Why can't I add a XML datasource in Expression Blend 3?

Answer 1

I just tried using blend 1.0. This version also had other options, such as 'infer xml schema'.

My guess is that there was some check added in later version to ensure that the file pointed to has an extension.

My suggestion is to edit the xaml directly. Note, this is old XAML from blend 1.0:

<Window.Resources>
 <XmlDataProvider x:Key="rssDS" d:IsDataSource="True" Source="http://www.tanguay.info/web/rss/"/&gt;
</Window.Resources>

Hope that helps, let me know.