WPF 绑定到附加属性 绑定到只读属性

绑定有两个需要注意的地方

1.如果绑定到 附加属性（Binding Attached Property），需要加上括号，这个比较特别，例如

<TextBox x:Name="tbUserName"
Width="200"
Grid.Column="0"
Margin="10,10,0,0"
Foreground="Gray"
nasSetting:TextBoxMaskHelper.MaskText="Please input username"
Text="{Binding Path=(nasSetting:TextBoxMaskHelper.MaskText),Mode=OneWay,ElementName=tbUserName}"
Height="30" CharacterCasing="Normal">

<TextBox.Foreground>
<MultiBinding Converter="{StaticResource MaskBrushConverter}">
<Binding Path="Text"   ElementName="tbUserName" UpdateSourceTrigger="PropertyChanged"/>
<Binding Path="(nasSetting:TextBoxMaskHelper.MaskText)"  ElementName="tbUserName" />
</MultiBinding>
</TextBox.Foreground>

2.如果绑定到只读的属性(Binding to readonly property)，例如IsFocused，需要加上 Mode = OneWay

<TextBox.Text>
<MultiBinding Converter="{StaticResource MaskTextConverter}" Mode="OneWay">
<Binding Path="(nasSetting:TextBoxMaskHelper.MaskText)"  ElementName="tbUserName" />
<Binding Path="IsFocused"   ElementName="tbUserName"  Mode="OneWay"/>
</MultiBinding>
</TextBox.Text>