Solve the binary conversion problem

luozx2504

The conversion is still wrong, saying that it is impossible to convert the DBNull value in byte []
But when I change the database type to char, the value is correct. Why is it empty when I use the binary type

lin3412048

What type of UserName field is in your database?

luozx2504

Is of type binary
But it can be changed to char type. If it is binary type, DBNull value cannot be converted to byte []

luozx2504

But the database requires binary type

lin3412048

Oh
You can judge whether it is empty before force conversion
byte [] b =
(cmd.Parameters ["@ UserName"]. value == null)?
new byte [0]:
(byte []) cmd.Parameters ["@ UserName"]. value;

weiranli00

But wrong, why is it empty, it should be valuable, I just want the value

mgcfox

Is the stored procedure UsmgcfoxsExict no return value?

luozx2504

So why do I have a return value when I change the field to char type?
database:
CREATE procedure UsmgcfoxsExict
@Uid binary (30),
@UserName binary (30) output
as
select @ UserName = UserName from AlbumLogin where Uid = @ Uid
GO
program:
SqlCommand cmd = new SqlCommand ("UsmgcfoxsExict", conn);
        cmd.CommandType = CommandType.StoredProcedure;

        cmd.Parameters.Add (new SqlParameter ("@ Uid", SqlDbType.Binary, 30));
        cmd.Parameters ["@ Uid"]. Value = Encoding.UTF32.GetBytes (paraUid);

        cmd.Parameters.Add (new SqlParameter ("@ UserName", SqlDbType.Binary, 30));
        cmd.Parameters ["@ UserName"]. Direction = ParameterDirection.Output;

        conn.Open ();
        cmd.ExecuteNonQuery ();
        conn.Close ();

      return Encoding.UTF32.GetString (((byte []) cmd.Parameters ["@ UserName"]. Value));

error:
Unable to cast object of type "System.DBNull" to type "System.Byte []"