Create DoubleAnimation and Animate a Button with Button BeginAnimation and Button WidthProperty
//File:Window.xaml.cs
using System.Windows;
using System.Windows.Controls;
using System.Windows.Media.Animation;
public partial class Window1 : Window
{
public Window1()
{
InitializeComponent();
DoubleAnimation a = new DoubleAnimation();
a.From = 50;
a.To = 100;
b.BeginAnimation(Button.WidthProperty, a);
}
}