在用户控件WPF中使用数据模板选择器时引发Null引用异常

本文关键字：选择器 Null 异常 引用 WPF 控件 用户 数据 | 更新日期: 2023-09-27 18:25:00

嗨，我遇到了一个问题，在我的xaml中，使用数据模板选择器为用户控件抛出了一个空引用异常，我真的不知道为什么。这是下面的代码，如果有任何帮助，我们将不胜感激。

更新它只发生在设计时，而不是运行时

数据模板选择器代码：

using System.Windows;
using System.Windows.Controls;
using UI.ViewModel;
namespace UI.DataTemplateSelectors
{
    public class CouponViewerDataTemplateSelector : DataTemplateSelector
    {
        public DataTemplate coupon1DataTemplateSelector { get; set; }
        public DataTemplate coupon2DataTemplateSelector { get; set; }
        public DataTemplate coupon3DataTemplateSelector { get; set; }

        public override DataTemplate SelectTemplate(object item, DependencyObject container)
        {
            if (item == null)
                return null;
            var CouponViewerViewModel = (CouponViewerViewModel)item;
            switch (CouponViewerViewModel.Coupon.CouponBarcode )
            {
                case "994460":
                    return Coupon1DataTemplateSelector;
                case "994470":
                    return Coupon2DataTemplateSelector;
                case "994480":
                    return Coupon3DataTemplateSelector;

            }
            return null;
        }

    }
}

Xaml（我已经折叠了不同的DataTemplates代码，因为Xaml文件是巨大的）

<UserControl x:Class="UI.View.CouponViewerView"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008" 
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        xmlns:viewModel="clr-namespace:UI.ViewModel"
        xmlns:converter="clr-namespace:UI.View.Converter"
        xmlns:converters="clr-namespace:UI.Converters"
        xmlns:Web="clr-namespace:Web;assembly=Web"
        xmlns:dataTemplateSelectors="clr-namespace:UI.DataTemplateSelectors"
        mc:Ignorable="d"
        d:DataContext="{d:DesignInstance Type=viewModel:CouponViewerViewModel, IsDesignTimeCreatable=True}"

        Height="1018" Width="720">
    <UserControl.Resources>
        <converter:EmptyStringToVisibilityConverter x:Key="EmptyStringToVisibilityConverter"/>
        <converters:AppendCharToBeginningAndEndOfStringConverter x:Key="AppendCharToBeginningAndEndOfStringConverter"/>
        <converters:HexToBackgroundColourConverter x:Key="HexToBackgroundColourConverter"/>
        <converters:HexToForegroundColourConverter x:Key="HexToForegroundColourConverter"/>
        <converters:MaxCharcterSelectionNameConverter x:Key="MaxCharcterSelectionNameConverter"/>
        <converters:DateTimeToSplitDataGridConverter x:Key="DateTimeToSplitDataGridConverter"/>
        <converters:HeaderFormatterConverter x:Key="HeaderFormatterConverter"/>
            <DataTemplate x:Key="Coupon1DataTemplateSelector" ...>
            <DataTemplate x:Key="Coupon2DataTemplateSelector" ...>
            <DataTemplate x:Key="Coupon3DataTemplateSelector" ...>
                 <dataTemplateSelectors:CouponViewerDataTemplateSelector
        Coupon1DataTemplateSelector="{StaticResource Coupon1DataTemplateSelector}"
        Coupon2DataTemplateSelector="{StaticResource Coupon2DataTemplateSelector}"
        Coupon3DataTemplateSelector ="{StaticResource Coupon3DataTemplateSelector}"
        x:Key="CouponViewerDataTemplateSelector" />
</UserControl.Resources>
 <StackPanel>
        <StackPanel x:Name="StackPanelToPrint">
            <StackPanel.ContextMenu >
                <ContextMenu>
                    <MenuItem Header="Print"  Click="PrintCurrentForm" />
                    <MenuItem Header="OverLay Image"  Command="{Binding Path=OverlayImageToCouponViewerCommand }" />
                </ContextMenu>
            </StackPanel.ContextMenu>
            <Grid Margin="0,0,0,0">
                <TextBlock Text="{Binding DesignTimeError}" Visibility="{Binding DesignTimeError, Converter={StaticResource EmptyStringToVisibilityConverter}}" Grid.ColumnSpan="2" Margin="0,0,0,34" />
                <Canvas Grid.ColumnSpan="2" Margin="0,0,0,0">
                    <Rectangle Fill="Black" Width="8.5" Height="8.5" Canvas.Left="11" Canvas.Top="128" />
                    <Rectangle Fill="Black" Width="8.5" Height="8.5" Canvas.Left="742.5" Canvas.Top="128" />
                    <TextBlock Canvas.Left="338" FontSize="9" Foreground="Red" Canvas.Top="1046"  Text="{Binding Path=MarkSenseCoupon.CouponBarcode}" />
                    <TextBlock FontFamily="Free 3 Of 9"  Canvas.Left="24"  Canvas.Top="1026" FontSize="110" Height="15" Text="{Binding Path=Coupon.CouponBarcode,  Converter={StaticResource AppendCharToBeginningAndEndOfStringConverter}}"/>
                    <TextBlock Canvas.Left="409" FontSize="9" Foreground="Red"  Canvas.Top="1046" Text="{Binding Path=MarkSenseCoupon.CouponInstance}" />
                    <TextBlock FontFamily="Free 3 of 9" Canvas.Left="409" Canvas.Top="1026"  FontSize="110" Height="15" Text="{Binding Path=Coupon.CouponInstance,  Converter={StaticResource AppendCharToBeginningAndEndOfStringConverter}}"/>
                    <Rectangle Fill="Black" Width="8.5" Height="8.5" Canvas.Left="11.5" Canvas.Top="1043" />
                    <Rectangle Fill="Black" Width="8.5" Height="8.5" Canvas.Left="742.5" Canvas.Top="1043" />

                </Canvas>
                <Image Canvas.ZIndex="1" Source="{Binding Coupon.OverlayImage}"  Margin="0,-21,-76,108" Stretch="Fill" x:Name="PrintImageContextMenu" />


                <StackPanel Margin="-105,146,66,0" Height="900" VerticalAlignment="Top" >
                    <ContentControl Content="{Binding}"  ContentTemplateSelector="{StaticResource CouponViewerDataTemplateSelector}"  />

                </StackPanel>
            </Grid>
        </StackPanel>
    </StackPanel>
</UserControl>

堆栈跟踪：

    at UI.DataTemplateSelectors.CouponViewerDataTemplateSelector.SelectTemplate(Object item, DependencyObject container)
   at System.Windows.Controls.ContentPresenter.ChooseTemplate()
   at System.Windows.Controls.ContentPresenter.EnsureTemplate()
   at System.Windows.Controls.ContentPresenter.OnPreApplyTemplate()
   at System.Windows.FrameworkElement.ApplyTemplate()
   at System.Windows.FrameworkElement.MeasureCore(Size availableSize)
   at System.Windows.UIElement.Measure(Size availableSize)
   at System.Windows.ContextLayoutManager.UpdateLayout()
   at System.Windows.UIElement.UpdateLayout()

在用户控件WPF中使用数据模板选择器时引发Null引用异常

我之前一直在做这项工作的同事在我没有注意到的情况下将这行代码添加到ViewModel中，这导致了所有问题，因为它在设计时数据加载之前返回

    public CouponViewerViewModel()
    {
        if (DesignerProperties.IsInDesignMode == true)
        {
            return;
        }
        if (DesignerProperties.IsInDesignMode)
        {
            LoadDesignTimeData();
        }
    }

为最后的一个愚蠢问题道歉

属性名称区分大小写。 
public DataTemplate coupon1DataTemplateSelector { get; set; }

与。

Coupon1DataTemplateSelector="{StaticResource Coupon1DataTemplateSelector}"
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