msdnの「ComboBoxControlTemplate」を使用できます。
Popupこのスタイルから編集する必要があります  。Gridそこに2行のコントロールを追加しました。私が追加した最初の行では、Extended WPF Toolkit(WatermarkTextBoxTextBox )からより良いオプションがここにあると思います。2行目には、アイテムがあります。WatermarkTextBoxScrollViewer
fromのTextBox最初の行は常に表示ComboBoxされます。アイテムが多数ある場合、これらの要素のみがスクロールコンテンツに含まれます。
以下は完全な例です。
<!-- Fill Brushes -->
<LinearGradientBrush x:Key="NormalBrush" StartPoint="0,0" EndPoint="0,1">
    <GradientBrush.GradientStops>
        <GradientStopCollection>
            <GradientStop Color="#FFF" Offset="0.0"/>
            <GradientStop Color="#CCC" Offset="1.0"/>
        </GradientStopCollection>
    </GradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="HorizontalNormalBrush" StartPoint="0,0" EndPoint="1,0">
    <GradientBrush.GradientStops>
        <GradientStopCollection>
            <GradientStop Color="#FFF" Offset="0.0"/>
            <GradientStop Color="#CCC" Offset="1.0"/>
        </GradientStopCollection>
    </GradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="LightBrush" StartPoint="0,0" EndPoint="0,1">
    <GradientBrush.GradientStops>
        <GradientStopCollection>
            <GradientStop Color="#FFF" Offset="0.0"/>
            <GradientStop Color="#EEE" Offset="1.0"/>
        </GradientStopCollection>
    </GradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="HorizontalLightBrush" StartPoint="0,0" EndPoint="1,0">
    <GradientBrush.GradientStops>
        <GradientStopCollection>
            <GradientStop Color="#FFF" Offset="0.0"/>
            <GradientStop Color="#EEE" Offset="1.0"/>
        </GradientStopCollection>
    </GradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="DarkBrush" StartPoint="0,0" EndPoint="0,1">
    <GradientBrush.GradientStops>
        <GradientStopCollection>
            <GradientStop Color="#FFF" Offset="0.0"/>
            <GradientStop Color="#AAA" Offset="1.0"/>
        </GradientStopCollection>
    </GradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="PressedBrush" StartPoint="0,0" EndPoint="0,1">
    <GradientBrush.GradientStops>
        <GradientStopCollection>
            <GradientStop Color="#BBB" Offset="0.0"/>
            <GradientStop Color="#EEE" Offset="0.1"/>
            <GradientStop Color="#EEE" Offset="0.9"/>
            <GradientStop Color="#FFF" Offset="1.0"/>
        </GradientStopCollection>
    </GradientBrush.GradientStops>
</LinearGradientBrush>
<SolidColorBrush x:Key="DisabledForegroundBrush" Color="#888" />
<SolidColorBrush x:Key="DisabledBackgroundBrush" Color="#EEE" />
<SolidColorBrush x:Key="WindowBackgroundBrush" Color="#FFF" />
<SolidColorBrush x:Key="SelectedBackgroundBrush" Color="#DDD" />
<!-- Border Brushes -->
<LinearGradientBrush x:Key="NormalBorderBrush" StartPoint="0,0" EndPoint="0,1">
    <GradientBrush.GradientStops>
        <GradientStopCollection>
            <GradientStop Color="#CCC" Offset="0.0"/>
            <GradientStop Color="#444" Offset="1.0"/>
        </GradientStopCollection>
    </GradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="HorizontalNormalBorderBrush" StartPoint="0,0" EndPoint="1,0">
    <GradientBrush.GradientStops>
        <GradientStopCollection>
            <GradientStop Color="#CCC" Offset="0.0"/>
            <GradientStop Color="#444" Offset="1.0"/>
        </GradientStopCollection>
    </GradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="DefaultedBorderBrush" StartPoint="0,0" EndPoint="0,1">
    <GradientBrush.GradientStops>
        <GradientStopCollection>
            <GradientStop Color="#777" Offset="0.0"/>
            <GradientStop Color="#000" Offset="1.0"/>
        </GradientStopCollection>
    </GradientBrush.GradientStops>
</LinearGradientBrush>
<LinearGradientBrush x:Key="PressedBorderBrush" StartPoint="0,0" EndPoint="0,1">
    <GradientBrush.GradientStops>
        <GradientStopCollection>
            <GradientStop Color="#444" Offset="0.0"/>
            <GradientStop Color="#888" Offset="1.0"/>
        </GradientStopCollection>
    </GradientBrush.GradientStops>
</LinearGradientBrush>
<SolidColorBrush x:Key="DisabledBorderBrush" Color="#AAA" />
<SolidColorBrush x:Key="SolidBorderBrush" Color="#888" />
<SolidColorBrush x:Key="LightBorderBrush" Color="#AAA" />
<!-- Miscellaneous Brushes -->
<SolidColorBrush x:Key="GlyphBrush" Color="#444" />
<SolidColorBrush x:Key="LightColorBrush" Color="#DDD" />
<ControlTemplate x:Key="ComboBoxToggleButton" TargetType="ToggleButton">
    <Grid>
        <Grid.ColumnDefinitions>
            <ColumnDefinition />
            <ColumnDefinition Width="20" />
        </Grid.ColumnDefinitions>
        <Border
            x:Name="Border" 
            Grid.ColumnSpan="2"
            CornerRadius="2"
            Background="{StaticResource NormalBrush}"
            BorderBrush="{StaticResource NormalBorderBrush}"
            BorderThickness="1" />
        <Border 
            Grid.Column="0"
            CornerRadius="2,0,0,2" 
            Margin="1" 
            Background="{StaticResource WindowBackgroundBrush}" 
            BorderBrush="{StaticResource NormalBorderBrush}"
            BorderThickness="0,0,1,0" />
        <Path 
            x:Name="Arrow"
            Grid.Column="1"     
            Fill="{StaticResource GlyphBrush}"
            HorizontalAlignment="Center"
            VerticalAlignment="Center"
            Data="M 0 0 L 4 4 L 8 0 Z"/>
    </Grid>
    <ControlTemplate.Triggers>
        <Trigger Property="ToggleButton.IsMouseOver" Value="true">
            <Setter TargetName="Border" Property="Background" Value="{StaticResource DarkBrush}" />
        </Trigger>
        <Trigger Property="ToggleButton.IsChecked" Value="true">
            <Setter TargetName="Border" Property="Background" Value="{StaticResource PressedBrush}" />
        </Trigger>
        <Trigger Property="IsEnabled" Value="False">
            <Setter TargetName="Border" Property="Background" Value="{StaticResource DisabledBackgroundBrush}" />
            <Setter TargetName="Border" Property="BorderBrush" Value="{StaticResource DisabledBorderBrush}" />
            <Setter Property="Foreground" Value="{StaticResource DisabledForegroundBrush}"/>
            <Setter TargetName="Arrow" Property="Fill" Value="{StaticResource DisabledForegroundBrush}" />
        </Trigger>
    </ControlTemplate.Triggers>
</ControlTemplate>
<ControlTemplate x:Key="ComboBoxTextBox" TargetType="TextBox">
    <Border x:Name="PART_ContentHost" Focusable="False" Background="{TemplateBinding Background}" />
</ControlTemplate>
<Style x:Key="{x:Type ComboBox}" 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="3,3,23,3"
                        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}">
                            <Grid.RowDefinitions>
                                <RowDefinition Height="30" />
                                <RowDefinition Height="{TemplateBinding MaxDropDownHeight}" />
                            </Grid.RowDefinitions>
                            <Border 
                                x:Name="DropDownBorder"
                                Background="{StaticResource WindowBackgroundBrush}"
                                BorderThickness="1"
                                BorderBrush="{StaticResource SolidBorderBrush}"
                                Grid.RowSpan="2"/>
                            <TextBox Margin="2" VerticalAlignment="Center" />
                            <ScrollViewer Grid.Row="1" Margin="4,6,4,6" SnapsToDevicePixels="True">
                                <StackPanel IsItemsHost="True" KeyboardNavigation.DirectionalNavigation="Contained" />
                            </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>
    <Style.Triggers>
    </Style.Triggers>
</Style>