MultiSelectComboBox.xaml

<UserControl x:Class="CrmCodeGenerator.Controls.MultiSelectComboBox"
             xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
             xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
             >
    <ComboBox
        x:Name="MultiSelectCombo"  
        SnapsToDevicePixels="True"
        OverridesDefaultStyle="True"
        ScrollViewer.HorizontalScrollBarVisibility="Auto"
        ScrollViewer.VerticalScrollBarVisibility="Auto"
        ScrollViewer.CanContentScroll="True"
        IsSynchronizedWithCurrentItem="True"
                           >
        <ComboBox.ItemTemplate>
            <DataTemplate>
                <CheckBox Content="{Binding Title}"
                          IsChecked="{Binding Path=IsSelected, Mode=TwoWay}"
                          Tag="{RelativeSource FindAncestor, AncestorType={x:Type ComboBox}}"
                           Click="CheckBox_Click"           />
            </DataTemplate>
        </ComboBox.ItemTemplate>
        <ComboBox.Template>
            <ControlTemplate TargetType="ComboBox">
                <Grid >
                    <ToggleButton 
                        x:Name="ToggleButton" 
                       Grid.Column="2" IsChecked="{Binding Path=IsDropDownOpen,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}"
                        Focusable="false"                           
                        ClickMode="Press" HorizontalContentAlignment="Left" >
                        <ToggleButton.Template>
                            <ControlTemplate>
                                <Grid>
                                    <Grid.ColumnDefinitions>
                                        <ColumnDefinition Width="*"/>
                                        <ColumnDefinition Width="18"/>
                                    </Grid.ColumnDefinitions>
                                    <Border
                  x:Name="Border" 
                  Grid.ColumnSpan="2"
                  CornerRadius="2"
                  Background="White"
                  BorderBrush="Black"
                  BorderThickness="1,1,1,1" />
                                    <Border 
                    x:Name="BorderComp" 
                  Grid.Column="0"
                  CornerRadius="2" 
                  Margin="1" 
                 Background="White"
                  BorderBrush="Black"
                  BorderThickness="0,0,0,0" >
                                        <TextBlock Text="{Binding Path=Text,RelativeSource={RelativeSource Mode=FindAncestor, AncestorType=UserControl}}" 
                                               Background="White" Padding="3" />
                                    </Border>
                                    <Path 
                  x:Name="Arrow"
                  Grid.Column="1"     
                  Fill="Black"
                  HorizontalAlignment="Center"
                  VerticalAlignment="Center"
                  Data="M 0 0 L 4 4 L 8 0 Z"/>
                                </Grid>
                            </ControlTemplate>
                        </ToggleButton.Template>
                         </ToggleButton>
                        <Popup 
                        Name="Popup"
                        Placement="Bottom"                        
                        AllowsTransparency="True" 
                        Focusable="False"  IsOpen="{TemplateBinding IsDropDownOpen}"
                        PopupAnimation="Slide">
                        <Grid 
                                  Name="DropDown"
                                  SnapsToDevicePixels="True"  
                            MinWidth="{TemplateBinding ActualWidth}"
                                  MaxHeight="{TemplateBinding MaxDropDownHeight}">
                            <Border 
                                    x:Name="DropDownBorder" 
                                   BorderThickness="1" Background="White"
                                    BorderBrush="Black"/>
                            <ScrollViewer Margin="4,6,4,6" SnapsToDevicePixels="True" DataContext="{Binding}">
                                <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 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>
                </ControlTemplate.Triggers>
            </ControlTemplate>
        </ComboBox.Template>
    </ComboBox>
</UserControl>