XAML DockPanel: The property content is set more than once - xaml

Question

I'm have this XAML code where it throws an error:
The property 'Content' is set more than once.
This whole code points a same kind of error. Help much appreciated.
EDIT: Added XAML of Window
<Window x:Class="AddressBook.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
Title="AddressBook"
Loaded="WindowLoaded"
SizeToContent="WidthAndHeight"
MinHeight="480"
MinWidth="640">
<Window.Resources>
<ObjectDataProvider x:Key="ContactList"
MethodName ="AddressBook.ContactList,AddressBook" />
<DataTemplate x:Key="ContactNameTemplate" >
<TextBlock Text=" {Binding Path=Firstname}" />
</DataTemplate>
</Window.Resources>
<Grid Background="White" Name="DocumentRoot">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="200"/>
<ColumnDefinition Width="*"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="Auto"/>
<!-- Menu -->
<RowDefinition Height="Auto"/>
<!-- Tool Bar -->
<RowDefinition Height="Auto"/>
<!-- Content Area -->
<RowDefinition Height="Auto"/>
<!-- Status Bar -->
</Grid.RowDefinitions>
</Grid>
<DockPanel Name="DockPanel_Menu" Grid.Column="0" Grid.ColumnSpan="2" Grid.Row="0">
<Menu Background="White">
<MenuItem Header="File">
<MenuItem Header="New Contact" Click="LaunchNewContactWizard"/>
<MenuItem Header="New Group" Click="NotImplementedMsg"/>
<Separator />
<MenuItem Header="Properties" Click="NotImplementedMSg"/>
<MenuItem Header="Delete" Click="NotImplementedMsg"/>
<MenuItem Header="Import">
<MenuItem Header="Address book (WAB)..."
Click="NotImplementedMsg"/>
<MenuItem Header="Business card (vCard)..."
Click="NotImplementedMsg"/>
</MenuItem>
<Separator />
<MenuItem Header="Exit" InputGestureText="Alt-F4"
Click="ExitApplication">
<MenuItem.ToolTip>
<ToolTip>
Click here to exit
</ToolTip>
</MenuItem.ToolTip>
</MenuItem>
</MenuItem>
</Menu>
<Menu Background="White">
<MenuItem Header="Edit">
<MenuItem Command="ApplicationCommands.Copy"/>
<MenuItem Command="ApplicationCommands.Paste"/>
</MenuItem>
</Menu>
</DockPanel>
</Window>
I can't really debug it for the last 3 days :(

Answer 1

You have to modify your code for example in the following way:
<Window>
...
<StackPanel>
<Grid Background="White" Name="DocumentRoot">
...
</Grid>
<DockPanel Name="DockPanel_Menu" Grid.Column="0" Grid.ColumnSpan="2" Grid.Row="0">
...
</DockPanel>
</StackPanel>
</Window>
Window can only have one child so you have to embed Grid and DockPanel within some container e.g. StackPanel, another DockPanel etc.