Imports System.Data.SqlClient
Imports System.Threading
Public Class FrmMultiThread
    Dim con As New SqlConnection
    Dim ad1, ad2, ad3 As SqlDataAdapter
    Dim ds As New DataSet
    Dim cmd As SqlCommand
    Dim dr As SqlDataReader
    Dim eid As Integer
    Dim th1 As Thread
    Dim th2 As New Thread(AddressOf ShowRecords)
    Dim th3 As New Thread(AddressOf Search)
    Private Sub FrmMultiThread_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load
        con.ConnectionString = "data source=seed;initial catalog=pankajp;user id=sa;password=sa;multipleactiveresultsets=true"
        con.Open()
        ComboBox.CheckForIllegalCrossThreadCalls = False
        MessageBox.Show("status of th2 = " & th2.ThreadState.ToString)
        MessageBox.Show("status of th3 = " & th3.ThreadState.ToString)
        th1 = New Thread(AddressOf FillCombo)
        th1.IsBackground = True
        th1.Start()  'FillCombo() is called as a therad
    End Sub
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        th2.Start() 'ShowRecords() is called as a therad
    End Sub
    Public Sub ShowRecords()  'represented as th3
        MessageBox.Show("status of th1 = " & th1.ThreadState.ToString)
        MessageBox.Show("status of th2 = " & th2.ThreadState.ToString)
        MessageBox.Show("status of th3 = " & th3.ThreadState.ToString)
        ad1 = New SqlDataAdapter("select * from Dept", con)
        ad1.Fill(ds, "Department")
        DataGridView1.DataSource = ds
        DataGridView1.DataMember = "Department"
    End Sub

    Public Sub FillCombo() 'represented as th1 which is a background thread
        MessageBox.Show("status of th1 = " & th1.ThreadState.ToString)
        MessageBox.Show("status of th2 = " & th2.ThreadState.ToString)
        MessageBox.Show("status of th3 = " & th3.ThreadState.ToString)
        MessageBox.Show("the background thread for filling combobox is started")
        cmd = New SqlCommand("select empno from employee order by empno", con)
        dr = cmd.ExecuteReader(CommandBehavior.CloseConnection)
        While dr.Read
            ComboBox1.Items.Add(dr(0))
            Thread.Sleep(3000) '3 seconds 'this thread is haulted its execution for 3000 mili seconds
        End While
        dr.Close()
        MessageBox.Show("the background thread is over! plz check the combobox now!!")
        Button2.Enabled = True
    End Sub

    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        th3.Start()
    End Sub

    Public Sub Search() 'represented as th1
        MessageBox.Show("status of th1 = " & th1.ThreadState.ToString)
        MessageBox.Show("status of th2 = " & th2.ThreadState.ToString)
        MessageBox.Show("status of th3 = " & th3.ThreadState.ToString)
        Dim qry As String
        qry = "select * from employee where empno=" & eid
        ad2 = New SqlDataAdapter(qry, con)
        ad2.Fill(ds, "employee")
        DataGridView2.DataSource = ds
        DataGridView2.DataMember = "employee"
    End Sub

    Private Sub ComboBox1_SelectedIndexChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles ComboBox1.SelectedIndexChanged
        eid = ComboBox1.SelectedItem
    End Sub
End Class