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VerticalSplitterWidth プロパティ

垂直分割の幅を取得または設定します。これは依存プロパティです。
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'宣言
 
Public Property VerticalSplitterWidth As Double
public double VerticalSplitterWidth {get; set;}
解説

このプロパティを使用することによって、Style を指定せずに（VerticalSplitterStyleKey プロパティで）、スプリッターの Width を設定することができます。

使用例
Private Sub XOBinit()

        xamOutlookBar1.VerticalSplitterLocation = VerticalSplitterLocation.Right
        xamOutlookBar1.VerticalSplitterResizeMode = SplitterResizeMode.Deferred

        xamOutlookBar1.IsVerticalSplitterVisible = True
        xamOutlookBar1.VerticalSplitterWidth = 10

        Dim vsplitterPreviewStyle As Style = New Style(GetType(VerticalSplitterPreview))
        vsplitterPreviewStyle.Setters.Add(New Setter(BackgroundProperty, New SolidColorBrush(Colors.Coral)))

        xamOutlookBar1.Resources.Add(XamOutlookBar.VerticalSplitterPreviewStyleKey, vsplitterPreviewStyle)

        Dim vsplitterStyle As Style = New Style(GetType(Thumb))
        vsplitterStyle.Setters.Add(New Setter(BackgroundProperty, New SolidColorBrush(Colors.Chocolate)))
        vsplitterStyle.Setters.Add(New Setter(WidthProperty, 10.0))

        xamOutlookBar1.Resources.Add(XamOutlookBar.VerticalSplitterStyleKey, vsplitterStyle)

End Sub
public IG_XamOutlookBar_Properties()
{
    InitializeComponent();

    xamOutlookBar1.VerticalSplitterLocation = VerticalSplitterLocation.Right;
    xamOutlookBar1.VerticalSplitterResizeMode = SplitterResizeMode.Deferred;

    xamOutlookBar1.IsVerticalSplitterVisible = true;
    xamOutlookBar1.VerticalSplitterWidth = 10;
    
    Style vsplitterPreviewStyle = new Style(typeof(VerticalSplitterPreview));
    vsplitterPreviewStyle.Setters.Add(new Setter(BackgroundProperty, new SolidColorBrush(Colors.Coral)));

    xamOutlookBar1.Resources.Add(XamOutlookBar.VerticalSplitterPreviewStyleKey, vsplitterPreviewStyle);

    Style vsplitterStyle = new Style(typeof(Thumb));
    vsplitterStyle.Setters.Add(new Setter(BackgroundProperty, new SolidColorBrush(Colors.Chocolate)));
    vsplitterStyle.Setters.Add(new Setter(WidthProperty, 10.0));

    xamOutlookBar1.Resources.Add(XamOutlookBar.VerticalSplitterStyleKey, vsplitterStyle);
}
<Window x:Class="XamOutlookBarApp.IG_XamOutlookBar_Properties"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:igOutlookBar="http://infragistics.com/OutlookBar"
    Title="IG_XamOutlookBar_Properties" Height="300" Width="300">
    <StackPanel Orientation="Horizontal">
        <igOutlookBar:XamOutlookBar Name="xamOutlookBar1" 
                                    IsVerticalSplitterVisible="True"
                                    VerticalSplitterLocation="Right" 
                                    VerticalSplitterResizeMode="Deferred"
                                    VerticalSplitterWidth="10"
                                    >
            <igOutlookBar:OutlookBarGroup Header="Group 1" Content="Group 1"/>
            <igOutlookBar:OutlookBarGroup Header="Group 2" Content="Group 2"/>
            <igOutlookBar:XamOutlookBar.Resources>
                <Style x:Key="{x:Static igOutlookBar:XamOutlookBar.VerticalSplitterPreviewStyleKey}" 
                           TargetType="{x:Type igOutlookBar:VerticalSplitterPreview}">
                    <Setter Property="Background" Value="#77339933"/>
                </Style>
                <Style x:Key="{x:Static igOutlookBar:XamOutlookBar.VerticalSplitterStyleKey}" 
                           TargetType="{x:Type Thumb}">
                    <Setter Property="Background" Value="Green"/>
                    <Setter Property="Width" Value="10"/>
                </Style>
            </igOutlookBar:XamOutlookBar.Resources>                            
        </igOutlookBar:XamOutlookBar>
        <TextBlock Text="Vertical Splitter Sample"  Background="AliceBlue"/>
    </StackPanel>
</Window>
参照