Question

I am generating a random number admission no and this is my DAL

 public static int randomgen()
   {
       int id=0;
       int number = r.Next(100);
       HttpContext.Current.Session["number"] = "SN" + (" ") + number.ToString();
       SqlConnection con = DBConnection.OpenConnection();
       try
       {
           string sql1 = "select admissionno from tblstudent_details";
           SqlCommand cmd=new SqlCommand(sql1,con);
           SqlDataReader dr = cmd.ExecuteReader();
           if (dr.Read())
          {
               id = Convert.ToInt32(dr[0]);

           }
            dr.Close();
           return id;
       }

       catch (Exception)
       {

           throw;
       }
   }

and i am checking if there is any duplicate is getting generated but i am getting an error like Input string is not in a correct format?Where i am doing wrong?Is any better way than this?

Answer 1

You asked if there is a better way…

From what I understand about the question what you are trying to do is pick a random value and then check the database to see if that value already exists. You want to return a value back to the UI to tell the UI whether the value exists or not…

Here is a couple alternatives to consider…

public static bool randomgen() 
{
    bool isFound = false;
    string admissionNumber = "SN " + r.Next(100); 
    HttpContext.Current.Session["number"] = admissionNumber; 
    using (SqlConnection con = new SqlConnection()) // use "using" to guarantee connection is closed
    {
        string sql1 = "SELECT CASE WHEN EXISTS(SELECT admissionno FROM tlblstudent_details WHERE admissionno = @admissionno) THEN 1 ELSE 0 END";
        using (SqlCommand cmd = con.CreateCommand())
        {
             cmd.CommandText = sql;
             cmd.Parameters.AddWithValue("@admissionno", number);
             using (SqlDataReader dr = cmd.ExecuteReader())
             {
                 if (dr.Read())
                 {
                     isFound = (Convert.ToInt32(dr[0]) == 1)
                  }
              }
         }
     }
     return isFound;
}

This way you let SQL Server check to see if the value exists.

Another approach…

Not sure if you are required to prompt the user if the value is not unique, if that is not a requirement then I would consider a different approach; Keep trying until you find a unique value…Like this…

public static int randomgen() 
{ 
    bool isFound = true;
    while (isFound)
    {
        string admissionNumber = "SN " + r.Next(100); 
        using (SqlConnection con = new SqlConnection()) // use "using" to guarantee connection is closed
        {
            string sql1 = "SELECT CASE WHEN EXISTS(SELECT admissionno FROM tlblstudent_details WHERE admissionno = @admissionno) THEN 1 ELSE 0 END";
            using (SqlCommand cmd = con.CreateCommand(sql1))
            {
                cmd.Parameters.AddWithValue("@admissionno", admissionNumber);
                using (SqlDataReader dr = cmd.ExecuteReader())
                {
                    if (dr.Read())
                    {
                        isFound = (Convert.ToInt32(dr[0]) == 1)
                     }
                 }
             }
        }
        return number;
    }

This keeps checking until a unique value is returned. Then you return that unique value to the calling method. Now you set HttpContent.Current session from the caller, leaving the responsibility of this method to only be finding a unique Admission Number. The downside to the second approach is that it may take a long time to find a unique value, depending on how many values are already used, especially considering you are only allowing 100 values.

Hope this gives you good alternatives to consider. Let me know if you have additional questions.