Как привязать иконку кнопки к выбранному элементу выпадающего списка?

На странице 17 пошагового руководства приведен пример того, чего вы пытаетесь достичь.

Вы можете скачать его здесь: http://fluent.codeplex.com/documentation

Взято из прохождения:

<fluent1:Ribbon>
    <fluent1:Ribbon.Menu>
        <fluent1:Backstage />
    </fluent1:Ribbon.Menu>
    <fluent1:RibbonTabItem Header="Home">
        <fluent1:RibbonGroupBox Header="Clipboard">
            <!--  The following code shows standard mode for color gallery  -->
            <fluent1:DropDownButton Header="Standard">
                <!--  It's possible to create custom icon to present selected color  -->
                <fluent1:DropDownButton.Icon>
                    <Grid Width="16" Height="16">
                        <Image Source="Images\FontColor.png" />
                        <Border Height="4"
                                VerticalAlignment="Bottom"
                                BorderThickness="0">
                            <Border.Background>
                                <SolidColorBrush
                                    Color="{Binding ElementName=ColorGalleryStandard, Path=SelectedColor, FallbackValue=Black}" />
                            </Border.Background>
                        </Border>
                    </Grid>
                </fluent1:DropDownButton.Icon>
                <fluent1:ColorGallery x:Name="ColorGalleryStandard"
                                        IsNoColorButtonVisible="False"
                                        SelectedColor="Red" />
                <fluent1:MenuItem Header="A Menu Item" Icon="Images\Pink.png" />
            </fluent1:DropDownButton>
        </fluent1:RibbonGroupBox>
    </fluent1:RibbonTabItem>
</fluent1:Ribbon>

ОБНОВЛЕНИЕ

Я не вижу ничего плохого в вашем коде, я вставил его, и он успешно запустился, вот он снова вставлен из моего рабочего теста.

<Window x:Class="WpfApplication8.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:fluent1="clr-namespace:Fluent;assembly=Fluent"
        xmlns:system="clr-namespace:System;assembly=mscorlib"
        Title="MainWindow"
        Width="525"
        Height="350">
    <Window.Resources>
        <ObjectDataProvider x:Key="colorsTypeOdp"
                            MethodName="GetType"
                            ObjectType="{x:Type system:Type}">
            <ObjectDataProvider.MethodParameters>
                <system:String>
                    System.Windows.Media.Colors, PresentationCore,
                    Version=3.0.0.0, Culture=neutral,
                    PublicKeyToken=31bf3856ad364e35
                </system:String>
            </ObjectDataProvider.MethodParameters>
        </ObjectDataProvider>

        <ObjectDataProvider x:Key="colorPropertiesOdp"
                            MethodName="GetProperties"
                            ObjectInstance="{StaticResource colorsTypeOdp}" />
    </Window.Resources>
    <fluent1:DropDownButton Name="btnCommentColor" Header="Comments">
        <fluent1:DropDownButton.LargeIcon>
            <Grid Width="32" Height="32">
                <Image Source="Icons\BlueLarge.png" />
                <Border Height="32"
                        VerticalAlignment="Bottom"
                        BorderThickness="0"
                        CornerRadius="2">
                    <Border.Background>
                        <SolidColorBrush
                            Color="{Binding ElementName=galCommentColor, Path=SelectedValue, FallbackValue=Green}" />
                    </Border.Background>
                </Border>
            </Grid>
        </fluent1:DropDownButton.LargeIcon>
        <fluent1:Gallery Name="galCommentColor"
                         ItemsSource="{Binding Source={StaticResource colorPropertiesOdp}}"
                         MaxItemsInRow="12"
                         SelectedValuePath="Name">
            <fluent1:Gallery.ItemTemplate>
                <DataTemplate>
                    <Border Width="25"
                            Height="25"
                            VerticalAlignment="Stretch"
                            Background="{Binding Name}"
                            BorderBrush="Black"
                            BorderThickness="1"
                            CornerRadius="2"
                            ToolTip="{Binding Path=Name}" />
                </DataTemplate>
            </fluent1:Gallery.ItemTemplate>
        </fluent1:Gallery>
    </fluent1:DropDownButton>

</Window>

Спасибо Айбе за подтверждение, что это не я. Я получил желаемый результат, используя преобразователь свойства LargeIcon кнопки DropDownButton.

Вот XAML:

<Fluent:DropDownButton Name="btnCommentColor" Header="Comments" HasTriangle="False" LargeIcon="{Binding ElementName=galCommentColor, Path=SelectedValue, Converter={StaticResource ColorNameToBorderConverter_Key}}">
    <Fluent:Gallery x:Name="galCommentColor" ItemsSource="{Binding Source={StaticResource colorPropertiesOdp}}" SelectedValuePath="Name" MaxItemsInRow="12" SelectedIndex="51">
        <Fluent:Gallery.ItemTemplate>
            <DataTemplate>
                <Border ToolTip="{Binding Name}" BorderThickness="1" CornerRadius="2" BorderBrush="Black" Width="25" Height="25" VerticalAlignment="Stretch" Background="{Binding Name}" /> 
            </DataTemplate>
        </Fluent:Gallery.ItemTemplate>
    </Fluent:Gallery>
</Fluent:DropDownButton>

И код:

Public Class ColorNameToBorderConverter
    Implements IValueConverter

    Public Function Convert(value As Object, targetType As Type, parameter As Object, culture As Globalization.CultureInfo) As Object Implements IValueConverter.Convert
        If ApplicationIsInDesignMode Then value = "Black"

        If TypeOf value Is String Then
            Return New Border With {
                .Height = 32,
                .BorderThickness = New Thickness(1),
                .BorderBrush = New SolidColorBrush(System.Windows.Media.Colors.Black),
                .CornerRadius = New CornerRadius(2, 2, 2, 2),
                .VerticalAlignment = VerticalAlignment.Bottom,
                .Background = New SolidColorBrush(ColorConverter.ConvertFromString(value))
            }
        Else
            Throw New InvalidOperationException("Unsupported type [" & value.GetType.ToString & "]")
        End If
    End Function

    Public Function ConvertBack(value As Object, targetType As Type, parameter As Object, culture As Globalization.CultureInfo) As Object Implements IValueConverter.ConvertBack
        Throw New NotImplementedException
    End Function

    Private Shared ReadOnly Property ApplicationIsInDesignMode() As Boolean
        Get
            Return CBool(DesignerProperties.IsInDesignModeProperty.GetMetadata(GetType(DependencyObject)).DefaultValue)
        End Get
    End Property
End Class