his AD fields. userName = Environment.UserName.ToString(); returns NT/Authority and I need the name of the user who is logged on, not the user running the service.

David Wilson

Re: How do I find username of logged on user from a service

eggsovereasy31-Oct-07 9:05

eggsovereasy

31-Oct-07 9:05

What if no one is logged in or multiple people are logged in?

Re: How do I find username of logged on user from a service

dave@dmwilson.net31-Oct-07 9:13

dave@dmwilson.net

31-Oct-07 9:13

If no one is logged in then the service will not do anything. The service will only be running on XP and Vista so multiple users will not be an issue.

Re: How do I find username of logged on user from a service

mav.northwind31-Oct-07 22:37

mav.northwind

31-Oct-07 22:37

Hi!
That's not true. You can swap users with XP and Vista, so chances are there actually are more than one user logged in.

Regards,
mav

--
Black holes are the places where God divided by 0...

Re: How do I find username of logged on user from a service

dave@dmwilson.net1-Nov-07 2:19

dave@dmwilson.net

1-Nov-07 2:19

In my environment that can't happen. Users are required to use their smartcard to logon. If the card is removed the terminal locks. There is only one card reader so only one user can be logged in at one time.

Re: How do I find username of logged on user from a service

mav.northwind1-Nov-07 7:14

mav.northwind

1-Nov-07 7:14

dave@dmwilson.net wrote:
In my environment that can't happen

I guess that's among the top ten statements that indicate that there'll be trouble later on.
Anyway, I think your best chance is to search for a Win32 function to retrieve the list of currently active sessions - I don't think there's something in the BCL to provide you with this information.

Regards,
mav

--
Black holes are the places where God divided by 0...

Extracting images from wlan ip webcam with activeX control

the_chabo31-Oct-07 3:03

the_chabo

31-Oct-07 3:03

Hello to everybody,

first of all i want to express how useful these forums are and thank everyone for his help.

Now to my problem: i need to connect a wlan webcam to a pda with wm5. the webcam sends avi files which can be accessed by a normal webbrowser through activeX. Actually i need the stream to grab some images to send them further to an object tracker. The object tracker works fine on a win ce 5.0 system. So all i want to know is if it`s possible to do so and how can i solve that issue.

Thank you in advance.By the way it`s very urgent Smile | :)

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