Achieve "slide down" animation in WPF

I am attempting to create my own template for an Expander control. When the control is expanded, I want the content to slide down slowly.

The desired height of the content is not known at compile time.

I thought we could define the slide down as an animation:

<Storyboard x:Key="ExpandContent">

    <DoubleAnimation 
        Storyboard.TargetName="_expanderContent" 
        Storyboard.TargetProperty="Height" 
        From="0.0" 
        To="{Binding ElementName=_expanderContent,Path=DesiredHeight}"
        Duration="0:0:1.0" />
</Storyboard>

But Unfortunately not. We get an error

Cannot freeze this Storyboard timeline tree for use across threads.

It appears that we cannot use binding when defining animation parameters. (Discussed also in this question.)

Does anyone have any ideas on how I can approach this? I'm wary of using LayoutTransform.ScaleY, because that would create a distorted image.

This is similar to this question, but this question has an answer involved writing code-behind, which I don't think is possible in a control template. I'm wondering if a XAML-based solution is achievable.

---

For what it's worth, here is the current state of my control template.

<ControlTemplate x:Key="ExpanderControlTemplate"  TargetType="{x:Type Expander}">
    <ControlTemplate.Resources>
            <!-- Here are the storyboards which don't work -->
            <Storyboard x:Key="ExpandContent">

                <DoubleAnimation 
                    Storyboard.TargetName="_expanderContent" 
                    Storyboard.TargetProperty="Height" 
                    From="0.0" 
                    To="{Binding ElementName=_expanderContent,Path=DesiredHeight}"
                    Duration="0:0:1.0" />
            </Storyboard>
            <Storyboard x:Key="ContractContent">

                <DoubleAnimation 
                    Storyboard.TargetName="_expanderContent" 
                    Storyboard.TargetProperty="Height" 
                    From="{Binding ElementName=_expanderContent,Path=DesiredHeight}"
                    To="0.0"
                    Duration="0:0:1.0" />

            </Storyboard>
        </ControlTemplate.Resources>
    <Grid Name="MainGrid" Background="White">
        <Grid.RowDefinitions>
            <RowDefinition Height="Auto" />
            <RowDefinition Name="ContentRow" Height="Auto" />
        </Grid.RowDefinitions>
        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="*" />
        </Grid.ColumnDefinitions>
        <Border>
            <Grid>
                <Grid.ColumnDefinitions>
                    <ColumnDefinition Width="*" />
                    <ColumnDefinition Width="Auto" />
                </Grid.ColumnDefinitions>
                <ContentPresenter ContentSource="Header" />
                <ToggleButton Template="{StaticResource ProductButtonExpand}"
                              Grid.Column="1"
                              IsChecked="{Binding Path=IsExpanded,Mode=TwoWay,RelativeSource={RelativeSource TemplatedParent}}" 
                              />
                <Rectangle Grid.ColumnSpan="2" Fill="#FFDADADA" Height="1" Margin="8,0,8,2" VerticalAlignment="Bottom"/>

            </Grid>
        </Border>

            <ContentPresenter Grid.Row="1" HorizontalAlignment="Stretch" Name="_expanderContent">

            </ContentPresenter>

    </Grid>
    <ControlTemplate.Triggers>
        <Trigger Property="IsExpanded" Value="True">
            <Setter TargetName="_expanderContent" Property="Height" Value="{Binding ElementName=_expanderContent,Path=DesiredHeight}" />

                <!-- Here is where I would activate the storyboard if they did work -->
                <Trigger.EnterActions>
                <!--<BeginStoryboard Storyboard="{StaticResource ExpandContent}"/>-->
            </Trigger.EnterActions>
                <Trigger.ExitActions>
                    <!--<BeginStoryboard x:Name="ContractContent_BeginStoryboard" Storyboard="{StaticResource ContractContent}"/>-->
                </Trigger.ExitActions>
        </Trigger>
            <Trigger Property="IsExpanded" Value="False">

                <Setter TargetName="_expanderContent" Property="Height" Value="0" />
            </Trigger>
        </ControlTemplate.Triggers>

</ControlTemplate>

If you can use Interactions with FluidLayout (Blend 4 SDK) you are in luck, it's really useful for those fancy animation things.

First set the content CP's Height to 0:

<ContentPresenter Grid.Row="1"
    HorizontalAlignment="Stretch"
    x:Name="_expanderContent"
    Height="0"/>

To animate this, the Height just needs to be animated to NaN in the VisualState that represents the expanded state (non-discrete animations would not let you use NaN):

xmlns:is="http://schemas.microsoft.com/expression/2010/interactions"

<Grid x:Name="MainGrid" Background="White">
    <VisualStateManager.CustomVisualStateManager>
        <is:ExtendedVisualStateManager/>
    </VisualStateManager.CustomVisualStateManager>
    <VisualStateManager.VisualStateGroups>
        <VisualStateGroup x:Name="ExpansionStates" is:ExtendedVisualStateManager.UseFluidLayout="True">
            <VisualStateGroup.Transitions>
                <VisualTransition GeneratedDuration="0:0:1"/>
            </VisualStateGroup.Transitions>
            <VisualState x:Name="Expanded">
                <Storyboard>
                    <DoubleAnimationUsingKeyFrames Storyboard.TargetProperty="(FrameworkElement.Height)"
                                                   Storyboard.TargetName="_expanderContent">
                        <DiscreteDoubleKeyFrame KeyTime="0" Value="NaN"/>
                    </DoubleAnimationUsingKeyFrames>
                </Storyboard>
            </VisualState>
            <VisualState x:Name="Collapsed"/>
        </VisualStateGroup>
    </VisualStateManager.VisualStateGroups>
    <!-- ... --->

That should be all that is necessary, the fluid layout will create the transition for you from there.

---

If you have a code-behind solution that would be fine, you can even use code-behind in dictionaries like this:

<!-- TestDictionary.xaml -->
<ResourceDictionary x:Class="Test.TestDictionary"
                    ...>

//TestDictionary.xaml.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Windows;

namespace Test
{
    partial class TestDictionary : ResourceDictionary
    {
        //Handlers and such here
    }
}