Thanks for your replies!
I tried this:
<ComboBox Grid.ColumnSpan="2" Margin="4,6" Name="ms_detail_combo" Grid.Column="2" Grid.Row="1" ItemsSource="{Binding ElementName=map, Path=Layers.[0].Graphics}" IsEnabled="False">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBox Text="{Binding [Messstelle]}"></TextBox>
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
But my ComboBox doesnt show any Items. I think the Path of my ItemSource is wrong, but iam not quiet sure.
I have a MXD with two Features, and i want to show the Attributes of the first Feature. So my ArcGISDynamicMapServiceLayer is the only Layer i got, but dont i have to set the Sublayer (My first Feature that has Attriubute Messsstelle) as ItemSource? How can i do that in XAML?