Creating Storyboard in code behind in WPF

When working in code, you don't need Storyboard really, just animations for basic things, like you show in your question. I made a little sample to show how easy it works.

This is the complete code behind of the mainwindow:

namespace WpfCSharpSandbox
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            WidenObject(150, TimeSpan.FromSeconds(1));
        }

        private void WidenObject(int newWidth, TimeSpan duration)
        {
            DoubleAnimation animation = new DoubleAnimation(newWidth, duration);
            rctMovingObject.BeginAnimation(Rectangle.WidthProperty, animation);
        }
    }
}

This is how the XAML looks like:

<Window x:Class="WpfCSharpSandbox.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="Sandbox" Height="350" Width="525">
    <Grid Background="#333333">
        <Rectangle x:Name="rctMovingObject" Fill="LimeGreen" Width="50" Height="50"/>
    </Grid>
</Window>

Put this in a WPF app and see how it works, experiment with it and try other animations/properties.

Adding djerry's comment sample code would look like this:

var anim = new DoubleAnimation {
                                From = 1920, 
                                To = 1, 
                               };

wnd.BeginAnimation(Window.LeftProperty, anim); 

and you would have to have this code in window loaded event handler. Hope this helps.