It is quite common to filter one combo box data source based on the value selected in another drop down editor. For instance, imagine a Country/City scenario, demonstrated in our ASP.NET demos here (remember the amount of code lines for this demo). 

eXpressApp Framework (XAF) allows you to write less code for such common scenarios with the help of various data annotation attributes, configurable application UI metadata, built-in modules and their Controllers. Specially for this particular scenario in XAF we have the Current Object Parameter (@This) feature, which can be used in the criteria set for the lookup's data source via DataSourceCriteriaAttribute. For instance, if there is an Order class with the Product and Accessory reference properties, you can implement the latter property as shown below (just a single line) to have it filtered by the selected product (assuming that the Accessory class has a back reference to the Product class):

    [DataSourceCriteria("Product.Oid = '@This.Product.Oid'")]
    public Accessory Accessory { ... }