Symptoms
Error 'Method could not be resolved' when accessing method : GetWorklistItems(filter)'

Diagnoses
The same DLL is working fine in old machine while it's not working in new machine used to develop the custom app (via K2 Blackpearl API).

Resolution
This is an issue with the 32bit and 64bit SourceCode.Workflow.Client.dll. It could be due to old machine which is running on 32bit while new one is running on 64bit. We have 2 SourceCode.Workflow.Client.dll in our Blackpearl folder:-
a) Program Files (x86)K2 blackpearlin (32bit)
b) Program Files (x86)K2 blackpearlHost Serverin (64bit)