How to set maxlength for combobox in WPF?

Question

Hi,

How do i set maxlength to combobox, which is having a style applied to it.

Thanks

Answer 1

You are correct. There is a maxlength for a Textbox, but not for a combobox. You have to roll your own using a Textbox as an intermediary. Here's some code:

public int MaxLength {get; set;}
protected override void OnGotFocus(System.Windows.RoutedEventArgs e)
{
    base.OnGotFocus(e);
    TextBox thisTextBox = (TextBox)base.GetTemplateChild("PART_EditableTextBox");
    if (thisTextBox != null)
        thisTextBox.MaxLength = MaxLength;
}

Answer 2

thisTextBox is coming null. the style given is as follows.

Source="Arrow.png" Margin="4,4,4,4" HorizontalAlignment="Center" VerticalAlignment="Center" /> -->

<ControlTemplate x:Key="ComboBoxTextBox" TargetType="TextBox">
    <Border x:Name="PART_ContentHost" Focusable="False" Background="{TemplateBinding Background}" />
</ControlTemplate>


<Style x:Key="ComboBoxStyle" TargetType="ComboBox">
    <Setter Property="SnapsToDevicePixels" Value="true"/>
    <Setter Property="OverridesDefaultStyle" Value="true"/>
    <Setter Property="ScrollViewer.HorizontalScrollBarVisibility" Value="Auto"/>
    <Setter Property="ScrollViewer.VerticalScrollBarVisibility" Value="Auto"/>
    <Setter Property="ScrollViewer.CanContentScroll" Value="true"/>
    <Setter Property="MinWidth" Value="120"/>
    <Setter Property="MinHeight" Value="20"/>
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="ComboBox">
                <Grid>
                    <ToggleButton 
        Name="ToggleButton" 
        Template="{StaticResource ComboBoxToggleButton}" 
        Grid.Column="2" 
        Focusable="false"
        IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}"
        ClickMode="Press">
                    </ToggleButton>
                    <ContentPresenter
        Name="ContentSite"
        IsHitTestVisible="False" 
        Content="{TemplateBinding SelectionBoxItem}"
        ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
        ContentTemplateSelector="{TemplateBinding ItemTemplateSelector}"
        Margin="9,2,28,2"
        VerticalAlignment="Center"
        HorizontalAlignment="Left" />
                    <TextBox x:Name="PART_EditableTextBox"
        Style="{x:Null}" 
        Template="{StaticResource ComboBoxTextBox}" 
        HorizontalAlignment="Left" 
        VerticalAlignment="Center" 
        Margin="3,3,23,3"
        Focusable="True" 
        Background="Transparent"
        Visibility="Hidden"
        IsReadOnly="{TemplateBinding IsReadOnly}"/>
                    <Popup 
        Name="Popup"
        Placement="Bottom"
        IsOpen="{TemplateBinding IsDropDownOpen}"
        AllowsTransparency="True" 
        Focusable="False"
        PopupAnimation="Slide">
                        <Grid 
          Name="DropDown"
          SnapsToDevicePixels="True"             
          MinWidth="{TemplateBinding ActualWidth}"
          MaxHeight="{TemplateBinding MaxDropDownHeight}">
                            <Border 
            x:Name="DropDownBorder"
             Background="#CCFFCC" 
             BorderBrush="#000080"
            BorderThickness="2"
            />
                            <ScrollViewer Margin="4,6,4,6" SnapsToDevicePixels="True">
                                <ScrollViewer.Resources>
                                    <sys:Double x:Key="{x:Static SystemParameters.VerticalScrollBarWidthKey}">25</sys:Double>
                                </ScrollViewer.Resources>                                    
                                <StackPanel IsItemsHost="True" />
                            </ScrollViewer>
                        </Grid>
                    </Popup>
                </Grid>
                <ControlTemplate.Triggers>
                    <Trigger Property="HasItems" Value="false">
                        <Setter TargetName="DropDownBorder" Property="MinHeight" Value="95"/>
                    </Trigger>
                    <Trigger Property="IsEnabled" Value="false">
                        <Setter Property="Foreground" Value="{StaticResource DisabledForegroundBrush}"/>
                    </Trigger>
                    <Trigger Property="IsGrouping" Value="true">
                        <Setter Property="ScrollViewer.CanContentScroll" Value="false"/>
                    </Trigger>
                    <Trigger SourceName="Popup" Property="Popup.AllowsTransparency" Value="true">
                        <Setter TargetName="DropDownBorder" Property="CornerRadius" Value="4"/>
                        <Setter TargetName="DropDownBorder" Property="Margin" Value="0,2,0,0"/>
                    </Trigger>
                    <Trigger Property="IsEditable"
           Value="true">
                        <Setter Property="IsTabStop" Value="false"/>
                        <Setter TargetName="PART_EditableTextBox" Property="Visibility"    Value="Visible"/>
                        <Setter TargetName="ContentSite" Property="Visibility" Value="Hidden"/>
                    </Trigger>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </Setter.Value>
    </Setter>
    <Setter Property="ItemContainerStyle" >
        <Setter.Value>
            <Style TargetType="{x:Type ComboBoxItem}">
                <Setter Property="FontSize" Value="20" />
            </Style>
        </Setter.Value>
    </Setter>
</Style>

Answer 3

Hey, using DependencyProperty, we can set the maxlength of the combo box without modifying your style/template.

public class EditableComboBox
{

    public static int GetMaxLength(DependencyObject obj)
    {
        return (int)obj.GetValue(MaxLengthProperty);
    }

    public static void SetMaxLength(DependencyObject obj, int value)
    {
        obj.SetValue(MaxLengthProperty, value);
    }

    // Using a DependencyProperty as the backing store for MaxLength.  This enables animation, styling, binding, etc...
    public static readonly DependencyProperty MaxLengthProperty = DependencyProperty.RegisterAttached("MaxLength", typeof(int), typeof(EditableComboBox), new UIPropertyMetadata(OnMaxLenghtChanged));

    private static void OnMaxLenghtChanged(DependencyObject obj, DependencyPropertyChangedEventArgs args)
    {
        var comboBox = obj as ComboBox;
        if (comboBox == null) return;

        comboBox.Loaded +=
            (s, e) =>
            {
                var textBox = comboBox.FindChild(typeof(TextBox), "PART_EditableTextBox");
                if (textBox == null) return;

                textBox.SetValue(TextBox.MaxLengthProperty, args.NewValue);
            };
    }
}

Usage example:

<ComboBox ComboboxHelper:EditableComboBox.MaxLength="50" />

Where ComboboxHelper is:

xmlns:ComboboxHelper="clr-namespace:yourNameSpace;assembly=yourAssembly"

comboBox.FindChild(...) method is posted here.

Answer 4

Or you can use the GotFocus or Loaded event of the combobox for setting the maxlength.If the maxlength doest change too much during runtime you can use loaded event or else use gotfocus event

<ComboBox Height="30" IsEditable="True" Loaded="ComboBox_Loaded"/>

and in the respective event...

   var obj = (ComboBox)sender;
    if (obj != null)
    {
        var myTextBox = (TextBox)obj.Template.FindName("PART_EditableTextBox",obj);
        if (myTextBox != null)
        {
            myTextBox.MaxLength = maxLength;
        }
    }