Setting a custom property within a WPF/Silverlight page

You can work with normal property without Dependency property if you create a Base class for your Page.

public class BaseWindow : Window
{
   public string MyProperty { get; set; }
}

<local:BaseWindow x:Class="BaseWindowSample.Window1" x:Name="winImp"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="clr-namespace:BaseWindowSample" 
    MyProperty="myproperty value"
    Title="Window1" Height="300" Width="300">

</local:BaseWindow>

And it works even though MyProperty is not a Dependency or Attached.

You would need to make it an attachable property as Pavel noted, then you can write something like this

<Page x:Class="JonSkeetTest.SkeetPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:JonSkeetTest="clr-namespace:JonSkeetTest" mc:Ignorable="d" 
      d:DesignHeight="300" d:DesignWidth="300"
       JonSkeetTest:SkeetPage.MyProperty="testar"
    Title="SkeetPage">
    <Grid>
        
    </Grid>
</Page>

However, with only this code-behind, you will get this error instead:

The attachable property 'MyProperty' was not found in type 'SkeetPage'.

The attached property 'SkeetPage.MyProperty' is not defined on 'Page' or one of its base classes.

---

Edit

Unfortunately, you have to use Dependency Properties. Here's a working example

Page

<Page x:Class="JonSkeetTest.SkeetPage"
      xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
      xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
      xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006" 
      xmlns:d="http://schemas.microsoft.com/expression/blend/2008" xmlns:JonSkeetTest="clr-namespace:JonSkeetTest" mc:Ignorable="d" 
      JonSkeetTest:SkeetPage.MyProperty="Testing.."
      d:DesignHeight="300" d:DesignWidth="300"
    Title="SkeetPage">
   
    <Grid>
        <Button Click="ButtonTest_Pressed"></Button>
    </Grid>
</Page>

Code-behind

using System.Windows;
using System.Windows.Controls;

namespace JonSkeetTest
{
    public partial class SkeetPage
    {
        public SkeetPage()
        {
            InitializeComponent();
        }

        public static readonly DependencyProperty MyPropertyProperty = DependencyProperty.Register(
          "MyProperty",
          typeof(string),
          typeof(Page),
          new FrameworkPropertyMetadata(null,
              FrameworkPropertyMetadataOptions.AffectsRender
          )
        );

        public static void SetMyProperty(UIElement element, string value)
        {
            element.SetValue(MyPropertyProperty, value);
        }
        public static string GetMyProperty(UIElement element)
        {
            return element.GetValue(MyPropertyProperty).ToString();
        }

        public string MyProperty
        {
            get { return GetValue(MyPropertyProperty).ToString(); }
            set { SetValue(MyPropertyProperty, value); }
        }

        private void ButtonTest_Pressed(object sender, RoutedEventArgs e)
        {
            MessageBox.Show(MyProperty);
        }
    }
}

If you press the button, you will see "Testing..." in a MessageBox.