How to make a control dock in a wpf

Question

I'm trying to make my menu bar fill the screen horizontally like a menu bar should look. When I run the program the window is set to maximize already but the menu bar only fills half the screen on top. I'm not sure how to fix this. So just to be clear I'm trying to make my controls fit to appearance based on the size of the window.

Heres the code:

  <Window x:Class="WpfApplication3.MainWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="MainWindow" Height="350" Width="525" xmlns:my="clr-namespace:Microsoft.Windows.Themes;assembly=PresentationFramework.Royale" WindowStyle="ThreeDBorderWindow" SizeToContent="Manual" WindowState="Maximized" xmlns:my1="clr-namespace:System;assembly=mscorlib">
<Window.Resources>
    <my1:Double x:Key="Width1">500</my1:Double>
</Window.Resources>
<Grid ShowGridLines="False" Width="Auto">
    <Grid.ColumnDefinitions>
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition Width="Auto" />
        <ColumnDefinition Width="Auto" />
    </Grid.ColumnDefinitions>
    <Menu Height="23" HorizontalAlignment="Left" Name="menu1" VerticalAlignment="Top"       Width="503" IsMainMenu="True" Grid.ColumnSpan="2">
        <MenuItem Header="File" HorizontalContentAlignment="Stretch">
            <MenuItem Header="New">
                <MenuItem Header="New Camper" />
            </MenuItem>
        </MenuItem>
        <MenuItem Header="Edit" />
        <MenuItem Header="View" />
        <MenuItem Header="Add" />
    </Menu>
   </Grid>
   </Window>

Answer 1

Using DockPanel :

http://www.wpftutorial.net/DockPanel.html

<DockPanel LastChildFill="True">
    <Button Content="Dock=Top" DockPanel.Dock="Top"/>
    <Button Content="Dock=Bottom" DockPanel.Dock="Bottom"/>
    <Button Content="Dock=Left"/>
    <Button Content="Dock=Right" DockPanel.Dock="Right"/>
    <Button Content="LastChildFill=True"/>
</DockPanel>

Answer 2

The simple answer - set your menu to use Grid.ColumnSpan="3" , since you have 3 columns.

However, I would recommend nesting your "main" grid inside of a DockPanel or a second Grid (with 2 rows). This way, as you add content (ie: add new columns to your grid), you won't have to constantly adjust your menu to compensate.