将存储过程的结果（OUTPUT）存储到Gridview C＃ASP.Net

Question

I am trying to return the Average value that is generated by a stored procedure into a new column in the Gridview

What should I put instead of the "!!!!" in my code below

I keep getting "Stored procedure expects parameter which was not supplied" Error although I defined that parameter as OUTPUT in the procedure (BELOW)

   protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
{
    GridView1.Columns[3].Visible = true;
    string Teachername = GridView1.SelectedRow.Cells[1].Text;

    string connStr = ConfigurationManager.ConnectionStrings["MS4ConnectionString"].ToString();
    SqlConnection conn = new SqlConnection(connStr);

    SqlCommand cmd = new SqlCommand("Overall_Rating", conn);
    cmd.CommandType = CommandType.StoredProcedure;

    SqlParameter Parent_Username = new SqlParameter("@parent_username", TextBox1.Text);
    SqlParameter Teacher_Username = new SqlParameter("@teacher_username", Teachername);
    SqlParameter Avg = new SqlParameter("@Avg", !!!!);
    Avg.Direction = ParameterDirection.ReturnValue;

    cmd.Parameters.Add(Parent_Username);
    cmd.Parameters.Add(Teacher_Username);
    cmd.Parameters.Add(Avg);

    string retunvalue = (string)cmd.Parameters["@Avg"].Value;
    System.Diagnostics.Debug.WriteLine(retunvalue);

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

@parent_username varchar(40), @teacher_username varchar(40), @Avg smallint output AS SET @Avg = ( select Avg(PRT.rating) FROM Parents_Rate_Teachers PRT INNER JOIN Employees_Name E ON PRT.teacher_username = E.employee_username WHERE E.employee_username= @teacher_username ) return @Avg

Answer 1

I think you just want ParameterDirection.Output instead of ReturnValue:

SqlParameter avg= new SqlParameter();
avg.ParameterName = "@Avg";
avg.Direction = ParameterDirection.Output;
cmd.Parameters.Add(avg);

Answer 2

There are also problem in row (in addition to previous answer)

string retunvalue = (string)cmd.Parameters["@Avg"].Value;

you should return int, because @Avg is smallint (smallint = Int16):

int retunvalue = (int)cmd.Parameters["@Avg"].Value;