Auto incrementing number during form load every time

Your query:

Select MAX(IS NULL(no),0)+1) from billdetail1

gives a result:

[no column name]
value

Next in the code, you are trying to read results from column named: no

TextBox1.Text = dr("no").ToString()

Take a look at result (above table) and tell me where is a field named no?

So... replace your query with:

Select ISNULL(MAX([no]),0)+1 AS [no] FROM billdetail1

Read more about: IDENTITY[^]

[EDIT name="second look"]
I see one potential risk in the way you are trying to get the number of billdetail. Let say 2 users in the same time open a form. Your query will return for them billdetail [no] = 5. What's happen when they will try to save data? You'll get duplicates.
That's why the most important is to set identity (increment) - go to above link, example A - for the column named [no]
Of course, before save data, you can re-query a [no] to get max+1, but it not guarantee non-duplicates numbers.[/EDIT]

If you're talking about an auto numbering field in a DataTable then have a look at this: http://msdn.microsoft.com/en-us/library/system.data.datacolumn.autoincrement.aspx[^]

Dim sqlText As String = "SELECT MAX(no) FROM billdetail1 "

Dim con3 As New SqlConnection("Data Source=CHET-PC\sqlexpress;Initial Catalog=invoice;Integrated Security=True")
Dim command As New SqlCommand(sqlText, con3)

Try
con3.Open()
Dim pcount As String = Convert.ToString(command.ExecuteScalar())
If pcount.Length = 0 Then

TextBox1.Text = "1"
Else
Dim pcount1 As Integer = Convert.ToInt32(pcount)
Dim pcountAdd As Integer = pcount1 + 1
TextBox1.Text = pcountAdd.ToString()

End If

Catch ex As Exception
MessageBox.Show(ex.Message)
Finally
con3.Close()
End Try