The setter Expression attribute is mutually exclusive to the Value attribute and allows a simple expression to be calulated ...

                              The setter Expression attribute is mutually exclusive to the Value attribute and allows a simple expression
                              to be calulated for the property being set.  For example the expression 'Math.Max(9,Math.Sqrt(LinesOfCode))'
                              calculates a number based on the LinesOfCode property equal to the square root of that property, and returns
                              no less than 9.  This result could then be assigned to the FontSize property to achieve node sizing based on code size.