Border Style in Visual Studio

I have to change the border style of a field based on an expression. I enter in "BorderStyle" property this expression

=iif(((Fields!field1.Value)-(Previous(Fields!field1.Value)))>1,"Solid","Default")

but it gives me the error 'BorderStyle' is not declared. It may be inaccessible due to its protection level.
How do I solve this problem? Thank you!