The Business Rules Engine of Windows Workflow Foundation does not appear to update value type fields that are part of a structure. When the THEN action of a rule is executed and a calculation updates a value type in a struct, the value of the structure member remains zero. Turning on tracing of the execution of individual business rules shows successful execution of the assignments, but the targeted field maintains its initial value of zero during its use in subsequent calculations. No exceptions are thrown and tipping the BRE via the "update()" command has no impact on this behavior. In my case, changing the struct to a class cured the BRE of this ailment.