Every ComboBox in a MahApps application is styled without any markup on your side, drop-down and items included. On top of that come a watermark, a clear button and the editable variant with auto-completion.

The implicit style

Styles/Controls.xaml contains keyless styles for ComboBox and ComboBoxItem. Merging that dictionary — which the quick start does — is all it takes:

<ResourceDictionary Source="pack://application:,,,/MahApps.Metro;component/Styles/Controls.xaml" />

To extend rather than replace, base your own style on the keyed one:

<Style BasedOn="{StaticResource MahApps.Styles.ComboBox}" TargetType="{x:Type ComboBox}">
    <Setter Property="mah:TextBoxHelper.ClearTextButton" Value="True" />
</Style>

The styles

The last two are building blocks of the template rather than something to set yourself.

Watermark

<ComboBox mah:TextBoxHelper.Watermark="Pick someone" />

The watermark shows while nothing is selected. UseFloatingWatermark keeps it above the box afterwards, which works for the editable variant too:

Clear button

ClearTextButton adds an ✕ that empties the box. Unlike a DatePicker — where the button hangs off ButtonCommand — the ComboBox binds its visibility straight to this property, so the button is there whenever the flag is set:

<ComboBox mah:TextBoxHelper.ClearTextButton="True" mah:TextBoxHelper.Watermark="Pick someone" />

Clearing sets SelectedItem to null and, on an editable box, empties Text as well — pushing both back through their bindings.

Only when something is selected

A permanently visible ✕ next to an empty box is noise. A trigger on the style hides it again while nothing is selected:

<Style BasedOn="{StaticResource MahApps.Styles.ComboBox}" TargetType="{x:Type ComboBox}">
    <Setter Property="mah:TextBoxHelper.ClearTextButton" Value="True" />
    <Style.Triggers>
        <DataTrigger Binding="{Binding SelectedItem, RelativeSource={RelativeSource Self}, Converter={x:Static mah:IsNullConverter.Instance}}"
                     Value="True">
            <Setter Property="mah:TextBoxHelper.ClearTextButton" Value="False" />
        </DataTrigger>
    </Style.Triggers>
</Style>

IsNullConverter is a singleton in the library, so it needs no resource of its own — {x:Static mah:IsNullConverter.Instance} is the whole reference. It reports whether the value is null, which is why the trigger switches the button off rather than on.

A button of your own

ButtonContent, ButtonContentTemplate, ButtonWidth and ButtonCommand reshape the same button:

<ComboBox mah:TextBoxHelper.ClearTextButton="True"
          mah:TextBoxHelper.ButtonCommand="{Binding SearchCommand}"
          mah:TextBoxHelper.ButtonContent="M42.5,22A12.5,12.5 0 0,1 55,34.5A12.5,12.5 0 0,1 42.5,47C40.14,47 37.92,46.34 36,45.24L26.97,54.27C25.8,55.44 23.9,55.44 22.73,54.27C21.56,53.1 21.56,51.2 22.73,50.03L31.8,40.96C30.66,39.08 30,36.86 30,34.5A12.5,12.5 0 0,1 42.5,22Z">
    <mah:TextBoxHelper.ButtonContentTemplate>
        <DataTemplate>
            <ContentControl Width="16" Height="16" Padding="3"
                            Content="{Binding Mode=OneWay}"
                            Style="{DynamicResource MahApps.Styles.ContentControl.PathIcon}" />
        </DataTemplate>
    </mah:TextBoxHelper.ButtonContentTemplate>
</ComboBox>

Editable

IsEditable="True" lets the user type, which with an ItemsSource gives auto-completion:

<ComboBox IsEditable="True"
          ItemsSource="{Binding Albums}"
          DisplayMemberPath="Title"
          mah:TextBoxHelper.Watermark="Album" />

ComboBoxHelper adds the two things a ComboBox lacks compared with a TextBox:

Both only do something while IsEditable is True. See ComboBoxHelper.

The drop-down

The drop-down is a list of ComboBoxItems, styled by MahApps.Styles.ComboBoxItem:

That style is where the item colours come from, and it sets eleven ItemHelper brushes — selected, hovered, disabled and the combinations. To recolour them, derive a container style rather than setting the properties on the ComboBox; a style setter on the item beats a value inherited from the box:

<ComboBox.ItemContainerStyle>
    <Style BasedOn="{StaticResource MahApps.Styles.ComboBoxItem}" TargetType="{x:Type ComboBoxItem}">
        <Setter Property="mah:ItemHelper.ActiveSelectionBackgroundBrush" Value="#2E7D32" />
    </Style>
</ComboBox.ItemContainerStyle>

ItemHelper has the full list of states.

Long lists

MahApps.Styles.ComboBox.Virtualized is the base style plus VirtualizingStackPanel.IsVirtualizing, IsVirtualizingWhenGrouping and recycling mode. Reach for it once the list is long enough that opening it stutters:

<ComboBox Style="{StaticResource MahApps.Styles.ComboBox.Virtualized}"
          ItemsSource="{Binding Albums}"
          DisplayMemberPath="Title"
          IsEditable="True" />

Grouping

Grouping is WPF's own GroupStyle, and works on the virtualized style because of IsVirtualizingWhenGrouping. The headers come out styled to match the rest of the drop-down:

<ComboBox Style="{StaticResource MahApps.Styles.ComboBox.Virtualized}"
          ItemsSource="{Binding GroupedAlbums}"
          DisplayMemberPath="Title">
    <ComboBox.GroupStyle>
        <GroupStyle>
            <GroupStyle.HeaderTemplate>
                <DataTemplate>
                    <TextBlock Margin="4 2" FontWeight="Bold" Text="{Binding Name}" />
                </DataTemplate>
            </GroupStyle.HeaderTemplate>
        </GroupStyle>
    </ComboBox.GroupStyle>
</ComboBox>

The grouping itself is not something the ComboBox does — it comes from the bound collection being a grouped view:

var view = new CollectionViewSource { Source = this.Albums };
view.GroupDescriptions.Add(new PropertyGroupDescription(nameof(Album.Genre)));

this.GroupedAlbums = view.View;

{Binding Name} in the header template is the group key — the genre string, in this case — not a property of your item type.

Helper properties

Four helpers reach a ComboBox, and their full property tables are on their own pages: TextBoxHelper for the watermark and the button, ComboBoxHelper for the typing rules, ControlsHelper for the border and corner radius, and ItemHelper for the drop-down rows.

What the styles themselves set:

Note the focus and mouse-over brushes are the ComboBox ones, not the TextBox ones the other input controls use.

Validation

Validation.ErrorTemplate is set to MahApps.Templates.ValidationError, so a failing rule on SelectedItem or Text is drawn the way it is on every other MahApps input control. How the popup behaves is ValidationHelper.