using (SqlConnection Conn = new SqlConnection(sConnectionString))
{
sSQL = @"SELECT userName FROM tblUsers WHERE userID = @id";
SqlCommand cmd = new SqlCommand(sSQL, Conn);
//Adicionar já com um valor
cmd.Parameters.AddWithValue("@id", 1);
//Alterar o valor
cmd.Parameters["@id"].Value = 2;
//Verificar se existe e remover
if (cmd.Parameters.IndexOf(cmd.Parameters["@id"]) != -1)
cmd.Parameters.Remove(cmd.Parameters["@id"]);
//Outra forma de adicionar
cmd.Parameters.Add("@id", SqlDbType.Int);
cmd.Parameters["@id"].Value = 1;
Conn.Open();
Response.Write(cmd.ExecuteScalar());
Conn.Close();
}