How do I show that a combobox has multiple entries?

Question

I'm working on a C# project, based on XAML.
I have a combobox, which mostly will contain just one entry, but in several cases, it might contain more.
I would like to warn the user about the possibility of multiple entries. I know I need a converter for that, but I'm just wandering if such a thing already exists.

Currently, my combobox looks as follows:

<ComboBox x:Name="cmb_Locations"
          Grid.Column="1" Grid.Row="1"
          Margin="2"               
          ItemsSource="{Binding FilteredLocations}"
          SelectedItem="{Binding SelectedLocation, Mode=TwoWay}"
          DisplayMemberPath="Name"
          Background="{converters:RatioConverter}">
</ComboBox>

(The mentioned converter is just bogus.)

Does anybody know about a simple converter for such a thing?

The proposal of Denis is great, but it changes the foreground colour instead of the background, but the name seems not to be Background, but what should it be?

Current situation: the green arrow shows what is changed, the red rectangle shows (a part) of what I want to change (the entire background, in fact):

Edit: ±progress
In the meantime I've been working further, and amongst others getting some help from ChatGPT, I now have the following code in my XAML file:

<ComboBox x:Name="cmb_Locations"
           Grid.Column="1" Grid.Row="1"
           Margin="2"               
            ItemsSource="{Binding FilteredLocations}"
            SelectedItem="{Binding SelectedLocation, Mode=TwoWay}"
            DisplayMemberPath="Name">
    <ComboBox.Style>
        <Style TargetType="ComboBox">
            <Setter Property="Foreground" Value="LightCoral"/>
            <Style.Triggers>
                <DataTrigger Binding="{Binding RelativeSource={RelativeSource Self}, Path=Items.Count}" Value="1">
                    <Setter Property="Foreground" Value="DarkGreen" />
                </DataTrigger>
            </Style.Triggers>
        </Style>
    </ComboBox.Style>
    <ComboBox.Resources>
        <Style TargetType="ComboBoxItem">
            <Setter Property="Background" Value="Yellow"/>
            <Style.Triggers>
                <MultiDataTrigger>
                    <MultiDataTrigger.Conditions>
                        <Condition Binding="{Binding RelativeSource={RelativeSource AncestorType=ComboBox}, Path=Items.Count}" Value="1" />
                        <Condition Binding="{Binding RelativeSource={RelativeSource Self}, Path=IsSelected}" Value="True" />
                    </MultiDataTrigger.Conditions>
                    <Setter Property="Background" Value="Violet" />
                </MultiDataTrigger>
            </Style.Triggers>
        </Style>
    </ComboBox.Resources>
</ComboBox>

This has the following effect:

When there is only one item, something happens, which is great!
The thing, which happens, can be seen here:

As you see, the "green" trigger has been executed, turning the foreground colour into green.

Where there are more items, something happens, which is great too!
The thing, which happens, can be seen here:

As you see, the "LightCoral" trigger has been executed, turning the foreground colour into lighcoral.
Also, the background colour of the items changes into yellow.

However, the thing which does not happen (and that's what I'm after), is the changing of the background colour of the very first item: that ALWAYS stays the same.

How can I achieve the very first item to have its background modified?

Another edit: I've done a test:
Just as a test, I've added a very simple other ComboBox to my XAML:

<ComboBox x:Name="cmb_test" Grid.Column="0" Grid.Row="4">
    <ComboBoxItem>1</ComboBoxItem>
    <ComboBoxItem>2</ComboBoxItem>
</ComboBox>

However, this gives the following result in the Live Visual Tree:

This gives me the impression that there is somewhere another object, causing that wrong behaviour, but where?

Answer 1

You can use triggers for this. In the example below the text is always red if there is more than one item, and black if there is exactly one item. Of cause just the color change is not very intuitive, but it gives you a starting point:

<ComboBox>
    <ComboBox.Style>
    <Style TargetType="ComboBox">
        <Setter Property="Foreground" Value="Red" />                    
        <Style.Triggers>
            <DataTrigger Binding="{Binding RelativeSource={RelativeSource Self}, Path=Items.Count}" Value="1">
                <Setter Property="Foreground" Value="Black" />
            </DataTrigger>
        </Style.Triggers>
    </Style>
    </ComboBox.Style>
    <ComboBoxItem>1</ComboBoxItem>
    <ComboBoxItem>2</ComboBoxItem>
</ComboBox>

You could also set the isEnabled property to false, this way the user knows there is only one option available.

Note from the question author: in order to completely understand this answer, it is necessary to read the comments and to check the other StackOverflow post, mentioned there.