Thank you, data type of the context property i was trying to assign the value returned by GetValueAt() method was wrong. I had it as EnsLib.HL7.Message. 

Yeah, error message was a bit confusing, it was pointing to Source property as undefined, but if i reference the source property as request.{MSH:9.2}  which was translated by Ensemble code generator as  

set (whatever the property or variable) = request.GetValueAt("MSH:9.2")

i was pretty confident request object does contain this property, and as per documentation it can be referenced by the above code, but when it was not generating the expected results, i got confused and thought maybe i am reference it incorrectly. 

Please if you or anyone has some additional documentations on BPLs, please share.

Thank you, data type of the context property i was trying to assign the value returned by GetValueAt() method was wrong. I had it as EnsLib.HL7.Message. 

Yeah, error message was a bit confusing, it was pointing to Source property as undefined, but if i reference the source property as request.{MSH:9.2}  which was translated by Ensemble code generator as  

set (whatever the property or variable) = request.GetValueAt("MSH:9.2")

i was pretty confident request object does contain this property, and as per documentation it can be referenced by the above code, but when it was not generating the expected results, i got confused and thought maybe i am reference it incorrectly. 

Please if you or anyone has some additional documentations on BPLs, please share.

Thank You for pointing it out, data type of the property was the issue. Now the property is set to %string ,  and i am able to assign the request.{MSH:9.2} value to context.x property.