In-Out Parameter for SqlCommand

I have the following parameter for SqlCommand. How do I make it to both in and out the paramter value for the Stored Procedure.

 SqlCommand mySqlCommand = new SqlCommand("aspInsertZipCode", mySqlConnection);
 mySqlCommand.CommandType = CommandType.StoredProcedure;
 mySqlCommand.Parameters.Add("@DataRows", dataStringToProcess.ToString());