WPF附加属性绑定TwoWay时Intellisense提示“No DataContext found”问题

问题现象 #

开发的UserControl中，ItemsPanelTemplate内的Grid使用了两个自定义附加属性local:GridSetUp.WeekView和local:GridSetUp.GridData，绑定代码如下：

<ItemsPanelTemplate>
 <Grid Name="WeekViewGrid"  Width="Auto" Height="Auto" HorizontalAlignment="Left"                                   
  local:GridSetUp.WeekView="{Binding TheWeek.Days, Mode=TwoWay}"
  local:GridSetUp.GridData="{Binding TheGridData,  Mode=TwoWay}"
  IsSharedSizeScope="True">
 </Grid>
</ItemsPanelTemplate>

Intellisense会标记绑定中的TwoWay，提示“No DataContext found for binding TwoWay”，但实际绑定到DataContext的TheWeek.Days和TheGridData功能完全正常，且Mode语法符合标准。

附加属性的注册代码如下：

WeekView附加属性 #

public static DependencyProperty WeekViewProperty = DependencyProperty.RegisterAttached( 
    "WeekView", 
    typeof(ObservableCollection<Day>), 
    typeof(GridSetUp), 
    new PropertyMetadata(new ObservableCollection<Day> { }, WeekViewChanged)); 

public static ObservableCollection<Day> GetWeekView(Grid grid)
{
    return (ObservableCollection<Day>)grid.GetValue(WeekViewProperty);
}

public static void SetWeekView(Grid grid, ObservableCollection<Day> value)
{
    grid.SetValue(WeekViewProperty, value);
}

GridData附加属性 #

public static DependencyProperty GridDataProperty = DependencyProperty.RegisterAttached(
    "GridData", 
    typeof(GridData), 
    typeof(GridSetUp), 
    new PropertyMetadata(new GridData(), GridDataChanged));

public static GridData GetGridData(Grid grid)
{
    return (GridData)grid.GetValue(GridDataProperty);
}

public static void SetGridData(Grid grid, GridData value)
{
    grid.SetValue(GridDataProperty, value);
}

public static void GridDataChanged(DependencyObject obj, DependencyPropertyChangedEventArgs e)
{            
}    

原因分析 #

这是Visual Studio Intellisense的静态分析误报，核心原因在于：

• 附加属性注册时使用的是PropertyMetadata，没有显式通过FrameworkPropertyMetadata标记该属性支持双向绑定（即未设置FrameworkPropertyMetadataOptions.BindsTwoWayByDefault）。
• Intellisense在解析XAML绑定表达式中的Mode=TwoWay时，出现了逻辑混淆，错误地将Mode本身当成了需要绑定到DataContext的目标属性，而非绑定表达式的配置项，因此抛出“找不到DataContext”的提示。
• 运行时WPF的绑定引擎能正确识别整个绑定表达式的结构，不受Intellisense静态分析的影响，所以实际功能完全正常。

消除提示的方案 #

如果要让Intellisense停止误报，可以修改附加属性的注册代码，使用FrameworkPropertyMetadata并添加BindsTwoWayByDefault选项，明确告知Intellisense该属性支持双向绑定：

// 修改WeekViewProperty注册
public static DependencyProperty WeekViewProperty = DependencyProperty.RegisterAttached(
    "WeekView", 
    typeof(ObservableCollection<Day>), 
    typeof(GridSetUp), 
    new FrameworkPropertyMetadata(
        new ObservableCollection<Day>(), 
        FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
        WeekViewChanged));

// 修改GridDataProperty注册
public static DependencyProperty GridDataProperty = DependencyProperty.RegisterAttached(
    "GridData", 
    typeof(GridData), 
    typeof(GridSetUp), 
    new FrameworkPropertyMetadata(
        new GridData(), 
        FrameworkPropertyMetadataOptions.BindsTwoWayByDefault,
        GridDataChanged));

内容的提问来源于stack exchange，提问作者user3486991