insert value list in paramter

Hi, i'm using a sql-select command (commandtyp:text) and the sql-query is
build with commands:

sql="... and PROD in ( :PRDCT_NAME )"
Cmd.Parameters.Add(":PRDCT_NAME", OracleType.VarChar, 0, "PRDCT_NAME")
Cmd.Parameters.Item(":PRDCT_NAME").Value = _Product

_Product contains a value list like "'a','b','c'"
an error occurs during executing the command.
where is the failure?

Thanks
jens