WPF DataGrid表头按钮绑定OpenWindowCommand报错排查

问题描述 #

在WPF项目中，在DataGridTemplateColumn.HeaderTemplate内的按钮上绑定OpenWindowCommand，运行时出现绑定路径错误，提示在String对象上找不到该属性，但ViewModel类中已定义public ICommand OpenWindowCommand属性。

相关代码 #

XAML代码：

<DataGridTemplateColumn.HeaderTemplate>
    <DataTemplate>
        <Grid >
            <Grid.RowDefinitions>
                <RowDefinition Height="*"/>
            </Grid.RowDefinitions>
            <Grid Grid.Column="1" >     
                <Button Command="{Binding OpenWindowCommand}">         
                   <Image Source="{StaticResource IconImage}" Width="20" Height="20" />   
                </Button> 
            </Grid>
            ...

ViewModel代码：

public ICommand OpenWindowCommand
{
    get
    {
           ...
    }
}

报错信息 #

System.Windows.Data Error: 40 : BindingExpression path error: 'OpenWindowCommand' property not found on 'object' ''String' (HashCode=1217782553)'. BindingExpression:Path=OpenWindowCommand; DataItem='String' (HashCode=1217782553); target element is 'Button' (Name=''); target property is 'Command' (type 'ICommand')

System.Windows.Data Information: 20 : BindingExpression cannot retrieve value due to missing information. BindingExpression:Path=OpenWindowCommand; DataItem='String' (HashCode=1217782553); target element is 'Button' (Name=''); target property is 'Command' (type 'ICommand')

System.Windows.Data Information: 21 : BindingExpression cannot retrieve value from null data item. This could happen when binding is detached or when binding to a Nullable type that has no value. BindingExpression:Path=OpenWindowCommand; DataItem='String' (HashCode=1217782553); target element is 'Button' (Name=''); target property is 'Command' (type 'ICommand')

System.Windows.Data Information: 10 : Cannot retrieve value using the binding and no valid fallback value exists; using default instead. BindingExpression:Path=OpenWindowCommand; DataItem='String' (HashCode=1217782553); target element is 'Button' (Name=''); target property is 'Command' (type 'ICommand')

报错原因 #

DataGridTemplateColumn.HeaderTemplate的默认DataContext并不是页面/窗口的ViewModel，而是该列的Header内容（通常是字符串类型的列标题文本）。所以按钮的Command绑定会尝试在String对象上查找OpenWindowCommand属性，自然找不到，从而抛出绑定错误。

解决方法 #

方法1：使用RelativeSource绑定到父级DataGrid的DataContext #

修改按钮的Command绑定，通过RelativeSource找到父级的DataGrid，再访问其DataContext中的命令：

<Button Command="{Binding DataContext.OpenWindowCommand, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}">
    <Image Source="{StaticResource IconImage}" Width="20" Height="20" />
</Button>

方法2：使用ElementName绑定 #

给DataGrid设置一个名称（比如x:Name="MainDataGrid"），然后通过ElementName直接绑定到它的DataContext：

<DataGrid x:Name="MainDataGrid" ...>
    <DataGridTemplateColumn.HeaderTemplate>
        <DataTemplate>
            <!-- 其他代码 -->
            <Button Command="{Binding DataContext.OpenWindowCommand, ElementName=MainDataGrid}">
                <Image Source="{StaticResource IconImage}" Width="20" Height="20" />
            </Button>
        </DataTemplate>
    </DataGridTemplateColumn.HeaderTemplate>
</DataGrid>

方法3：显式设置HeaderTemplate根元素的DataContext #

在HeaderTemplate的根容器（比如Grid）上直接设置DataContext为父级DataGrid的DataContext，这样内部所有控件的绑定都会默认使用这个上下文：

<DataGridTemplateColumn.HeaderTemplate>
    <DataTemplate>
        <Grid DataContext="{Binding DataContext, RelativeSource={RelativeSource AncestorType={x:Type DataGrid}}}">
            <Grid.RowDefinitions>
                <RowDefinition Height="*"/>
            </Grid.RowDefinitions>
            <Grid Grid.Column="1" >     
                <Button Command="{Binding OpenWindowCommand}">         
                   <Image Source="{StaticResource IconImage}" Width="20" Height="20" />   
                </Button> 
            </Grid>
            ...

内容的提问来源于stack exchange，提问作者AniAxe