How to make a StackPanel Width that of another StackPanel?

Question

I have two dockpanels which each have a left StackPanel.

The width of the bottom StackPanel is determined by the width of the text is in it.

The width of the top StackPanel should be the same as the width of the bottom StackPanel.

I've tried to bind the Width of the top StackPanel to the Width of the bottom StackPanel via ElementName but this doesn't work.

How can I make the top width the same as the bottom width?

<StackPanel>
    <DockPanel LastChildFill="True" Height="100" >
        <StackPanel Width="{Binding ElementName=LeftMenuText, Path=Width}" 
                    DockPanel.Dock="Left"
                    Background="Yellow">
            <TextBlock
                Text="This is some text."/>
        </StackPanel>
        <StackPanel DockPanel.Dock="Right"
                    Background="Orange">
        </StackPanel>
    </DockPanel>

    <DockPanel 
        Height="3"
        Background="Black"></DockPanel>

    <DockPanel LastChildFill="True" Height="100">
        <StackPanel Name="LeftMenuWrapper"
                    DockPanel.Dock="Left"
                    Background="Yellow">
            <TextBlock 
                    Text="This is some text that is longer."/>
        </StackPanel>
        <StackPanel DockPanel.Dock="Right"
                    Background="Blue">
        </StackPanel>
    </DockPanel>
</StackPanel>

Answer 1

Bind it to ActualWidth of LeftMenuWrapper:

 <StackPanel>
    <DockPanel LastChildFill="True" Height="100" >
        <StackPanel Width="{Binding ElementName=LeftMenuWrapper, Path=ActualWidth}" 
                    DockPanel.Dock="Left"
                    Background="Yellow">
            <TextBlock
                Text="This is some text."/>
        </StackPanel>
        <StackPanel DockPanel.Dock="Right"
                    Background="Orange">
        </StackPanel>
    </DockPanel>

    <DockPanel 
        Height="3"
        Background="Black"></DockPanel>

    <DockPanel LastChildFill="True" Height="100">
        <StackPanel Name="LeftMenuWrapper"
                    DockPanel.Dock="Left"
                    Background="Yellow">
            <TextBlock 
                    Text="This is some text that is longer."/>
        </StackPanel>
        <StackPanel DockPanel.Dock="Right"
                    Background="Blue">
        </StackPanel>
    </DockPanel>
</StackPanel>

Just to add to your arsenal another way to do this. You can also use Grid's IsSharedScope property:

<Page xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml">
   <StackPanel Grid.IsSharedSizeScope="True">
      <Grid Height="100">
         <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" SharedSizeGroup="TextHolder"/>
            <ColumnDefinition Width="*"/>
         </Grid.ColumnDefinitions>
         <Border Background="Yellow">
            <TextBlock Text="This is some text."/>
         </Border>
         <Border Grid.Column="1" Background="Orange"/>
      </Grid>
      <Border Height="3" Background="Black"/>
      <Grid Height="100">
         <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" SharedSizeGroup="TextHolder"/>
            <ColumnDefinition Width="*"/>
         </Grid.ColumnDefinitions>
         <Border Background="Yellow">
            <TextBlock Text="This is some text that is longer."/>
         </Border>
         <Border Grid.Column="1" Background="Blue"/>
      </Grid>
   </StackPanel>
</Page>

Answer 2

You can do this using Grids with a SharedSizeGroup instead of DockPanels. I.e.

<StackPanel Grid.IsSharedSizeScope="True">
    <Grid Height="100" >

        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" SharedSizeGroup="A"/>
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>

        <StackPanel Grid.Column="0" Width="{Binding ElementName=LeftMenuText, Path=Width}"
                                DockPanel.Dock="Left"
                                Background="Yellow">
            <TextBlock
                    Text="This is some text."/>
        </StackPanel>
        <StackPanel Grid.Column="1" DockPanel.Dock="Right"
                                Background="Orange">
        </StackPanel>
    </Grid>

    <DockPanel
      Height="3"
      Background="Black"></DockPanel>

    <Grid Height="100">

        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="Auto" SharedSizeGroup="A"/>
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>

        <StackPanel Grid.Column="0" Name="LeftMenuWrapper"
                                DockPanel.Dock="Left"
                                Background="Yellow">
            <TextBlock
                            Text="This is some text that is longer."/>
        </StackPanel>
        <StackPanel Grid.Column="1" DockPanel.Dock="Right"
                                Background="Blue">
        </StackPanel>
    </Grid>
</StackPanel>

The key things to remember are to give each column inside your grids a SharedSizeGroup with the same name ("A" in this example), and add Grid.IsSharedSizeScope="True" to a shared parent of the Grids (the StackPanel containing the Grids in this example)