@id int OUTPUT,

SET @id = 3
SELECT @id

private void BLogin_Click(object sender, RoutedEventArgs e)
        {

            SqlConnectionStringBuilder connect = new SqlConnectionStringBuilder();

            connect.ConnectionString = @"Data Source=.\SQLEXPRESS;Initial Catalog=SB;" +
          "Integrated Security=SSPI;Pooling=False";


            using (SqlConnection cn = new SqlConnection())
            {
                cn.ConnectionString = connect.ConnectionString;
                try
                {
                    cn.Open();

                    SqlCommand myCommand = new SqlCommand("US_LOGIN", cn);


                    myCommand.CommandType = CommandType.StoredProcedure;
                    ... Тут аналогичный код с передачей других параметров только с пераметром Input.
                    SqlParameter param1 = new SqlParameter();
                    param1.Direction = ParameterDirection.Output;
                    myCommand.Parameters.Add("@id", SqlDbType.Int).Value = 8;

                     myCommand.ExecuteReader();
                    // Возврат выходного параметра.
                    label1.Content = Convert.ToString(myCommand.Parameters["@id"].Value);
                }

                catch (SqlException ex)
                {
                    MessageBox.Show("Ошибка: " + ex);
                }
                finally
                {
                    cn.Close();
                   
                }
            }
        }

ALTER PROCEDURE test 
	-- Add the parameters for the stored procedure here
  @testparam nvarchar(1),
  @outparam nvarchar(1) output
AS
BEGIN
	-- SET NOCOUNT ON added to prevent extra result sets from
	-- interfering with SELECT statements.
	SET NOCOUNT OFF;

    -- Insert statements for procedure here
	SET @outparam = '1'
	select @outparam
END
GO

using (SqlConnection cn = new SqlConnection())
            {
                cn.ConnectionString = connect.ConnectionString;
                try
                {

                    cn.Open();

                    SqlCommand myCommand = new SqlCommand("test", cn);


                    myCommand.CommandType = CommandType.StoredProcedure;
                    // Входной параметр.
                    
                    SqlParameter param1 = new SqlParameter();
                    param1.Direction = ParameterDirection.Input;
                    myCommand.Parameters.Add("@testparam", SqlDbType.NVarChar, 20).Value = "1";

                    SqlParameter param2 = new SqlParameter();
                    param2.Direction = ParameterDirection.Output;
                    myCommand.Parameters.Add("@outparam", SqlDbType.NVarChar).Value = '8';

                    // Выполнение хранимой процедуры.
                    myCommand.ExecuteReader();
                    // Возврат выходного параметра
                    label1.Content = Convert.ToString(myCommand.Parameters[1].Value);
                }

                catch (SqlException ex)
                {
                    MessageBox.Show("Ошибка: " + ex);
                }

SqlParameter @outparam = new SqlParameter();
@outparam.Direction = ParameterDirection.Output;
myCommand.Parameters.Add("@outparam", SqlDbType.NVarChar).Value = '8';