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RadDropDownList provides flexible auto-completion options that suggest and append text from choices in the list as the user types.


The RadDropDownList.AutoCompleteMode property controls auto-complete behavior and can be set to None, Suggest, Append and SuggestAppend:

All auto-complete modes depend on the value of the Boolean CaseSensitive property.

  • None: Nothing happens when a user begins to type into the text box part of the control. If the user types the whole text of an item and presses Enter, the item is selected.

Figure 1: AutoCompleteMode.None

WinForms RadDropDownList AutoCompleteMode None


this.radDropDownList1.AutoCompleteMode = AutoCompleteMode.None;

Me.radDropDownList1.AutoCompleteMode = AutoCompleteMode.None

  • Suggest: As the user types an entry into the text box, the drop-down part of the control is shown, and the displayed items are filtered according to the entered text.


this.radDropDownList1.AutoCompleteMode = AutoCompleteMode.Suggest;

Me.radDropDownList1.AutoCompleteMode = AutoCompleteMode.Suggest

Figure 2: AutoCompleteMode.Suggest

WinForms RadDropDownList AutoCompleteMode Suggest

  • Append: As the user types, the next item in the list that matches the user input is automatically appended to the characters the user has already typed. The drop-down list is not shown without the user clicking the arrow.

Figure 3: AutoCompleteMode.Append

WinForms RadDropDownList AutoCompleteMode Append


this.radDropDownList1.AutoCompleteMode = AutoCompleteMode.Append;

Me.radDropDownList1.AutoCompleteMode = AutoCompleteMode.Append

  • SuggestAppend: Similar to the Append setting, but the drop-down list is shown and the suggested item is highlighted.


this.radDropDownList1.AutoCompleteMode = AutoCompleteMode.SuggestAppend;

Me.radDropDownList1.AutoCompleteMode = AutoCompleteMode.SuggestAppend

Figure 4: AutoCompleteMode.SuggestAppend

WinForms RadDropDownList AutoCompleteMode SuggestAppend

Auto-Complete Helpers

RadDropDownList internally uses auto-complete helpers to perform the auto-complete functionality.

* AutoCompleteSuggestHelper: it is created when the AutoCompleteMode property is set to AutoCompleteMode.Suggest or AutoCompleteMode.SuggestAppend. You can find below the AutoCompleteSuggestHelper's properties:

  • SuggestMode: determines whether the items are auto-completed considering whether the text starts with or contains the searched text.


this.radDropDownList1.DropDownListElement.AutoCompleteSuggest.SuggestMode = Telerik.WinControls.UI.SuggestMode.Contains;

Me.radDropDownList1.DropDownListElement.AutoCompleteSuggest.SuggestMode = Telerik.WinControls.UI.SuggestMode.Contains

Figure 5: SuggestMode.Contains

WinForms RadDropDownList SuggestMode Contains

  • AutoCompleteDataSource: specifies the data structure to be bound.

  • AutoCompleteDisplayMember: specifies the particular field in the data source which will be used from the items in AutoCompleteSuggestHelper for their Text.

* AutoCompleteAppendHelper: it is created when the AutoCompleteMode property is set to AutoCompleteMode.Append or AutoCompleteMode.SuggestAppend. The LimitToList property controls whether the user is blocked to enter invalid string in the editable part.

Limit the user to enter only valid values

this.radDropDownList1.DropDownListElement.AutoCompleteAppend.LimitToList = true;

Me.radDropDownList1.DropDownListElement.AutoCompleteAppend.LimitToList = True

Customize auto-complete helpers

By default, the items displayed in the AutoCompleteSuggestHelper’s pop-up are sorted alphabetically. The following example demonstrates how to manipulate the sort order considering the item’s Text.Length property:

Custom comparer

public void ApplyComparer()
    this.radDropDownList1.AutoCompleteMode = AutoCompleteMode.Suggest;        
    this.radDropDownList1.DropDownListElement.AutoCompleteSuggest = new CustomAutoCompleteSuggestHelper(this.radDropDownList1.DropDownListElement);
    this.radDropDownList1.DropDownListElement.AutoCompleteSuggest.SuggestMode = SuggestMode.Contains;

public class CustomAutoCompleteSuggestHelper : AutoCompleteSuggestHelper
    public CustomAutoCompleteSuggestHelper(RadDropDownListElement owner) : base(owner)

    public override void ApplyFilterToDropDown(string filter)
        this.DropDownList.ListElement.DataLayer.DataView.Comparer = new CustomComparer();

public class CustomComparer: IComparer<RadListDataItem>
    public int Compare(RadListDataItem x, RadListDataItem y)
        return x.Text.Length.CompareTo(y.Text.Length);

Public Sub ApplyComparer()
    Me.radDropDownList1.AutoCompleteMode = AutoCompleteMode.Suggest
    Me.radDropDownList1.DropDownListElement.AutoCompleteSuggest = New CustomAutoCompleteSuggestHelper(Me.radDropDownList1.DropDownListElement)
    Me.radDropDownList1.DropDownListElement.AutoCompleteSuggest.SuggestMode = SuggestMode.Contains
End Sub
Public Class CustomAutoCompleteSuggestHelper
    Inherits AutoCompleteSuggestHelper
    Public Sub New(owner As RadDropDownListElement)
    End Sub
    Public Overrides Sub ApplyFilterToDropDown(filter As String)
        Me.DropDownList.ListElement.DataLayer.DataView.Comparer = New CustomComparer()
    End Sub
End Class
Public Class CustomComparer
    Implements IComparer(Of RadListDataItem)
    Public Function [Compare](x As RadListDataItem, y As RadListDataItem) As Integer Implements IComparer(Of RadListDataItem).[Compare]
        Return x.Text.Length.CompareTo(y.Text.Length)
    End Function
End Class

Figure 6: Custom comparer

WinForms RadDropDownList Custom Comparer

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