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Implement Custom Sorting

Sorting items by Value

By default, RadComboBox sorts items by their Text property. This article will show you how to implement custom sorting - the items will be sorted by their Value property.

For this purpose, we will use the IComparer interface which exposes a method that compares two objects.

Here are the two steps that you need to perform:

  1. Create a class that implements IComparer for your custom sorting:
C#
	     
public class SortComboItemsByValue : IComparer
{
   public int Compare(object x, object y)
   {
	   RadComboBoxItem p1 = new RadComboBoxItem();
	   RadComboBoxItem p2 = new RadComboBoxItem();
	   if (x is RadComboBoxItem)
		   p1 = x as RadComboBoxItem;
	   else
		   throw new ArgumentException("Object is not of type RadComboBoxItem.");
	   if (y is RadComboBoxItem)
		   p2 = y as RadComboBoxItem;
	   else
		   throw new ArgumentException("Object is not of type RadComboBoxItem.");
	   int cmp = 0;
	   if (p1.ComboBoxParent.Sort == RadComboBoxSort.Ascending)
	   {
		   //here we compare the Values of the items
		   cmp = String.Compare(p1.Value, p2.Value, !p1.ComboBoxParent.SortCaseSensitive);
	   }
	   if (p1.ComboBoxParent.Sort == RadComboBoxSort.Descending)
	   {
		   //here we compare the Values of the items
		   cmp = String.Compare(p1.Value, p2.Value, !p1.ComboBoxParent.SortCaseSensitive) * -1;
	   }
	   return cmp;
   }
} 			
  1. Call the overloaded SortItems or Items.Sort methods passing your IComparer as a parameter:
C#
	     
RadComboBox2.Sort = RadComboBoxSort.Ascending;
RadComboBox2.SortItems(new SortComboItemsByValue());
				

That's it. Now the combobox items will be sorted by their Values instead of Text.

Custom sorting with checked items on top

When using RadComboBox with enabled checkbox, common issue is how to bring the checked items on top of the sorted dropdown. This can be achieved by adding additional check in the custom comparer class from the sample above

Sample declaration:

ASP.NET
<telerik:RadComboBox runat="server" AutoPostBack="true" OnItemChecked="RadComboBox1_ItemChecked" ID="RadComboBox1" CheckBoxes="true">
    <Items>
        <telerik:RadComboBoxItem Text="Item 1" Value="1" />
        <telerik:RadComboBoxItem Text="Item 2" Value="2" />
        <telerik:RadComboBoxItem Text="Item 3" Value="3" />
        <telerik:RadComboBoxItem Text="Item 4" Value="4" />
        <telerik:RadComboBoxItem Text="Item 5" Value="5" />
        <telerik:RadComboBoxItem Text="Item 6" Value="6" />
    </Items>
</telerik:RadComboBox>

Code-behind:

C#
protected void Page_Load(object sender, EventArgs e)
{
    RadComboBox1.Sort = RadComboBoxSort.Ascending;
    RadComboBox1.SortItems(new SortCheckedComboItemsFirst());
}
public class SortCheckedComboItemsFirst : IComparer
{
    public int Compare(object x, object y)
    {
        RadComboBoxItem p1 = new RadComboBoxItem();
        RadComboBoxItem p2 = new RadComboBoxItem();
        if (x is RadComboBoxItem)
            p1 = x as RadComboBoxItem;
        else
            throw new ArgumentException("Object is not of type RadComboBoxItem.");
        if (y is RadComboBoxItem)
            p2 = y as RadComboBoxItem;
        else
            throw new ArgumentException("Object is not of type RadComboBoxItem.");
        int cmp = 0;

        if (p1.Checked)
        {
            if (p2.Checked == false)
            {
                return -1;
            }
            // uncomment for skipping the sorting of the checked items
            //else
            //{                   
            //    return 1;
            //}
        }
        else if (p2.Checked)
        {
            return 1;
        }

        if (p1.ComboBoxParent.Sort == RadComboBoxSort.Ascending)
        {
            //here we compare the Values of the items
            cmp = String.Compare(p1.Value, p2.Value, !p1.ComboBoxParent.SortCaseSensitive);
        }
        if (p1.ComboBoxParent.Sort == RadComboBoxSort.Descending)
        {
            //here we compare the Values of the items
            cmp = String.Compare(p1.Value, p2.Value, !p1.ComboBoxParent.SortCaseSensitive) * -1;
        }
        return cmp;
    }
}

protected void RadComboBox1_ItemChecked(object sender, RadComboBoxItemEventArgs e)
{

}

See Also