FIM -IIF Custom Expression

Hi,

How can i write a Custom IIF expression , where the value returned can be assigned to a "reference type attribute " for a user.

Ex-  IIF(Eq(abc,"123"),"accountname1","accountname2")--->  xyz

xyz-reference attribute.

abc- user's attribute used in IIF case

This is not working as the workflow gives error "conversion failedwhen converting from a character string.

Regards

shakti