c# - 将空值绑定(bind)到 wpf 控件的宽度和高度时出现绑定(bind)错误

Question

我们的项目需要绑定(bind)控件的许多属性，例如Height、Width、MinHeight、Row、Column、rowspan 等。在这样做的同时，我们观察到这些值是 null 时的绑定(bind)错误，我们将从数据库。

为了说明，我的 MainWindow.xaml.cs 看起来像这样。

public partial class MainWindow : Window
{
    public MainWindow()
    {
        InitializeComponent();
        //TextWidth  id null
        TextBlockSize1 = new ItemSize() { TextHeight=20 };
        //TextWidth is null
        TextBlockSize2 = new ItemSize() { TextWidth = 40 };
        //TextHeight is null and TextWidth is null
        TextBlockSize3 = new ItemSize() { TextWidth = 40 };
        textblock1.DataContext = TextBlockSize1;
        textblock2.DataContext = TextBlockSize2;
        textblock3.DataContext = TextBlockSize3;
    }
    public ItemSize TextBlockSize1 { get; set; }

    public ItemSize TextBlockSize2 { get; set; }

    public ItemSize TextBlockSize3 { get; set; }
}

public class ItemSize
{
    public double? TextHeight { get; set; }
    public double? TextWidth { get; set; }
}

和 MainWindow.xaml 看起来像这样。

<Window x:Class="WPfAppln1.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:ctrl="clr-namespace:WPfAppln1.Controls"
    Title="MainWindow" Height="350" Width="525">
    <StackPanel >
         <TextBlock Name="textblock1" Text=" TextBlock 1"  Width="{Binding TextWidth}"  Height="{Binding TextHeight}"></TextBlock>
         <TextBlock Name="textblock2" Text=" TextBlock 2"  Width="{Binding TextWidth}"  Height="{Binding TextHeight}"></TextBlock>
         <TextBlock Name="textblock3" Text=" TextBlock 3"  Width="{Binding TextWidth, TargetNullValue=Auto}"  Height="{Binding TextHeight, TargetNullValue=Auto}"></TextBlock>     
   </StackPanel>
</Window>

在输出窗口中为此显示以下绑定(bind)错误:

System.Windows.Data Error: 5 : 'WPfAppln1.vshost.exe' (Managed (v4.0.30319)): Loaded 'C:\Windows\Microsoft.Net\assembly\GAC_MSIL\System.Numerics\v4.0_4.0.0.0__b77a5c561934e089\System.Numerics.dll', Skipped loading symbols. Module is optimized and the debugger option 'Just My Code' is enabled.
Value produced by BindingExpression is not valid for target property.; Value='<null>' BindingExpression:Path=TextWidth; DataItem='ItemSize' (HashCode=43929715); target element is 'TextBlock' (Name='textblock1'); target property is 'Width' (type 'Double')
System.Windows.Data Error: 5 : Value produced by BindingExpression is not valid for target property.; Value='<null>' BindingExpression:Path=TextHeight; DataItem='ItemSize' (HashCode=57104612); target element is 'TextBlock' (Name='textblock2'); target property is 'Height' (type 'Double')
System.Windows.Data Error: 5 : Value produced by BindingExpression is not valid for target property.; Value='<null>' BindingExpression:Path=TextHeight; DataItem='ItemSize' (HashCode=59587750); target element is 'TextBlock' (Name='textblock3'); target property is 'Height' (type 'Double')
    The thread '<No Name>' (0x2c60) has exited with code 0 (0x0).

由于这些错误，应用程序需要很长时间才能加载屏幕。

因此，问题是如何在绑定(bind)到 wpf 控件时容纳可为 null 的值，以及当绑定(bind)值为 null 时如何为控件的宽度属性提供默认值，例如“auto”。

Answer 1

您可以使用 TargetNullValue (如果源为空)或 FallbackValue (以防绑定(bind)失败，例如 DataContext 为 null)

更新:

感谢 Dean 指出这一点，我错误地认为 Auto 会起作用(即 TypeConverter 会负责转换)。

但是，您仍然可以使用自动属性并使用 x:Static Markup Extension 在 XAML 中提供 Auto 值像这样 -

<TextBlock Name="textblock1" Text=" TextBlock 1"  
    Height="{Binding TextHeight, TargetNullValue={x:Static System:Double.NaN}, 
               FallbackValue={x:Static System:Double.NaN}}">
</TextBlock> 

Value="{x:Static System:Double.NaN}" 也可以像这样用在 DataTrigger 方法中 -

<TextBlock.Style>
    <Style>
       <Setter Property="Control.Width" Value="{Binding Path=TextWidth}" />
        <Style.Triggers>
            <DataTrigger
               Binding="{Binding Path=TextWidth}"
               Value="{x:Null}">
               <Setter Property="Control.Width" Value="{x:Static System:Double.NaN}" />
            </DataTrigger>
        </Style.Triggers>
    </Style>
</TextBlock.Style>

注意:将需要这个命名空间-

xmlns:System="clr-namespace:System;assembly=mscorlib" 

旧代码:

<TextBlock Name="textblock1" Text=" TextBlock 1"  Width="{Binding TextWidth}"  
    Height="{Binding TextHeight, TargetNullValue=Auto, FallbackValue=Auto }">
</TextBlock> 

另一种解决方案是使用这样的触发器 -

<TextBlock.Style>
    <Style>
        <Style.Triggers>
            <DataTrigger
                Binding="{Binding Path=TextWidth}"
                Value="{x:Null}">
                <Setter Property="Control.Width" Value="Auto" />
            </DataTrigger>
            <DataTrigger
                Binding="{Binding Path=TextHeight}"
                Value="{x:Null}">
                <Setter Property="Control.Height" Value="Auto" />
            </DataTrigger>
        </Style.Triggers>
    </Style>
</TextBlock.Style>