The EnableAutoFilter property can be used to limit the visible items in the dropdown area to those that start with the text entered into the WebDropDown (the textbox above the dropdown list).  Here is an example of how to set the properties programmatically for the WebDropDown control wdd.

'setup control
wdd.DisplayMode = Infragistics.Web.UI.ListControls.DropDownDisplayMode.DropDown
wdd.EnableAutoCompleteFirstMatch = True
wdd.EnableAutoFiltering = Infragistics.Web.UI.ListControls.AutoFiltering.Client
'setup data binding
wdd.TextField = "Title"
wdd.ValueField = "Value"
wdd.DataSource = source
wdd.DataBind()

Of course, the properties could have been setup in the .aspx file

Note the following...

• DisplayMode must equal "DropDown" (you can't type into the textbox of the control otherwise)
• EnableAutoFiltering="Client" is the key to having the client do the filtering
• EnableAutoCompleteFirstMatch="true" does the autocomplete.  If set to "false", then the dropdown will still be filtered but you will need to click on an item to select it.

Advanced Stuff...

• By default, filtering is done on a "StartsWith" basis.  That is, when you type the letter "a", all of the items that begin with the letter "a" are displayed.  However you can change this behavior by setting the AutoFilterQueryType.  Possible values are (StartsWith, EndsWith, Contains, DoesNotContain, Equals, DoesNotEqual).  "Contains" offers some interesting possibilities.
• You can change the order that the filtered items are displayed using the AutoFilterSortOrder property.
• You can also change the maximum number of items displayed using the AutoFilterResultSize property.

More Information: https://www.infragistics.com/help/aspnet/webdropdown-autocomplete-and-auto-filtering

Hope this helps

ROB