how can I out a parameter as varchar2 in oracle

There are no less than 10 overloads of the OracleParameterCollection.Add method. It looks like you have accidentally called the wrong one.

I believe the one you want is Add(string, OracleDbType, int, object, ParameterDirection), in which case you're just missing a value for the object parameter. This parameter should contain an initial value for the Oracle parameter you're using. In your case, however, the initial value doesn't matter as it's an out parameter. Add null after 50 and your stored procedure call should succeed.

The one you have called is Add(string, OracleDbType, object, ParameterDirection). The size 50 has been interpreted as an initial value for the parameter. I'm not sure how to interpret the error that Oracle returns ("numeric or value error") – that implies to me that Oracle has tried to convert a string to a number and failed. Perhaps the value 50 overrides the type OracleDbType.Varchar2 and so Oracle expects a number rather than a string?

There were another couple of problems I found:

• Should command.Parameters["SP_REQ_NUM"] be command.Parameters["SP_NUM"]?
• Your stored procedure doesn't return a number; calling Convert.ToInt32 on a string such as 201405 001 will fail.