how to initialize auto genrated sr.no using vb.net

The logic is: NextToken = MAX(Token)+1 Where Month = Current_Month

Let's say you have a class like this:

Public Class TokenData
	Private iToken as Integer = 0
	Private dDate as Date
	
	Sub New(ByVal _Token As Integer, ByVal _Date As Date) 
		iToken = _Token
		dDate = _Date
	End Sub
	
	Public Property Token As Integer
		Get
			Return iToken
		End Get
		Set (ByVal _Token As Integer)
			iToken = _Token
		End Set
	End Property

	Public Property mDate As Date
		Get
			Return dDate
		End Get
		Set (ByVal _Date As Date)
			dDate = _Date
		End Set
	End Property
End Class

To be able to get next Token number from List(of TokenData) for the current month, you need to create linq statement this way:

Sub Main
    'initial Token
    Dim iToken As Integer = 555
    'initial date
    Dim dDate As Date = New Date(2014, 10, 15)

    'declare list of TokenData
    Dim MyData As List(Of TokenData) = New List(Of TokenData)

    'Add initial value
    MyData.Add(New TokenData(iToken, dDate))

    'get next tokens
    Dim i As Integer = 4
    Do While dDate < Date.Today
        dDate = dDate.AddDays(i)
        iToken = MyData.Where(Function(c) c.mDate.Month = dDate.Month And c.mDate.Year=dDate.Year).Select(Function(c) c.Token).DefaultIfEmpty().Max() + 1
        MyData.Add(New TokenData(iToken, dDate))
    Loop

End Sub

Result:

Token     mDate
555       2014-10-15 00:00:00
556       2014-10-19 00:00:00
557       2014-10-23 00:00:00
558       2014-10-27 00:00:00
559       2014-10-31 00:00:00
1         2014-11-04 00:00:00
2         2014-11-08 00:00:00
3         2014-11-12 00:00:00
4         2014-11-16 00:00:00
5         2014-11-20 00:00:00
6         2014-11-24 00:00:00
7         2014-11-28 00:00:00
1         2014-12-02 00:00:00
2         2014-12-06 00:00:00