Work with If-Modified-Since and Last-Modified in ASP.Net. 

• Response.ContentType is "text/html" by default. So if you want to write .gif images by ASPX, you have to set right Content-Type http header according to the file type (image/gif).
• There is a good idea to handle at least Last-Modified and If-Modified-Since headers to reduce traffic and increase performance. Otherways, the file is sent each time.

  Function SendFileOverHTTP(ByVal FileName) As String
    'Get If-Modified-Since header from request.
    Dim sIfModifiedSince As String = Request.Headers("If-Modified-Since")

    'Convert the header To date.
    Dim IfModifiedSince As Date = DateFromHTTP(sIfModifiedSince)

    'Get a time when the file was last modified.
    Dim LastModified As Date = FileDateTime(FileName).ToUniversalTime

    'round the date To seconds.
    LastModified = New Date(LastModified.Year, LastModified.Month, LastModified.Day, _
      LastModified.Hour, LastModified.Minute, LastModified.Second)

    'Check If the last modified time of the file is greater than If-Modified-Since
    If LastModified > IfModifiedSince Then '200 OK - file was modified.
      Response.StatusCode = "200"
      Response.StatusDescription = "OK"

      'Set Last-Modified header
      Response.AddHeader("Last-Modified", DateToHTTPDate(LastModified))

      Dim ContentType As String = GetContentType(FileName)
      'Set Content-type http header (from registry)
      Response.ContentType = ContentType
      Response.WriteFile(FileName)
      Return "Send: " & FileLen(FileName) & "," & ContentType & "," & FileName
    Else '304 - file is Not modified.
      Response.StatusCode = "304"
      Response.StatusDescription = "Not Modified"
      Return "304: " & LastModified & "," & IfModifiedSince & "," & FileName
    End If
  End Function

  'returns a content-type of the file by file extensions
  Public Function GetContentType(ByRef FileName As String) As String
    'get the file ext
    Dim Ext As String = System.IO.Path.GetExtension(FileName)

    'open a the file type registry key 
    Dim Reg As Microsoft.Win32.RegistryKey = _
      Microsoft.Win32.Registry.ClassesRoot.OpenSubKey(Ext)

    'read Content Type value.
    Return Reg.GetValue("Content Type")
  End Function


  Public Function GetFileLastModified(ByVal FileName) As String
    Return DateToHTTPDate(FileDateTime(FileName))
  End Function



  'Converts date (19991022 11:08:38)
  'to http form (Fri, 22 Oct 1999 12:08:38 GMT)
  Function DateToHTTPDate(ByVal OleDATE As Date) As String
    On Error Resume Next
    OleDATE = OleDATE.ToUniversalTime
    Return engWeekDayName(OleDATE) & _
     ", " & Right("0" & Day(OleDATE), 2) & " " & engMonthName(OleDATE) & _
     " " & Year(OleDATE) & " " & Right("0" & Hour(OleDATE), 2) & _
     ":" & Right("0" & Minute(OleDATE), 2) & ":" & Right("0" & Second(OleDATE), 2) & " GMT"
  End Function

  Function engWeekDayName(ByVal dt As Date) As String
    Dim Out As String
    Select Case WeekDay(dt, 1)
      Case 1 : Out = "Sun"
      Case 2 : Out = "Mon"
      Case 3 : Out = "Tue"
      Case 4 : Out = "Wed"
      Case 5 : Out = "Thu"
      Case 6 : Out = "Fri"
      Case 7 : Out = "Sat"
    End Select
    Return Out
  End Function

  Function engMonthName(ByVal dt As Date) As String
    Dim Out As String
    Select Case Month(dt)
      Case 1 : Out = "Jan"
      Case 2 : Out = "Feb"
      Case 3 : Out = "Mar"
      Case 4 : Out = "Apr"
      Case 5 : Out = "May"
      Case 6 : Out = "Jun"
      Case 7 : Out = "Jul"
      Case 8 : Out = "Aug"
      Case 9 : Out = "Sep"
      Case 10 : Out = "Oct"
      Case 11 : Out = "Nov"
      Case 12 : Out = "Dec"
    End Select
    Return Out
  End Function

  Public Function DateFromHTTP(ByVal HTTPDate As String) As Date
    Dim Swd As String, d As String, Sm As String, y As String, h As String
    Dim m As String, s As String, g As String, Out As Date
    HTTPDate = LCase$(HTTPDate)

    If Mid$(HTTPDate, 27, 3) = "gmt" Then
      Swd = Left$(HTTPDate, 3)
      d = Mid$(HTTPDate, 6, 2)
      Sm = Mid$(HTTPDate, 9, 3)
      y = Mid$(HTTPDate, 13, 4)
      h = Mid$(HTTPDate, 18, 2)
      m = Mid$(HTTPDate, 21, 2)
      s = Mid$(HTTPDate, 24, 2)
      Out = New Date(y, mFromSm(Sm), d, h, m, s)
      Out = Out.ToLocalTime
    End If

    Return Out
  End Function

  Function wdFromSwd(ByVal Swd As String) As Integer
    Dim Out As Integer
    Select Case LCase$(Swd)
      Case "sun" : Out = 1
      Case "mon" : Out = 2
      Case "tue" : Out = 3
      Case "wed" : Out = 4
      Case "thu" : Out = 5
      Case "fri" : Out = 6
      Case "sat" : Out = 7
    End Select
    Return Out
  End Function

  Function mFromSm(ByVal Sm As String) As Integer
    Dim Out As Integer
    Select Case LCase$(Sm)
      Case "jan" : Out = 1 : Case "feb" : Out = 2
      Case "mar" : Out = 3 : Case "apr" : Out = 4
      Case "may" : Out = 5 : Case "jun" : Out = 6
      Case "jul" : Out = 7 : Case "aug" : Out = 8
      Case "sep" : Out = 9 : Case "oct" : Out = 10
      Case "nov" : Out = 11 : Case "dec" : Out = 12
    End Select
    Return Out
  End Function

<A
 Href="http://www.motobit.com/tips/detpg_net-last-modified/"
 Title="Short support vb.Net functions to convert
  date from/to http string date
  and sample code to send
  files with 304 http code"
>Work with If-Modified-Since and Last-Modified in ASP.Net.</A>