.NET Listview Refresh

Question

I have the following code, which basically takes values from a database and populates a listview.

using (IDataReader reader = cmd.ExecuteReader())
{                    
    lvwMyList.Items.Clear();
    while (reader.Read())
    {
        ListViewItem lvi = lvwMyList.Items.Add(reader["Value1"].ToString());
        lvi.SubItems.Add(reader["Value2"].ToString());                    
    }
}

The problem that I have is that this is repeatedly executed at short intervals (every second) and results in the items in the listview continually disappearing and re-appearing. Is there some way to stop the listview from refreshing until it’s done with the updates? Something like below:

using (IDataReader reader = cmd.ExecuteReader())
{                    
    lvwMyList.Items.Freeze(); // Stop the listview updating
    lvwMyList.Items.Clear();
    while (reader.Read())
    {
        ListViewItem lvi = lvwMyList.Items.Add(reader["Value1"].ToString());
        lvi.SubItems.Add(reader["Value2"].ToString());                    
    }
    lvwMyList.Items.UnFreeze(); // Refresh the listview
}

Answer 1

Like this:

try
{
    lvwMyList.BeginUpdate();
    //bla bla bla

}
finally
{
    lvwMyList.EndUpdate();
}

Make sure that you invoke lvwMyList.Items.Clear() after BeginUpdate if you want to clear the list before filling it.

Answer 2

This is my first time posting on StackOverflow, so pardon the messy code formatting below.

To prevent locking up the form while updating the ListView, you can use the method below that I've written to solve this issue.

Note: This method should not be used if you expect to populate the ListView with more than about 20,000 items. If you need to add more than 20k items to the ListView, consider running the ListView in virtual mode.

 public static async void PopulateListView<T>(ListView listView, Func<T, ListViewItem> func, 
        IEnumerable<T> objects, IProgress<int> progress) where T : class, new()
    {
        if (listView != null && listView.IsHandleCreated)
        {
            var conQue = new ConcurrentQueue<ListViewItem>();

            // Clear the list view and refresh it
            if (listView.InvokeRequired)
            {
                listView.BeginInvoke(new MethodInvoker(() =>
                    {
                        listView.BeginUpdate();
                        listView.Items.Clear();
                        listView.Refresh();
                        listView.EndUpdate();
                    }));
            }
            else
            {
                listView.BeginUpdate();
                listView.Items.Clear();
                listView.Refresh();
                listView.EndUpdate();
            }

            // Loop over the objects and call the function to generate the list view items
            if (objects != null)
            {
                int objTotalCount = objects.Count();

                foreach (T obj in objects)
                {
                    await Task.Run(() =>
                        {
                            ListViewItem item = func.Invoke(obj);

                            if (item != null)
                                conQue.Enqueue(item);

                            if (progress != null)
                            {
                                double dProgress = ((double)conQue.Count / objTotalCount) * 100.0;

                                if(dProgress > 0)
                                    progress.Report(dProgress > int.MaxValue ? int.MaxValue : (int)dProgress);
                            }
                        });
                }

                // Perform a mass-add of all the list view items we created
                if (listView.InvokeRequired)
                {
                    listView.BeginInvoke(new MethodInvoker(() =>
                        {
                            listView.BeginUpdate();
                            listView.Items.AddRange(conQue.ToArray());
                            listView.Sort();
                            listView.EndUpdate();
                        }));
                }
                else
                {
                    listView.BeginUpdate();
                    listView.Items.AddRange(conQue.ToArray());
                    listView.Sort();
                    listView.EndUpdate();
                }
            }
        }

        if (progress != null)
            progress.Report(100);
    }

You don't have to provide an IProgress object, just use null and the method will work just as well.

Below is an example usage of the method.

First, define a class that contains the data for the ListViewItem.

public class TestListViewItemClass
{
    public int TestInt { get; set; }

    public string TestString { get; set; }

    public DateTime TestDateTime { get; set; }

    public TimeSpan TestTimeSpan { get; set; }

    public decimal TestDecimal { get; set; }
}

Then, create a method that returns your data items. This method could query a database, call a web service API, or whatever, as long as it returns an IEnumerable of your class type.

public IEnumerable<TestListViewItemClass> GetItems()
{
    for (int x = 0; x < 15000; x++)
    {
        yield return new TestListViewItemClass()
        {
            TestDateTime = DateTime.Now,
            TestTimeSpan = TimeSpan.FromDays(x),
            TestInt = new Random(DateTime.Now.Millisecond).Next(),
            TestDecimal = (decimal)x + new Random(DateTime.Now.Millisecond).Next(),
            TestString = "Test string " + x,
        };
    }
}

Finally, on the form where your ListView resides, you can populate the ListView. For demonstration purposes, I'm using the form's Load event to populate the ListView. More than likely, you'll want to do this elsewhere on the form.

I've included the function that generates a ListViewItem from an instance of my class, TestListViewItemClass. In a production scenario, you will likely want to define the function elsewhere.

private async void TestListViewForm_Load(object sender, EventArgs e)
{     
    var function = new Func<TestListViewItemClass, ListViewItem>((TestListViewItemClass x) =>
    {
        var item = new ListViewItem();

        if (x != null)
        {
            item.Text = x.TestString;
            item.SubItems.Add(x.TestDecimal.ToString("F4"));
            item.SubItems.Add(x.TestDateTime.ToString("G"));
            item.SubItems.Add(x.TestTimeSpan.ToString());
            item.SubItems.Add(x.TestInt.ToString());
            item.Tag = x;

            return item;
        }

        return null;
    });

       PopulateListView<TestListViewItemClass>(this.listView1, function, GetItems(), progress);

 }

In the above example, I created an IProgress object in the form's constructor like this:

progress = new Progress<int>(value =>
{
    toolStripProgressBar1.Visible = true;

    if (value >= 100)
    {
        toolStripProgressBar1.Visible = false;
        toolStripProgressBar1.Value = 0;
    }
    else if (value > 0)
    {
        toolStripProgressBar1.Value = value;
    }
 });

I've used this method of populating a ListView many times in projects where we were populating up to 12,000 items in the ListView, and it is extremely fast. The main thing is you need to have your object fully built from the database before you even touch the ListView for updates.

Hopefully this is helpful.

I've included below an async version of the method, which calls the main method shown at the top of this post.

public static Task PopulateListViewAsync<T>(ListView listView, Func<T, ListViewItem> func,
        IEnumerable<T> objects, IProgress<int> progress) where T : class, new()
{
    return Task.Run(() => PopulateListView<T>(listView, func, objects, progress));
}