Currently, the RadExpressionEditor control shows a wavy underline and a red color to indicate that an error is present.

We could introduce API to modify this behavior, which will ultimately be used by the ChangeUnderlineDecoration and ChangeUnderlinesColor methods of the internal RadRichTextBox control.

Respect DisplayNameAttribute of the added fields from fields list in ExpressionTextBox.

When creating an expression that has a decimal, if it doesn't have a zero in front of the decimal, it doesn't evaluate the expression:
eg:  0.02 is valid , .02 is invalid

Allow developers to add their own functions to the list of available functions in the RadExpressionEditor and then pass these functions to the LINQ runtime when the expression tree is built. Add support for custom Categories as well.

Scheduled for: 
The feature will be available in R1 2019, scheduled for the mid of January 2019.