Serial Number Increment by 1 linked with Sql Server 2008

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VB

Visual-Studio

Dear Coders,

I had a simple programming to fetch data from sql server and put serial number automatically.

Form name - Form1

Serial No - Textbox1.text

Project ID - Textbox2.text

Project Name - Combobox1.selected value

But When I run the program it actually works in a way. The Problem is when i select the project for the first time it is putting the value as "1" and if I select the Project for the Second time it fetches data from sql and add one to the max of serial number. I have Even Given the Full code Below. Please help.

Imports System.Data.SqlClient

Public Class Form1

    Dim con As New SqlConnection
    Dim myconstring As String = "Data Source=ETA-TESTSRVR;Initial Catalog=Procurement;Integrated Security=True"

    Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged

        TextBox2.Text = ComboBox1.SelectedValue
        con.ConnectionString = myconstring
        Dim da As New SqlDataAdapter("SELECT max(sl_no)+1 as max FROM e1 where project_id='" + TextBox2.Text + "'", con)
        Dim dt As New DataTable
        da.Fill(dt)
        If TextBox1.Text = "" Then
            TextBox1.Text = 1
        Else
            TextBox1.Text = dt.Rows(0).Item("max").ToString
        End If
    End Sub

    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        'TODO: This line of code loads data into the 'ProcurementDataSet.Project' table. You can move, or remove it, as needed.
        Me.ProjectTableAdapter.Fill(Me.ProcurementDataSet.Project)

    End Sub
End Class