CODEWITHSUNDEEP
c#
Filip Ekberg
Filip Ekberg

Reputation: 36287

Understanding Delegates

It's not the first time I come across delegates and I am as confused as I were the last time and the time before that. So once and for all I want to get the delgate-confusion cleared up.

My problem is as followed

Having a Graphical User Interface which only displays a ListView with some boud items, I want to load the data from a Data Connection which takes some time, to increase the comfort of using the application I have instancieted a BackgroundWorker and in the doWork-method I want to fetch the data and display it.

This is how I want it

Now this is Cross-Thread-Invoking and I wanted to solve this with Delegates which brings me here. Following this tutorial, I got a working Delegate, However it did not solve the problem, inside my delegate I cannot change my ListView, it still says it is on another thread.

I want to find an Easy explenation on delegates and how to use them to solve my problem. Also, should I think or design my software different?

Upvotes: 4

Views: 939

Answers (3)

IanT8
IanT8

Reputation: 2217

Checkout this code, it does what you need:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
    }

    private void btnFill_Click(object sender, EventArgs e)
    {
        backgroundWorker1.DoWork += new DoWorkEventHandler(backgroundWorker1_DoWork);
        backgroundWorker1.RunWorkerAsync();
    }

    private delegate void AddItemToListViewDelegate(ListView view, ListViewItem item);

    private void AddItemToListView(ListView view, ListViewItem item)
    {
        if (InvokeRequired)
        {
            Invoke(new AddItemToListViewDelegate(AddItemToListView), new object[] { view, item });
            return;
        }

        view.Items.Add(item);
    }

    private delegate void ClearListViewItemsDelegate(ListView view);

    private void ClearListView(ListView view)
    {
        if (InvokeRequired)
        {
            Invoke(new ClearListViewItemsDelegate(ClearListView), new object[] { view });
            return;
        }

        view.Items.Clear();
    }

    void backgroundWorker1_DoWork(object sender, DoWorkEventArgs e)
    {
        for (int i = 0; i < 100; i++)
        {
            if (i == 0)
                ClearListView(listView1);

            var item = new ListViewItem();
            item.Name = i.ToString();
            item.Text = item.Name;
            AddItemToListView(listView1, item);
        }
    }
}

And for WPF something similar is required. Note this is not working code. As I don't use WPF I can't vouch that this is solid code, but it should give you an idea. You may need to create an type derived from EventArgs to encapsulate your listview and listviewitems.

If I get time, I'll edit this post so that it works, but that will have to wait until this evening!

using System.Windows.Threading;
...


if (listView1.Dispatcher.Thread != Thread.CurrentThread)
{
    listView1.Dispatcher.BeginInvoke(
        DispatcherPriority.Normal,
        new EventHandler<ListViewAddEventArgs>(AddItemToListView), sender, new object[] { e } );
    return;
}
listView1.Items.Add(e.File);

Upvotes: 0

Jon Skeet
Jon Skeet

Reputation: 1499800

Normally BackgroundWorker communicates with the UI thread using ReportProgress. You would hook up a delegate to receive those progress events before launching the background worker, and then the progress would be reported on the UI thread, where you're safe to change your ListView.

The other alternative in Windows Forms is to call Control.Invoke or Control.BeginInvoke, passing in a delegate which will update the UI. That delegate will be executed on the UI thread. For an example of this, see my threading tutorial or Joe Albahari's.

The equivalent of this in WPF is the Dispatcher - again, Invoke and BeginInvoke. You can access the dispatcher for a control with the Dispatcher property.

Upvotes: 6

Kirschstein
Kirschstein

Reputation: 14868

You can't change a ui control from a different thread directly, you need to check the Control.InvokeRequired property before you make a change.

See this example on msdn

Upvotes: 2

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