Trying to change value cells in a background worker thread

Question

I'm trying to update a Ultragridrow cell in a background worker, but this is throwing a InvalidOperation Exception when this is called more than 1 time.

Here you have the method that starts the RunWorkerAsync.

    private void RefreshGridCacheStart()
    {
        try
        {
            if (this.uGridCache.Rows.Count == 0)
            {
                return;
            }

            if(!workerThread.IsBusy)
            {
                workerThread.DoWork += LookUpHostnames;
                workerThread.ProgressChanged += UpdateCacheHostCell;
                workerThread.RunWorkerCompleted += WorkerCompleted;
                workerThread.WorkerReportsProgress = true;
                workerThread.RunWorkerAsync();
            }
        }
        catch (Exception ex)
        {
            Debug.WriteLine(ex.Message + "\n" + ex.Source + "\n" + ex.ToString());
        }
    }

This is the DoWork method:

    private void LookUpHostnames(object sender, DoWorkEventArgs e)
    {
        var rowValues = new object[2];

        try
        {
            foreach (UltraGridRow row in uGridCache.Rows)//here is were I get an invalid operation exception
            {
                string cellValue = row.Cells["Host"].Text;
                if (Globals.cNet.isValidIP(cellValue))
                {
                    rowValues[0] = row;
                    rowValues[1] = cellValue;

                    workerThread.ReportProgress(0, rowValues);

                    string resolvedHostname = Globals.cIPLookup.LookupHostFromIP(cellValue);
                    rowValues[1] = resolvedHostname;

                    workerThread.ReportProgress(0, rowValues);
                }
            }
        }
        catch (Exception ex)
        {
            Debug.WriteLine(ex.Message + "\n" + ex.Source + "\n" + ex.ToString());
        }

    }

And this is the Report Progress method:

    private void UpdateCacheHostCell(object sender, ProgressChangedEventArgs e)
    {
        var rowValues = e.UserState as object[];
        var row = (UltraGridRow) rowValues[0];
        var sMesage = (string) rowValues[1];

        row.Cells["Host"].Value = sMesage;
    }

Answer 1

You can find your answer here Different question but ultimately the same problem. Your are changing data inside a foreach loop which invalidates the enumerator.
There are 2 possible solutions I see from reading your code

• Save all changes that need to be made to an List of changes and only report progress once after the foreach loop. This might not be a very good solution though since you are processing in the background. If there is other code running that could also change the data in the grid you would get the same error again.
• Since you are not adding rows you could easily change the foreach to a for loop. This might also lead to an issue if code on the main thread could add, or worse, remove rows

Answer 2

Sounds like something must be changing the underlying row collection hence invalidating your enumerable.

If you convert your enumerable to a list using .ToList() (this will cause the enumerable to iterate and give you a new list containing the items in the original) you will be able to iterate over this new enumerable and changes in the source won't affect you.

foreach (UltraGridRow row in uGridCache.Rows.ToList())
{
    ....
    workerThread.ReportProgress(0, rowValues);
}

You will have to be aware that if something else is changing the rows on the grid, your ReportProgress might be reporting progress of something that no longer exists in the grid, you might want to check in your ReportProgress handler whether reporting on progress for that item is still valid before doing whatever you do.

Answer 3

The MSDN documentation on DoWork states the following: "You must be careful not to manipulate any user-interface objects in your DoWork event handler. Instead, communicate to the user interface through the BackgroundWorker events.".

You can view the full details of the DoWork method here: http://msdn.microsoft.com/en-us/library/system.componentmodel.backgroundworker.dowork.aspx

Accessing the UltraGridRows from this event is causing you to access the UltraGrid from another thread and windows forms controls aren't thread safe.

Note that this isn't limited to accessing properties of the control. If you were to set values in the data source that the UltraGrid is bound to you would have the same issue as the change notifications would then happen on the background thread and the UI would still be manipulated from the background thread.

Note that there are only a few members that are actually thread safe on windows forms controls and these are documented in the section on Thread Safety for Control on MSDN: http://msdn.microsoft.com/en-us/library/system.windows.forms.control.aspx

Safe, Simple Multithreading in Windows Forms is a good resource for threading in windows forms even though it is older:

• Part 1: http://msdn.microsoft.com/en-us/library/ms951089.aspx
• Part 2: http://msdn.microsoft.com/en-us/library/ms951109.aspx
• Part 3: http://msdn.microsoft.com/en-us/library/ms993020.aspx

How to: Make Thread-Safe Calls to Windows Forms Controls is also a good resource http://msdn.microsoft.com/en-us/library/ms171728.aspx