Calculated columns that refer to relationships

[Rythm Music]

I have seen examples and samples such that you can show the Related
Tables Data in the current Table.

The syntax is something like:
' add a new calculated column to the Categories table, with the number
of child
' products
ds.Tables("Cate gories").Column s.Add("Products Count", GetType(Integer ), _
"Count(Child(Ca tProducts).Prod uctID)")

http://www.devx.com/vb2themax/Tip/18863

However, whenever I try this or any other such expression, I get an
InvalidCast Exception in a Typed DataSet.

I want to know if anyone has tried this and has it worked?
Also which is better, to show the value in this manner or do a join in
the SQL statement.

Any help will be great.

--Rythm

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