Re: Writing to ListBox from multiple threads - C# Discussion Boards

Re: Calculate the age in years,months and days using the datetimepicker.

7-Apr-09 1:04

If you google for 'age calculation c#' you will get several pages of hits. Some are more accurate than others, but they should give you some ideas at least. Smile | :)

Luc Pattyn7-Apr-09 3:50

Writing to ListBox from multiple threads

7-Apr-09 3:50

others have given the answer already.

Schdev wrote:
int ageYears=Datetime.Now.Year-datetimepicker1.Value.Year;

is plain wrong unless today is December 31.

Smile | :)

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Bruce Coward7-Apr-09 0:00

7-Apr-09 0:00

I have a fairly standard Form1.cs with a listbox to display comms traffic. The comms traffic comes from two different interfaces each of which is handled by it's own background thread. I want to log the traffic in the listbox on Form1 and have set up a delegate / Invoke routine as below in the Form1 class:

//*********************************************************************
// public void log(string message)
// This function appends the message to the list box
//*********************************************************************
public delegate void LogDelegate(string s);
public void log(string message)
{
    if (InvokeRequired)
    {
        Invoke(new LogDelegate(log), new object[] { message });
    }
    else
    {
        lb.Items.Add(message);
        lb.TopIndex = lb.Items.Count - 1;
    }
} // End of void log()

When I try to call log(string) from either of the backgound threads the compiler isn't finding this function and I cant for the life of me see why not. I did try making it a static but that threw lots of errors so I took that away.

What am I doing wrong please?

Many thanks, Bruce Confused | :confused:

Nicholas Butler7-Apr-09 0:52

Nicholas Butler

7-Apr-09 0:52

I suspect this problem has nothing to do with threads.

public void log( string message ) is a normal ( instance ) method in Form1, so you need to call it using an instance: myForm1.log( message ).

Nick

----------------------------------
Be excellent to each other Smile | :)

Bruce Coward7-Apr-09 1:04

7-Apr-09 1:04

Hi Nick, Thanks for your response. The thread will see Form1.log() if I make the log method a static. However if I make it a static the compiler kicks on just about every line of log() with "Object refernece required for ..." I just am not sure what it is looking for that I haven't provided.
Cheers, Bruce

Nicholas Butler7-Apr-09 1:19

Nicholas Butler

7-Apr-09 1:19

Assuming Form1 is your main form and you only have one instance of it, put this in Form1:

static Form1 _Instance = null;

public Form1()
{
  if ( _Instance != null ) [ throw an exception or assert or something ]
  _Instance = this;
}

public static void LogMessage( string message )
{
  _Instance.log( message );
}

Then use this from your threads:

Form1.LogMessage( message );

Nick

----------------------------------
Be excellent to each other Smile | :)

Luc Pattyn7-Apr-09 3:53

7-Apr-09 3:53

Yep, that is why Nick said myForm1.log, not Form1.log.
myForm1 should be a variable pointing to your Form1 instance.

Smile | :)

Luc Pattyn [Forum Guidelines] [My Articles]

Henry Minute7-Apr-09 1:15

7-Apr-09 1:15

Hi Bruce,

I think that the problem might be with the:

if (InvokeRequired)<br />

line. Try:

if (this.lb.InvokeRequired)<br />

and then

lb.BeginInvoke(new LogDelegate(log), new object[] { message });<br />

Hope this helps. Smile | :)

Nicholas Butler7-Apr-09 1:23

Nicholas Butler

7-Apr-09 1:23

The Form and ListBox both run on the same ( UI ) thread, so it doesn't matter which you use.

Nick

----------------------------------
Be excellent to each other Smile | :)

Henry Minute7-Apr-09 1:35

7-Apr-09 1:35

Yes, of course! How dumb. Blush | :O

Bruce Coward7-Apr-09 1:24

7-Apr-09 1:24

Thanks Henry.

That is still throwing compile errors. I am beginning to suspect that the problem is because the two background threads are each in their own class file and that they cannot see the log(message) method in Form1 unless I make that method a static. Having made it a static the compiler is having trouble with pretty much every line of the method as follows:

Error 2 An object reference is required for the non-static field, method, or property 'System.Windows.Forms.Control.Invoke(System.Delegate, params object[])' \\Server1\srvr\Code\PC Programs\SYSTest\SYSTest\Form1.cs 60 17 SYSTest

Can anybody confirm whether I can make the log(message) a static or do I have to restructure my solution to put the background threads inside the Form1 class so I do not have to make the function into a static?

Many thanks for all your valuable input,

Bruce

Henry Minute7-Apr-09 1:35

7-Apr-09 1:35

The two background threads certainly need to be able to 'see' Form1, the best bet is probably as Nick suggested by passing a reference to Form1 in the constructor of the threads.

The problems caused by making log() static are that log() itself will not be able to see the listbox.

Bruce Coward7-Apr-09 1:40

7-Apr-09 1:40

"The problems caused by making log() static are that log() itself will not be able to see the listbox."

That makes sense. The compiler errors when the method is static could very well be caused by that.

I will try Nick's suggestion and let you know.

Many, many thanks

Bruce

DaveyM697-Apr-09 2:22

DaveyM69

7-Apr-09 2:22

What is spawning the threads? If it's the same class then you shouldn't be having an issue. If it's a seperate class then the easiest way is to raise an event in that class, that the form can subscribe to, and in the handler invoke the method required.

Luc Pattyn7-Apr-09 4:05

Re: Writing to ListBox from multiple threads - OT

7-Apr-09 4:05

Hi Bruce,

the code shown is perfect. However the problem is elsewhere. You have to help those threads to find and use your log method. Especially for logging purposes I tend to do that with static events, like so
(incomplete and unchecked code):

class Form1 : Form {
    public Form() {
        ...
        Class2.Logger+=log;    // one statement per "log client" class
    }

    public void log(string s) {...}
}

and the "log clients" look like this:

class Class2 {
    public static event Action<string> Logger;
    private void log(string s) {
        if (Logger!=null) Logger(s);
    }
}

BTW: the event does not have to be static, but it allows for doing Logger+=log before the actual client objects have been created.

PS: I always like to have a log(Exception) method too; I either solve that by doing the above stuff twice, or by adding the following code in every log client class:

private void log(Exception exc) {
    foreach(string s in exc.ToString().Split('\r','\n')) if (s.Length!=0) log(s);
}

Luc Pattyn [Forum Guidelines] [My Articles]

DaveyM697-Apr-09 5:14

DaveyM69

7-Apr-09 5:14

Don't want to hijack the thread but...

I experimented briefly with static events a while ago and came across a slight issue of objects not getting disposed.

Normally, if a ClassA instance subscribes to an event in a ClassB instance, when ClassB instance goes out of scope, it will allow ClassA instance to be disposed if there are no other references. With a static event in ClassB, that never happens of course and ClassA instance will continue to exist for the application's lifetime unless it explicitly unsubscribes from the static event.

Worth bearing in mind as otherwise unexpected things can happen, as well as potential memory pressures.

Re: Writing to ListBox from multiple threads - OT

Luc Pattyn7-Apr-09 5:31