Parallel processing a loop and UI updating for large data

Question

I would like to use a CollectionView of items in the view to real-time update based on a remote XML file.

I am using an ObservableCollection because it is fast. I have a large list of 25,000 records and have other code that will be running in each parallel thread that should take 1 second for each, hence why I chose to parallel

ServerList.cs

[Serializable]
public class ServerList
{
    [Serializable]
    public class ServerList
    {
        public List<Server> Servers { get; set; }
    }

    [Serializable]
    public class Server
    {
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public int Phone { get; set; }
        [XmlIgnore]
        public string FullName { get; set; }
    }
}

ListViewModel.cs

// constructors
public ServerList.Server ServerListViewItem;
private ObservableCollection<ServerList.Server> serverListViewItem;
public ObservableCollection<ServerList> ServerListView
{
    get
    {
        return serverListView;
    }
    set
    {
        this.serverListView = value;
        this.RaisePropertyChanged("serverListView");
    }
}
public ServerList serverList { get; set; }

// load list of servers from XML and parse into collection
this.serverList = new ServerList();
this.ServerListView = new ListCollectionView(new List<ServerList>());
XmlDocument xmlFetch = new XmlDocument();
xmlFetch.LoadXml(new WebClient().DownloadString("http://domain.com/pathto.xml"));
XmlNode xmlNode = xmlFetch;
XmlReader xmlReader = new XmlNodeReader(xmlNode);
XmlSerializer mySerializer = new XmlSerializer(typeof(ServerList));
this.serverList = (ServerList)mySerializer.Deserialize(xmlReader);

// parallel method should update UI as results are finished
private void LoadAll()
{
    Parallel.ForEach(
    this.serverList,
    new ParallelOptions { MaxDegreeOfParallelism = 25 },
    row =>
        {
            this.ServerListViewItem = new ServerList.Server();
            string fullName = row.FirstName + " " + row.LastName;
            this.ServerListViewItem.FullName = fullName;
            this.ServerListViewItem.FirstName = FirstName;
            this.ServerListViewItem.LastName = LastName;
            this.ServerListViewItem.Phone = Phone;
            ServerListView.Add(ServerListViewItem); // problematic!
        }
    );
}

// allow usage of the UI while parallel thread is running
Thread ParallelProcessThread = new Thread(this.LoadAll);
ParallelProcessThread.Start();

List.xaml

<UserControl.Resources>
    <Style x:Key="lvStyle" TargetType="{x:Type ListView}">
        <Setter Property="VirtualizingStackPanel.IsVirtualizing" Value="True"/>
        <Setter Property="VirtualizingStackPanel.VirtualizationMode" Value="Recycling"/>
        <Setter Property="ScrollViewer.IsDeferredScrollingEnabled" Value="True"/>
        <Setter Property="ListView.ItemsSource" Value="{Binding Path=ServerListView}"/>
        <Setter Property="BorderThickness" Value="0"/>
        <Setter Property="Background" Value="#202020" />
        <Setter Property="ListView.View">
            <Setter.Value>
                <GridView AllowsColumnReorder="False">
                    <GridViewColumn Header="FullName" Width="150">
                        <GridViewColumn.CellTemplate>
                            <DataTemplate>
                                <TextBlock Text="{Binding FullName}"/>
                            </DataTemplate>
                        </GridViewColumn.CellTemplate>
                    </GridViewColumn>
                    <GridViewColumn Header="FirstName" Width="150">
                        <GridViewColumn.CellTemplate>
                            <DataTemplate>
                                <TextBlock Text="{Binding FirstName}"/>
                            </DataTemplate>
                        </GridViewColumn.CellTemplate>
                    </GridViewColumn>
                    <GridViewColumn Header="LastName" Width="150">
                        <GridViewColumn.CellTemplate>
                            <DataTemplate>
                                <TextBlock Text="{Binding LastName}"/>
                            </DataTemplate>
                        </GridViewColumn.CellTemplate>
                    </GridViewColumn>
                    <GridViewColumn Header="Phone" Width="Auto">
                        <GridViewColumn.CellTemplate>
                            <DataTemplate>
                                <TextBlock Text="{Binding Phone}"/>
                            </DataTemplate>
                        </GridViewColumn.CellTemplate>
                    </GridViewColumn>
                </GridView>
            </Setter.Value>
        </Setter>
        <Style.Triggers>
            <DataTrigger Binding="{Binding IsLoading}" Value="True">
                <Setter Property="ListView.Cursor" Value="Wait"/>
                <Setter Property="ListView.Background" Value="LightGray"/>
            </DataTrigger>
        </Style.Triggers>
    </Style>
</UserControl.Resources>

.. the call in List.xaml

<ListView Style="{DynamicResource lvStyle}" />

I get this error:

This type of CollectionView does not support changes to its SourceCollection from a thread different from the Dispatcher thread.

Answer 1

2 things:

1. The error you get is because the worker thread (non-UI thread) can't update the UI. To update the UI use the dispatcher. See Using the C# Dispatcher.
2. The scenario and code you mentioned, I don't think you are getting any performance benefit from it. The code in your multi-threaded section only sets the properties and doesn't do some heavy compute-bound or background work. Since, ultimately for updating the UI, you will have to use dispatcher for ServerListView.Add(ServerListViewItem);, the way it's written, the performance would likely only suffer. Simply remove the parallel and if you really want to make it appear fast, make the fetching from remote xml part fast e.g. fetching as one record (or a batch of records) at a time.

PS: Couldn't post as comment due to char limit.