Re: Delegate / Event Behaviour - C# Discussion Boards

17-Jun-06 9:55

Hi
Not sure how to explain this, but here goes. I have a method 'ShowFormForLong' which recieves a long as a parameter. It checks a Dictionary to see if the dictionary contains the long as a key. If it does, the object stored is a form, and the form is 'shown'. If the dictionary doesn't contain the long, a new form is created and added to the dictionary, and shown. Works a treat when the long is entered by the user, they press 'Go' button, and the onButton event calls the ShowFormForLong method.

But...and there always is a but....my app allows sockets to connect, and the socket can recieve a 'long'. I read a long from the socket, and fire an event, which my main form subscribes to. The eventargs contain the 'long' and I call 'ShowFormForLong' passing the long from the eventarg. But this time, the form is shown, but it seems as though the message loop isn't running in the form, as only the background is drawn, no controls are visible, and then the app crashes.

I will get the code on Monday, and post if nobody has any ideas, but is there something I should be doing? Or is it something as simple as I can't create multiple forms...

From memory...

Dictionary< long , MyFormClass>formDictionary;

void ShowFormForLong( long myLong )
{
if (formDictionary.Contains(myLong))
{
   MyFormClass myForm = (MyFormClass)formDictionary[myLong];
   myForm.Show();
}
else
{
   MyFormClass myForm = new MyFormClass();
   formDictionary.Add( myLong , myForm );
   myForm.Show();
}
}

Regards

Angel

*********************************************
The sooner you fall behind, the longer you have to catch up.

-- modified at 16:05 Saturday 17th June, 2006 - typos

Jun Du17-Jun-06 10:55

Jun Du

17-Jun-06 10:55

Angel1058 wrote:
But this time, the form is shown, but it seems as though the message loop isn't running in the form, as only the background is drawn, no controls are visible, and then the app crashes.

Do you know when the crash happens, the long you got from the socket matches an existing one in the dictionary, or is a new one added to the dictionary? Or it crashes in both cases?

- It's easier to make than to correct a mistake.

Malcolm Smart17-Jun-06 11:19

17-Jun-06 11:19

hi Jun

It crashes in the show with a CLR exceptino - Unhandling exception of type "System Exception" occurred in Unknown Module or something like that. The long can exist or not, crashes on both cases, but I think Roberts suggestion below is the answer - cross threading!!

I love this place. New to C# / .NET. Good at C++ and MFC, but this CLR is all good stuff!!

Thanks for your time anyway, much appreciated

Regards

Rose | [Rose]

Angel

*********************************************
The sooner you fall behind, the longer you have to catch up.

Robert Rohde17-Jun-06 11:00

Robert Rohde

17-Jun-06 11:00

Not sure on this but may the event coming from the socket connection be called from a different thread then the GUI was created on. If yes use Control.Invoke to transfer the call to the appropriate thread.

Malcolm Smart17-Jun-06 11:16

17-Jun-06 11:16

Yes it is, the app listens to the socket on its own thread, so the user can do what they want without the app blocking on the receive (not up to async sockets yet!!). So, that could well be the problem. Haven't come across Control.Invoke, so off to google now.

Thanks to both of you for answering. I will post back with the results.

Regards

Rose | [Rose]

Angel

*********************************************
The sooner you fall behind, the longer you have to catch up.

Malcolm Smart17-Jun-06 11:59

17-Jun-06 11:59

Hi
Just been messing with some code - the main application form (which created the second forms based on the 'long' is in thread 1. I output Thread.CurrentThread.GetHashCode just before the second form is displayed to determine this.

When the socket event is fired, the thread is now Thread 3. This is where it gets confusing. Calling a method on the main from (created from thread 1) from thread 1, works, and creates the second form a treat. Calling a method from the main form (created from thread 1) from thread 3, fails, and the second form doesn't work.

Is this what you're saying? I should only call methods from the main form, from thread 1?

I've looked at Invoke, but not got a clue how to use it with events.

If you can confirm the above I will go away relatively happy and know what the problem is.

Cheers

Regards

Rose | [Rose]

Angel

*********************************************
The sooner you fall behind, the longer you have to catch up.

Robert Rohde17-Jun-06 13:29

Robert Rohde

17-Jun-06 13:29

Yes that is what I mean. Its generally unsafe to change GUI elements from another thread than they were created in. Like in your case this will normally not result in an immediate exception but some weird behaviour. This even depends on which OS you are using.
The following snippet will demonstrate how to use Invoke with the standard EventHandler signature:

//in the main form
private void OnEvent(object sender, EventArgs e)
{
    if (base.InvokeRequired) {
        base.Invoke(new EventHandler(OnEvent), 
                    new object[] {sender, e});
    }
    else
    {
        //Do what ever you want to do
    }
}

A bit of explanation:
InvokeRequired just checks if the current thread is the one the form was created in or not. Invoke will then transfer the call over the thread boundaries. Thus this function (when called from a different thread) will call itself again within the correct thread. Note that this call may block if the GUI thread is busy and wait till its free. You could also use BeginInvoke. Then the call would not block while the GUI thread is busy and the thread could keep running. The call would than be made the next time the GUI thread is free.

Malcolm Smart17-Jun-06 20:46

17-Jun-06 20:46

Blinding answer Robert - thanks.

This fixed the problem a treat, only to highlight a second problem (with similar results). When the 'long' comes in via socket, I fire an event. The main form subscribes to display the second form (discussed above), but so does another form, which the user can enter a long manually to keep the app in sync. Unfortunately, when I update the long on the users form with the long from the socket, an event gets fired which creates the second form. So, when a socket long arrives, two separate evenhandlers attempt to create the second form.....hmmm.

Needless to say, they don't now!!! But I'd have never have got there without the above code anyway.

Many thanks for your answer and your time. Much appreciated.

Regards

Rose | [Rose]

Angel

*********************************************
The sooner you fall behind, the longer you have to catch up.

LongRange.Shooter17-Jun-06 18:15

LongRange.Shooter

17-Jun-06 18:15

The problem is probably coming from the fact that the socket request is from a remote system and you cannot do a Show() across that boundary remote boundary. ( The exact same behaviour occurs if you setup a remote agent and call a method remotely that would show a form. )

Basically what you have to do is marshal the assembly across the boundaries so that the form is on the local machine and have the caller issue a CreateInstance and call Show() for this to work.

The reason it does not work is that your Show() opens up a message pump on your machine, the empty form opens up on the remote machine, the message pump on the remote machine issues an Activate only there is nothing on the remote machine listening for it on the message pump. Meanwhile your message pump is sitting there getting bored because it is not recieving any messages.

To start research on what it would take to make it work, read about MarshalByRefObject and then think about the impact of shipping Windows Forms all over the network when doing this.

Malcolm Smart17-Jun-06 20:48

ADO.NET - Inserting a row and getting the ID

17-Jun-06 20:48

No, the second thread is on the local machine. The remote machine is a console app, no gui. Just servers a stream of data as and when. Robert's answer about messing about with GUIs across different threads was the problem, but thanks for the info on remoting. I thought C# / .NET would be a walk in the park after 10+ years of C++ / MFC. It's no park! (Or possibly Central Park, NY after 2300 hrs).

Regards

Rose | [Rose]

Angel

*********************************************
The sooner you fall behind, the longer you have to catch up.

AJ12317-Jun-06 9:44

AJ123

17-Jun-06 9:44

Hi There

This has got to be an easy one... I am coming to ADO.NET from an ADO background, so i appologise if it comes across as totally stupid!

All i want to do is insert a new row into a table and pull back the autonumber primary key assigned to it.

What i don't want to have to do is:

1) Fill a Dataset in order to get a DataTable definition.
2) Use stored procedures.

Anybody know the answer off the top of their head? Guess i really need to buy a good ADO.NET book.

Thanks In Advance
Rich

Re: ADO.NET - Inserting a row and getting the ID

LongRange.Shooter17-Jun-06 18:17

LongRange.Shooter

17-Jun-06 18:17

You might get a faster answer if you post this question on the ADO.NET board instead of the C# board.

Re: ADO.NET - Inserting a row and getting the ID

AJ12317-Jun-06 23:08

AJ123

17-Jun-06 23:08

Good Point! - Thanks

Found my answer at :

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnadonet/html/manidcrisis.asp[^]

Creating a Database Connection with C#

v3nn17-Jun-06 9:19

17-Jun-06 9:19

hello, this is my first post here Smile | :)

i want to create a mysql database connection on a remote machine with c#.

well i tried to work with the SqlDataAdapter to create a connection to my www site but it always says no connection possible Confused | :confused:

after looking for some tutorials i didnt found anything usefull and i hope someone here could help me out Smile | :)

to make it a bit more clearly - how do i connect?

thanks in advance and sry for my bad english Poke tongue | ;-P

Re: Creating a Database Connection with C# [modified]

Guffa17-Jun-06 9:40

Guffa

17-Jun-06 9:40

A more specific error message would be helpful...

Have you installed an OleDb connector for MySQL?

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v3nn17-Jun-06 9:55

17-Jun-06 9:55

oh well i never did that before :->

first i made a new project, just dragged the SqlDataAdapter in my Design and then the Assistent popped up. i chosed "new connection" and entered the infos in the textboxes below.

the error that appeared was the following: [DBNETLIB][ConnectionOpen (Connect()).]SQL Server does not exist or access denied (happens when i want to test the connection or pick a database on the server)

thats all what i did Unsure | :~

Guffa17-Jun-06 9:59

Guffa

17-Jun-06 9:59

Have you configured the database for remove access? By default it's only accessible from the same server.

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v3nn17-Jun-06 11:01

17-Jun-06 11:01

yes its configured but the other reply has got the info i was looking for
(SqlAdapter is only for SQL not MySQL)

thanks anyway for your help Smile | :)

Guffa17-Jun-06 11:45

Guffa

17-Jun-06 11:45

That is right.

I thought that the SqlConnection could handle any OleDb data source, but as I double checked this (as I often do when faced with contradictory information), I see that the SqlConnection only handles SQL Server, and the passage about OleDb data sources in the reference about SqlConnection is a reference to the OleDbConnection.

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Colin Angus Mackay17-Jun-06 10:02

Colin Angus Mackay

17-Jun-06 10:02

v3nn wrote:
i tried to work with the SqlDataAdapter

That will only work with SQL Server, not MySQL

You can download the MySQL Data Provider here[^] and the documentation is here[^]

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v3nn17-Jun-06 10:58