At design time you can open RadExpressionEditor
by clicking the ellipsis button of the columns'
At design time the “Result Preview” functionality is not working,
because there is no real data to evaluate.
In addition, the expression is checked for validity. However, it cannot be verified
according to actual data and value types. Because of this, it is possible to save an invalid
expression at design time. If an invalid expression is created,
an exception message will be shown when the project is ran.