您可以在ComboBox的Template没有项时替换它，例如，使用TextBlock或任何其他可视元素。

<ComboBox>
    <ComboBox.Style>
        <Style TargetType="ComboBox">
            <Style.Triggers>
                <Trigger Property="HasItems" Value="False">
                    <Setter Property="Template">
                        <Setter.Value>
                            <ControlTemplate TargetType="ComboBox">
                                <TextBlock Text="No Items"/>
                            </ControlTemplate>
                        </Setter.Value>
                    </Setter>
                </Trigger>
            </Style.Triggers>
        </Style>
    </ComboBox.Style>
</ComboBox>

试试这个（我不能自己检查--我“手边”没有电脑）：

<ComboBox>
    <ComboBox.Resources>
        <CompositeCollection x:Key="noItems">
            <sys:String>No Items</sys:String>
        </CompositeCollection>
    </ComboBox.Resources>
  
    <ComboBox.Style>
        <Style TergetType="ComboBox">
            <Setter Property="ItemsSource" Value="{Binding MyCollection}"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding MyCollection.Count}"
                             Value="0">
                    <Setter Property="ItemsSource" Value="{DynamicResource noItems}"/>
                    <Setter Property="SelectedIndex" Value="0"/>
                </DataTrigger>
            </Style.Triggers>
        </Style>
    <ComboBox.Style>
</ComboBox>

或TargetNullValue的此类变量：

<ComboBox>
    <ComboBox.ItemsSource>
        <Binding Path="MyCollection">
            <Binding.TargetNullValue>
                <CompositeCollection>
                    <sys:String>No Items</sys:String>
                </CompositeCollection>
            </Binding.TargetNullValue>
        </Binding>
    </ComboBox.ItemsSource>
</ComboBox>