DropDownList item Visibility

[Emanuel Varga]

Hello,

In the old ListControl i was using the item.Visibility Collapsed to hide some items while some conditions are not met, in the new version with the DropDownList i've noticed that the visibility property no longer applies to the item, it applies to the visual item, and it no longer works as before, and even more than this the IsVisible property of the item is readOnly, can someone please tell me how should i hide an item and not leave a blank space in the list ?

Thank you very much,

Best Regards,
Emanuel Varga

[Niklas]

I got the same problem...

//Niklas

[Victor]

Hello Emanuel Varga, Niklas,

Thank you for writing.

The approach you were using with the previous combo box and list box do not work with RadListControl and RadDropDownList because of the newly introduced UI virtualization. Now the UI items are transient and you can not rely directly on them to set their state as they are sometimes recreated and/or recycled.
 
In order to hide items that do not match your criteria you should use the filtering feature. You simply set a Predicate<RadListDataItem> method that you have defined to the Filter property of RadDropDownList. This method will be called for every item in your drop down list and the items for which you return false from the predicate will be hidden from view.

Write again if you need further assistance.
 
Kind regards,
Victor
the Telerik team

[Jason Parrish]

Can you provide an example?

[Emanuel Varga]

Hello Jason,

Please take a look at the following example:

using System;
using System.Collections.Generic;
using System.Windows.Forms;
using Telerik.WinControls.UI;

public partial class Form1 : Form
{
    private RadListControl radListControl1;
    private List<string> availableItems = new List<string>
    {
        "a1", "a2", "a3", "b1", "b2", "c1"
    };

    private List<string> selectionItems = new List<string>
    {
        "a", "b", "c"
    };

    private RadDropDownList radDropDownList;

    public Form1()
    {
        InitializeComponent();

        this.Controls.Add(radListControl1 = new RadListControl());
        radListControl1.Dock = DockStyle.Fill;
        radListControl1.Filter += OnListFilter;

        this.Controls.Add(radDropDownList = new RadDropDownList());
        radDropDownList.Dock = DockStyle.Top;
        radDropDownList.SelectedValueChanged += new EventHandler(radDropDownList_SelectedValueChanged);
    }

    protected override void OnLoad(EventArgs e)
    {
        base.OnLoad(e);
        radDropDownList.DataSource = selectionItems;
        radListControl1.DataSource = availableItems;
    }

    private bool OnListFilter(RadListDataItem item)
    {
        return !string.IsNullOrEmpty(radDropDownList.SelectedText) && item.Text.Contains(radDropDownList.SelectedText);
    }

    void radDropDownList_SelectedValueChanged(object sender, EventArgs e)
    {
        radListControl1.Rebind();
    }
}

Hope this helps, if you have any other questions or comments, please let me know,

Best Regards,
Emanuel Varga
Telerik WinForms MVP

[Peter]

Hi Jason,

Thank you for writing.

You can also use the following quick solution to hide a particular item:

this.radDropDownList1.VisualListItemFormatting += new VisualListItemFormattingEventHandler(radDropDownList1_VisualListItemFormatting);
this.radDropDownList1.AutoSizeItems = true;

void radDropDownList1_VisualListItemFormatting(object sender, VisualItemFormattingEventArgs args)
{
     if (args.VisualItem.Data.RowIndex == 2)//collapse the element with index 2
            args.VisualItem.Visibility = ElementVisibility.Collapsed;
}

As you can see in the code snippet above, you can handle the VisualListItemFormatting event and set item Item visibility to collapsed depending on a custom condition. In case you decide to follow this approach, you should also set AutoSizeItems to true as well.

I hope this helps.

 

Kind regards,
Peter
the Telerik team