Solved This Myself..
Just Changed the Code Like this.
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)
TextBox1.Text = dt.Rows(0).Item("max").ToString
If TextBox1.Text = "" Then
TextBox1.Text = 1
End If
End Sub
Anyways Thanks for your Help Coders..