c# - odataV4 filter contains with case sensitive and incase sensitive -

i have tried filter contains case sensitive in odata. not working. please find query string

http://services.odata.org/v4/northwind/northwind.svc/employees/?$filter=contains(toupper(firstname),%27c%27)  http://services.odata.org/v4/northwind/northwind.svc/employees/?$filter=contains(tolower(firstname),%27c%27) 

it return both lower , upper case . possible achieve?