Cannot copy one List to another

Pavel

Hello,

In my application I pass List lst1 into ProgressChanged handler of the a BackGroundWorker.
Inside of handler I need to exploit lst1 for refreshing datagridview. But lst1 is changing in permanence inside of background process.
So I can't directly use lst1, because it's reference-type object and any changes in background process will impact ProgressChanged handler, where lst1 is supposed to be constant.
The solution is: once inside of ProgressChanged handler, assign lst1 to some local list lst2.
Surprisingly, none of the List copying methods works.
I've tried the following:
1. List lst2 = new List(lst1);

2. List lst2 = lst1.ToList();

3. List lst2 = new List();
lst2.AddRange(lst1);

In all cases ArgumentException is generated with the following message:
Destination array was not long enough. Check destIndex and length, and the array's lower bounds.

Thanks in advance.

Pavel.

18 Replies

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Pavel14y ago
Problems resolved when doing all the critical job inside of lock block in BackGround Process:
if (bins_count % 1000 == 0 && bins_count > 0)
{
      lock (padLock)
      {
           List routed_best_case_item = new List(Routed_Best_Cases);
            progr_struct_item.bins_count = bins_count;
            progr_struct_item.routed_cases = routed_best_case_item;
            progr_struct_item.routed_layers = routed_layers_best;
            bw.ReportProgress(1, progr_struct_item);
       }
}

Thanks Vulpes !
Vulpes14y ago
Basically, I think you have two problems here :

1. Trying to speed up the display of up to 100 rows in the DGV.

2. Preventing the BGW from copying the first list into the second until the DGV has displayed the previous records in the list.

On the first point, one thing you could try is not to clear the DGV and then recreate new rows but to simply overwrite the columns in the existing ones. If there are less than 100 rows, then the remainder could all be blanked.

On the second point, you could set a bool variable to tell the BGW whether it's safe to copy the list and call the ReportProgress method and, if it isn't safe, the BGW could block in the meantime. The code would be essentially on these lines:

// add bool field to class

private volatile bool safeToCopy = false;

// in BGW

while(!safeToCopy)
{
Thread.Sleep(100);
}

// code to copy from first list to second list
safeToCopy = false;
// report progress

// in ProgressChanged handler

// code to add data to DGV
// perhaps delay a little with Thread.Sleep to allow user to scan them
safeToCopy = true;
Pavel14y ago
Individually. Here is real code from Handler (sorry for format). If in 1st line I replace 100 by 10, gridview can refresh, but for me 10 is too few.

int row_max = (Routed_Best_Cases.Count <= 100) ? Routed_Best_Cases.Count : 100;

dgv_BestCases.Rows.Clear();

for (int i = 0; i < row_max; i++)

{

dgv_BestCases.Rows.Add(1);

for (int n = 0; n < SFP.Length; n++)

{

try

{

dgv_BestCases.Rows[i].Cells[n].Value = Routed_Best_Cases[i].SFP_layout[n];

dgv_BestCases.Rows[i].Cells[n].Style.ForeColor = Routed_Best_Cases[i].SFP_mirror[n] ?
Color.Blue : Color.Red;

}

catch (ArgumentOutOfRangeException)

{

Debug.WriteLine("SFP i = {0};\tn = {1};\tRows = {2};\trouted_cases = {3}", i, n, dgv_BestCases.Rows.Count, Routed_Best_Cases.Count);

}

}

for (int n = 0; n < Molex_Conn.Length; n++)

{

try

{

dgv_BestCases.Rows[i].Cells[SFP.Length + n].Value = Routed_Best_Cases[i].Track_Order[n].type ==
tx_rx.TX ? "TX" : "RX" + Routed_Best_Cases[i].Track_Order[n].channel.ToString();

dgv_BestCases.Rows[i].Cells[SFP.Length + n].Style.ForeColor = Routed_Best_Cases[i].Track_Order[n].tap !=
taps.DOWN ? Color.Red : Color.Blue;

}

catch (ArgumentOutOfRangeException)

{

Debug.WriteLine("Molex i = {0};\tn = {1};\tRows = {2};\trouted_cases = {3}", i, n, dgv_BestCases.Rows.Count, Routed_Best_Cases.Count);

}

}

}
Vulpes14y ago
Are you using data-binding to bind the list to the DGV or are you adding the rows individually?
Pavel14y ago
Second solution passes ... at least in the sense, that there is no exception, but another problem arises. In the ProgressHandler I use the list for filling datagridview. If class member list is used for this, I should reset datagridview rows and fill them with new values each time Handler is called. As result - the CPU is at 100% and datagridview remains empty - there is no sufficient time before consequtive Handler calls.
Pavel14y ago
Thanks Vulpes !

Maybe I've missed something, but 1st solution doesn't work. Here is piece of code from bg-process:

if (bins_count % 1000 == 0)

                            {

                                progr_struct_item.bins_count = bins_count;

                                progr_struct_item.routed_cases = Routed_Best_Cases;

                                bw.ReportProgress(1, progr_struct_item);

                            }

As you see, I assign the list (routed_cases field in structure) just before calling handler. The esception is the same as mentionned in my previous message.

Will try to follow your second proposition.

Thanks.

P.S. Once more what a ridiculuous past feature in this forum. I participated in many of forums... Never seen code formatting as here. Or putting a code just not supported??? I've contacted site administration. Still no answer.
Vulpes14y ago
Well, I think that's what you'll have to do - create a copy of the list in the background worker, just before you call the ReportProgress method, so that you can be sure that no other code will be manipulating the list at the same time.

However, I don't see any need to pass the copied list as a parameter to the ProgressChanged handler. I'd just define it as a field of your class so that it's accessible from any method in the class.
Pavel14y ago
Yes, in the background thread I do some manipulations with list:
- add new elements
- when some condition is met, the list is cleared
What I can't understand why there is no mean to create some "fixed detached copy" of list in the background thread and pass it as parameter into Progress handler ?
Vulpes14y ago
In the background thread do you ever insert (as opposed to add), remove or change elements in the list?

If you're just adding elements (and possibly reading them ), then we might be able to attack this in a different way though I'll refrain from producing any code till I hear from you on this.
Pavel14y ago
Inside of handler:

List<Routed_Case> curr_routed_cases = null;


lock (padLock)

            {

                curr_routed_cases =
new List<Routed_Case>(((ProgressStruct)e.UserState).routed_cases);

            }

The exception is generated inside of lock(padLock).

P.S. The message editing facilities are terrible. Maybe something wrong in my browser ...
Vulpes14y ago
Unfortunately, that hasn't worked either.

Can you either try again or just post any code where you're doing something with the list.
Pavel14y ago
Image uploading feature doesn't work !!!
Please see zip-attachment.
Pavel14y ago
Pavel14y ago
Unfortunately it didn't work. The same exception. Please, see the image in attachment (sorry for the real names, but not SomeType1, lst1, etc.)
Vulpes14y ago
It's feasible if you use a lock as that will prevent the background thread from altering the list whilst you're trying to copy it. The other thread will wait until the lock is released.

You'll need to add this static field to your form:

private static object padLock = new object();

Now change the code which copies the list to the following:

List lst2 = null;

lock(padLock)
{
lst2 = new List(lst1);
}

Notice that this only creates a 'shallow copy' in that the SomeType1 objects within the list are not themselves copied - only the references to them are copied.
Pavel14y ago
Thanks for answer Vulpes.
No, I don't do this.
First, I don't master well multithreading.
Second, I try to find some technique, that allow to do create "instant flash copy" lst2 of a list lst1. lst1 is continually changing in the bg-process, but lst2 should be equal to the lst1 in the moment of assignement, and then detached. This detached value I want to pass in handler.

Is this second approach feasible ?

Regards.
Vulpes14y ago
This definitely looks like a concurrency issue to me as List is not thread-safe.

When you attempt to copy the list are you using a lock, to prevent the background worker from changing the list concurrently?
Pavel14y ago
Even following construction doesn't work:

SomeType1[] some_type1_arr = new SomeType1[some_type1_lst.Count];
some_type1_lst.CopyTo(some_type1_arr);

While 1st instruction is executing, the List some_type1_lst changes its lenght in the background process: when exception occurs and I check the length of some_type1_arr and count of some_type1_lst, the values are: 422 and 475 correspondingly.

So the main problem is how to pass some_type1_lst into ProgressChanged handler by value !!!

Regards.

Pavel.