Generally, collections in RadExpressionEditor are treated as IQueryable (you can see that in the LambdaExpression the editor produces). Consequently, all the Queryeable extensions methods are support, except for those such as Cast<T>(), which require a type argument. We don't have plans to implement support for calling generic methods in the ExpressionEditor, but we will consider making it possible to cast values.
Unfortunately, the only thing I can suggest right now is to add properties or methods on your model class that can expose the DataValue property as a double.
We will release thorough documentation of the expression parser with our Q2 beta release, which should be out in a couple of weeks.
All the best,
the Telerik team
Do you want to have your say when we set our development plans? Do you want to know when a feature you care about is added or when a bug fixed? Explore the Telerik Public Issue Tracking
system and vote to affect the priority of the items