In Combo Box I Loaded The Values When Selecting Any One Of The Value From Combo The Other Text Fields Values Sholud Come.How To Do ?Here I Will Give The Code.

protected void btn1_Click(object sender, EventArgs e)
    {
        lbl3.Visible = true;
        //lbl3.Text = "OPEN";

        try
        {
            SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["SQLDbConnection"].ToString());
            SqlCommand command = new SqlCommand("gas2", con);
            command.CommandType = CommandType.StoredProcedure;
            command.Parameters.Add("@action", SqlDbType.VarChar).Value = ddl1.SelectedValue.Trim();
            command.Parameters.Add("@serial_no", SqlDbType.VarChar).Value = ddl2.SelectedValue.Trim();
            command.Parameters.Add("@gas_name", SqlDbType.VarChar).Value = txtGasname.Text.Trim();
            con.Open();

            command.ExecuteNonQuery();
            fillSerialNo();
            con.Close();
        }
        catch (SqlException ex)
        {
            Console.WriteLine("SQL Error" + ex.Message.ToString());
            ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "alertMessage", "alert('" + ex.Message.ToString() + "')", true);
            //lbl3.Text = ex.Message.ToString();
        }      
    }    
    #endregion

    #region Page Method
    private void fillSerialNo()
    {
        SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["SQLDbConnection"].ToString());
        string queryString = "Select serial_no,gas_name from tbl_cyl_gas_master ";
        SqlCommand command = new SqlCommand(queryString, con);
        SqlDataAdapter adapter = new SqlDataAdapter(queryString, con);
        con.Open();
        SqlDataReader dr = command.ExecuteReader(CommandBehavior.CloseConnection);
        DataTable dt = new DataTable();
        dt.Load(dr);
        ddl2.DataSource = dt;
        ddl2.DataTextField = "serial_no";
        ddl2.DataValueField = "serial_no";
        ddl2.DataBind();
        ddl2.Items.Insert(0, new ListItem("-- Add --", "0"));
    }
    #endregion

alter procedure gas2
(
	@action			varchar(20),
	@serial_no		int,
	@gas_name		varchar(50)
)
AS
BEGIN
	declare @Newserial_no int
	declare @tran_stat varchar(10)
	SET @tran_stat=''
	IF(@tran_stat ='') OR (@tran_stat =null)
		SET @tran_stat = CASE when @action='add' then 'FR' END
 
	
	IF(@action ='add')
	BEGIN
		IF (@serial_no = 0)
		begin
			IF NOT EXISTS(SELECT * FROM tbl_Cyl_gas_master WHERE gas_name = @gas_name AND tran_stat = 'FR') 
				INSERT INTO tbl_Cyl_gas_master(gas_name,tran_stat,tran_date) VALUES (@gas_name,@tran_stat,GEtDATE())
				--SET @Newserial_no = SCOPE_IDENTITY()
		end	
		ELSE
		begin
			Raiserror('Gas Name or transaction exist',16,1)
		end
	END
	ELSE IF @action='update'
	BEGIN
		IF EXISTS(SELECT * FROM tbl_Cyl_gas_master WHERE serial_no = @serial_no AND tran_stat = 'FR') 
		begin
			--select serial_no ,gas_name from tbl_Cyl_gas_master where serial_no=@serial_no
			UPDATE tbl_Cyl_gas_master set gas_name=@gas_name,Modified_date=GETDATE() WHERE serial_no=@serial_no
			select serial_no ,gas_name from tbl_Cyl_gas_master where serial_no=@serial_no
		end
	ELSE
		RAISERROR ('Transaction not is fresh',16,1);
	END 
	ELSE IF @action='authorise'
	BEGIN
	IF EXISTS(SELECT * FROM tbl_Cyl_gas_master WHERE serial_no = @serial_no AND tran_stat = 'FR') 
		begin
			
			UPDATE tbl_Cyl_gas_master SET gas_name=@gas_name,tran_stat='AU',Modified_date=getdate() 
			WHERE serial_no=@serial_no
		end
	ELSE
	begin
			--return -1;
		RAISERROR ('Transaction already Authorised or Deleted',16,1)
	end
	ENd
	ELSE if @action='delete'
	begin
		if exists( select * from tbl_cyl_gas_master where serial_no=@serial_no and tran_stat='FR')
		begin
		update tbl_Cyl_gas_master set tran_stat='DE',gas_name='Deleted',
			Modified_date=getdate() where serial_no=@serial_no
	--DELETE gas_name from tbl_Cyl_gas_master where serial_no=@serial_no
		end
		else
			begin
				RAISERROR('Cannot delete.,Transaction is in Authorised Status',16,1)
			end
	end
	else
		select serial_no,gas_name,tran_stat from tbl_cyl_gas_master where serial_no=@serial_no
END