Thank you for writing.
In order to add an expression to filter by a text column where the value is an empty string, you can use the Equals
operator with an empty value. Please refer to the attached gif file which illustrates how to filter all rows where the ShipName
property is an empty string or is null. I have attached my sample project for your reference. Additional information about getting started with RadDataFilter
is available here: http://docs.telerik.com/devtools/winforms/datafilter/getting-started
If you are still experiencing any further difficulties, it would be greatly appreciated if you can specify what is the exact goal that you are trying to achieve. Thus, we would be able to assist you further.
I hope this information helps. Should you have further questions I would be glad to help.