I want Primary key as a auto generate value....

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SQL-Server-2005

.NET3.5

, +

HI!!
I create table named "LateFee" and RctNo as Primary key and also i write one function "Autoge" and call it on button's (Add) clilk event but it throw exception "cast specified is not valid" what should i do for generating number as auto generate ..

C#

public void Autogen()
       {
           #region Autogen

           cmd = new SqlCommand("select max(RctNo) from LateFee ",con);
           cmd.Connection = con;
           int i=0;
           try
           {
               if (con.State == ConnectionState.Closed)
               {
                   con.Open();
               }
              i =(int)cmd.ExecuteScalar();
               i = i++;
               txtrctno.Text =i.ToString();
               con.Close();
           }
           catch(InvalidCastException ex)
           {
               MessageBox.Show(ex.Message);
               con.Close();
           }
           #endregion

       }

thanks

Posted 25-Mar-13 1:10am

Arifa S

Updated 25-Mar-13 1:16am

v2

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2 solutions

Solution 2

in your table what is data type of "RctNo" column. You can try this
Corrct this Line.
cmd = new SqlCommand("select max(Cast(RctNo as Integer)) from LateFee ",con);

Posted 25-Mar-13 1:28am

Johirul Islam Tarun

Comments

Arifa S 25-Mar-13 7:31am

RctNo datatype is Int

OriginalGriff 25-Mar-13 7:50am

That is a very dangerous thing to do: see my answer!

Arifa S 25-Mar-13 8:02am

Means its better to use simple code instead of using auto generated code...
right sir
thanks

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OriginalGriff · Accepted Answer · 2013-03-25T01:24:00

That is a very, very bad idea.
Even if it worked, getting the highest number from the database and incrementing it by one does not ensure that teh number is unique - indeed, it may not be unique by the time you exit the method. Remember that SQL server is designed to work in a multiuser environment, so you have to assume that another user or users will be doing the exact same code at the exact same time. You should never, ever try to predict the "next value" of an identity field in advance - get the value for the row after you have inserted a new row and you are fine, but getting it in advance is just asking for complicated trouble in production.

"Then Sir what will be the command to be execute for getting next row????"

You don't. You only get the id of a row after you have created it. The easy way is to do the insert and select as a single operation:

SQL

INSERT INTO MyTable (UserName, UserRole) VALUES ('My Name', 666);
SELECT @@IDENTITY

Because this is executed as a single statement by SQL, it will return the value given to the Identity field by the insert.