WPF MVVM中ComboBox数据不在列表时显示“未找到数据”

你之前写的Trigger没起作用，核心原因是：当数据库返回的值不在ComboBox的选项列表里时，SelectedItem确实会变成null，但ComboBox的Text属性会优先显示绑定的原始值，而非你通过Trigger设置的提示文本——因为绑定的优先级高于Trigger的Setter。

下面给你几个以XAML为主的可行方案：

方案1：验证规则+DataTrigger组合 #

利用WPF的验证机制，当绑定的值不在选项中时触发错误，再用DataTrigger显示提示文本。

第一步：后台编写验证规则类 #

public class ComboBoxItemValidationRule : ValidationRule
{
    public override ValidationResult Validate(object value, System.Globalization.CultureInfo cultureInfo)
    {
        var comboBox = (ComboBox)ValidationStepHelper.GetCurrentObject();
        if (comboBox.ItemsSource != null && !comboBox.ItemsSource.Cast<object>().Contains(value))
        {
            return new ValidationResult(false, "Not found data");
        }
        return ValidationResult.ValidResult;
    }
}

第二步：XAML配置ComboBox #

<ComboBox x:Name="MyComboBox"
          ItemsSource="{Binding Options}"
          SelectedValue="{Binding SelectedData}">
    <ComboBox.Resources>
        <Style TargetType="ComboBox">
            <Style.Triggers>
                <DataTrigger Binding="{Binding (Validation.HasError), ElementName=MyComboBox}" Value="True">
                    <Setter Property="Text" Value="Not found data"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </ComboBox.Resources>
    <ComboBox.SelectedValueBinding>
        <Binding Path="SelectedData">
            <Binding.ValidationRules>
                <local:ComboBoxItemValidationRule ValidationStep="UpdatedValue"/>
            </Binding.ValidationRules>
        </Binding>
    </ComboBox.SelectedValueBinding>
</ComboBox>

方案2：MultiBinding+转换器 #

直接判断绑定的值是否在选项列表里，无需验证机制，逻辑更简洁。

第一步：后台编写转换器 #

public class ComboBoxItemExistConverter : IMultiValueConverter
{
    public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
    {
        var selectedValue = values[0];
        var itemsSource = values[1] as IEnumerable;
        if (selectedValue == null || itemsSource == null)
            return selectedValue;
        
        return itemsSource.Cast<object>().Contains(selectedValue) ? selectedValue : "Not found data";
    }

    public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
    {
        throw new NotImplementedException();
    }
}

第二步：XAML配置 #

<Window.Resources>
    <local:ComboBoxItemExistConverter x:Key="ItemExistConverter"/>
</Window.Resources>

<ComboBox ItemsSource="{Binding Options}">
    <ComboBox.Text>
        <MultiBinding Converter="{StaticResource ItemExistConverter}">
            <Binding Path="SelectedData"/>
            <Binding Path="ItemsSource" RelativeSource="{RelativeSource Self}"/>
        </MultiBinding>
    </ComboBox.Text>
    <ComboBox.SelectedValue>
        <Binding Path="SelectedData"/>
    </ComboBox.SelectedValue>
</ComboBox>

方案3：修改ComboBox模板（最灵活） #

直接修改控件模板，在没有匹配项时显示自定义文本块，完全通过XAML控制显示逻辑：

<ComboBox ItemsSource="{Binding Options}" SelectedValue="{Binding SelectedData}">
    <ComboBox.Template>
        <ControlTemplate TargetType="ComboBox">
            <Grid>
                <ToggleButton x:Name="ToggleButton" 
                              Template="{StaticResource ComboBoxToggleButtonTemplate}"/>
                <ContentPresenter x:Name="ContentPresenter"
                                  Content="{TemplateBinding SelectedItem}"
                                  ContentTemplate="{TemplateBinding SelectionBoxItemTemplate}"
                                  Margin="3,3,23,3"
                                  VerticalAlignment="Center"/>
                <TextBlock x:Name="NotFoundText"
                           Text="Not found data"
                           Margin="3,3,23,3"
                           VerticalAlignment="Center"
                           Visibility="Collapsed"/>
                <Popup x:Name="Popup"
                       Placement="Bottom"
                       IsOpen="{TemplateBinding IsDropDownOpen}"
                       AllowsTransparency="True"
                       Focusable="False">
                    <Border x:Name="DropDownBorder"
                            Background="White"
                            BorderBrush="{DynamicResource {x:Static SystemColors.WindowFrameBrushKey}}"
                            BorderThickness="1">
                        <ScrollViewer x:Name="DropDownScrollViewer">
                            <ItemsPresenter x:Name="ItemsPresenter"
                                            KeyboardNavigation.DirectionalNavigation="Contained"/>
                        </ScrollViewer>
                    </Border>
                </Popup>
            </Grid>
            <ControlTemplate.Triggers>
                <MultiDataTrigger>
                    <MultiDataTrigger.Conditions>
                        <Condition Binding="{Binding SelectedItem, RelativeSource={RelativeSource Self}}" Value="{x:Null}"/>
                        <Condition Binding="{Binding SelectedValue, RelativeSource={RelativeSource Self}}" Value="{x:Null}" Negate="True"/>
                    </MultiDataTrigger.Conditions>
                    <Setter TargetName="ContentPresenter" Property="Visibility" Value="Collapsed"/>
                    <Setter TargetName="NotFoundText" Property="Visibility" Value="Visible"/>
                </MultiDataTrigger>
                <!-- 可保留ComboBox默认模板中的其他触发器，如IsEnabled、IsMouseOver等交互效果 -->
            </ControlTemplate.Triggers>
        </ControlTemplate>
    </ComboBox.Template>
</ComboBox>

内容的提问来源于stack exchange，提问作者user19161290