The IsValid property indicates whether the control's expression is valid/syntactically correct. The Expression/Text property assigns a new expression to the control.  The EvaluationText property specifies the values for variables in the expression to be evaluated ( per line ). The EvaluationResult property specifies the result for each evaluation ( line ). Use the Evaluate property to get the result of evaluation of the control's expression. Use the ForeColorInvalid property to specify a different color to show the control's expression while it is not valid.