Question

   public DataTable selectcat(int catid)
{
    SqlConnection con = new SqlConnection(DBHelper.connection);
    SqlCommand com = new SqlCommand();
    com.Connection = con;
    con.Open();
    com.CommandType = CommandType.StoredProcedure;
    com.CommandText = "selectcatalog";
    com.Parameters.AddWithValue("@catid", catid);
    SqlDataReader dr = com.ExecuteReader();
    DataTable dt = new DataTable();  
        dt.Load(dr);
        con.Close();
        return dt;
}

then i call the function in dropdownlist event (SelectedIndexChanged)

 protected void catselectddl_SelectedIndexChanged(object sender, EventArgs e)
{
   DataTable dt;

    DAcatagory obj = new DAcatagory();
    dt = obj.selectcat(catselectddl.SelectedValue);
    DataRow dr = dt.Rows[0];
    catnametxt.Text = dr["catname"].ToString();
    catdestxt.Text = dr["catdescription"].ToString();
}

but it give me invalid arguments error at dt = obj.selectcat(catselectddl.SelectedValue);
PS: the dropdownlist take his display and the value from database

Answer 1

Just convert it to int:

dt = obj.selectcat(Convert.ToInt32(catselectddl.SelectedValue));