return value, output parameter,

Return Value

https://docs.microsoft.com/en-us/sql/t-sql/language-elements/return-transact-sql?view=sql-server-2017

https://stackoverflow.com/questions/706361/getting-return-value-from-stored-procedure-in-c-sharp

Mehrdad makes some good points, but the main thing I noticed is that you never run the query...

SqlParameter retval = sqlcomm.Parameters.Add("@b", SqlDbType.VarChar);
retval.Direction = ParameterDirection.ReturnValue;
sqlcomm.ExecuteNonQuery(); // MISSING
string retunvalue = (string)sqlcomm.Parameters["@b"].Value;

Using a Stored Procedure with Output Parameters

https://docs.microsoft.com/en-us/sql/connect/jdbc/using-a-stored-procedure-with-output-parameters?view=sql-server-2017

https://stackoverflow.com/questions/10905782/using-stored-procedure-output-parameters-in-c-sharp

ParameterDirection Enum

Input	1	The parameter is an input parameter.
InputOutput	3	The parameter is capable of both input and output.
Output	2	The parameter is an output parameter.
ReturnValue	6	The parameter represents a return value from an operation such as a stored procedure, built-in function, or user-defined function.