Custom control binding issue in silverlight

I'm trying to set up the binding of a custom control in Silverlight 3 and I'm having weird problems with it.

My xaml for the custom control:

<UserControl x:Class="StronicoMain.GenericSmallIcon"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation" 
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml" 
    Width="400" Height="300" >
    <Canvas x:Name="canGSI">

    </Canvas>
</UserControl>

      

        
        
        
      

    

The codename for the custom control is

private string _EntityTypeID;

    public string EntityTypeID
    {
        get
        {
            return _EntityTypeID;
        }

        set
        {
            _EntityTypeID = value;
        }
    }

    public GenericSmallIcon()
    {
        InitializeComponent();
        Loaded += new RoutedEventHandler(Page_Loaded);
    }        
    public void Page_Loaded(object sender, RoutedEventArgs e)
    {
        icoMale icoMale = new icoMale();
        icoFemale icoFem = new icoFemale();

        if (EntityTypeID == null)
        {
            canGSI.Children.Add(icoMale);
        }
        else if (EntityTypeID == "1")
        {
            canGSI.Children.Add(icoMale);
        }
        else if (EntityTypeID == "2")
        {
            canGSI.Children.Add(icoFem);
        }
        else
        {
            canGSI.Children.Add(icoMale);
        }            
    }

      

        
        
        
      

    

I call it from the DataGridSelection adapter (taken from the Microsoft Toolkit sample example for AutoCompleteBox-Datagrid version) - the relevant part looks like this:

<Stron:DataGridSelectionAdapter x:Name="SelectionAdapter" AutoGenerateColumns="False" IsReadOnly="False">
                                                    <Stron:DataGridSelectionAdapter.Columns>
                                                        <data:DataGridTemplateColumn>
                                                            <data:DataGridTemplateColumn.CellTemplate>
                                                                <DataTemplate><Stron:GenericSmallIcon EntityTypeID="{Binding EntityTypeID}"></Stron:GenericSmallIcon></DataTemplate>
                                                            </data:DataGridTemplateColumn.CellTemplate>
                                                        </data:DataGridTemplateColumn>
                                                            <data:DataGridTextColumn Header="Contact Name" FontWeight="Bold" Foreground="#CC000000" Binding="{Binding EntityName}" />
                                                        <data:DataGridTextColumn Header="Tags" Binding="{Binding EntityTags}" />                                                        
                                                    </Stron:DataGridSelectionAdapter.Columns>
                                                </Stron:DataGridSelectionAdapter>

      

        
        
        
      

    

I run the code and I get the error "ManagedRuntimeError # 4004" - if I try to use a custom control when manually configuring the data binding, it works fine, if I try to rely on the values ​​that are the data binding I get the error. How do I create a custom data binding event in a custom control? I think it is a problem that the page is being loaded before the values ​​are passed to it.

Thanks everyone.

~ Steve

Update, here is a working, modified code as per the accepted answer

public static readonly DependencyProperty EntityTypeIDProperty = DependencyProperty.Register("EntityTypeID", typeof(string), typeof(GenericSmallIcon), new PropertyMetadata(new PropertyChangedCallback(GenericSmallIcon.OnEntityTypeIDPropertyChanged)));
public string EntityTypeID
{
    get { return (string)GetValue(EntityTypeIDProperty); }
    set { SetValue(EntityTypeIDProperty, value); }
}
private static void OnEntityTypeIDPropertyChanged(
    DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    GenericSmallIcon control = d as GenericSmallIcon;
    string b = (string)e.NewValue;
}