HttpWebRequest.IfModifiedSince Property

Gets or sets the value of the If-Modified-Since HTTP header.

Namespace:  System.Net
Assembly:  System (in System.dll)

Public Property IfModifiedSince As DateTime

Property Value

Type: System.DateTime
A DateTime that contains the contents of the If-Modified-Since HTTP header. The default value is the current date and time.

The IfModifiedSince property is a standard System.DateTime object and can contain a System.DateTimeKind field of DateTimeKind.Local, DateTimeKind.Utc, or DateTimeKind.Unspecified. Any kind of time can be set when using the IfModifiedSince property. If DateTimeKind.Unspecified is set or retrieved, the IfModifiedSince property is assumed to be DateTimeKind.Local (local time).

The classes in the System.Net namespace always write it out the IfModifiedSince property on the wire during transmission in standard form using GMT (Utc) format.

If the IfModifiedSince property is set to DateTime.MinValue, then the If-Modified-Since HTTP header is removed from the Headers property and the WebHeaderCollection.

If the IfModifiedSince property is DateTime.MinValue, this indicates that the If-Modified-Since HTTP header is not included in the Headers property and the WebHeaderCollection.


The value for this property is stored in WebHeaderCollection. If WebHeaderCollection is set, the property value is lost.

The following code example checks the IfModifiedSince property.

' Create a new 'Uri' object with the mentioned string. 
Dim myUri As New Uri("")
' Create a new 'HttpWebRequest' object with the above 'Uri' object. 
Dim myHttpWebRequest As HttpWebRequest = CType(WebRequest.Create(myUri), HttpWebRequest)
' Create a new 'DateTime' object. 
Dim targetDate As DateTime = DateTime.Now
myHttpWebRequest.IfModifiedSince = targetDate

  ' Assign the response object of 'HttpWebRequest' to a 'HttpWebResponse' variable. 
  Dim myHttpWebResponse As HttpWebResponse = CType(myHttpWebRequest.GetResponse(), HttpWebResponse)
  Console.WriteLine("Response headers for recently modified page" + ControlChars.Cr + "{0}" + ControlChars.Cr, myHttpWebResponse.Headers)
  Dim streamResponse As Stream = myHttpWebResponse.GetResponseStream()
  Dim streamRead As New StreamReader(streamResponse)
  Dim readBuff(256) As [Char]
  Dim count As Integer = streamRead.Read(readBuff, 0, 256)
  Console.WriteLine(ControlChars.Cr + "The contents of Html Page are :  " + ControlChars.Cr)
  While count > 0
    Dim outputData As New [String](readBuff, 0, count)
    count = streamRead.Read(readBuff, 0, 256)
  End While 

  ' Close the Stream object.
  ' Release the HttpWebResponse Resource.
  Console.WriteLine(ControlChars.Cr + "Press 'Enter' key to continue.................")
Catch e As WebException
  If e.Response IsNot Nothing 

    If CType(e.Response,HttpWebResponse).StatusCode = HttpStatusCode.NotModified
      Console.WriteLine((ControlChars.Cr + "The page has not been modified since " + targetDate))
      Console.WriteLine(ControlChars.Cr + "Unexpected status code = " + Ctype(e.Response,HttpWebResponse).StatusCode)
    End If 
    Console.WriteLine(ControlChars.Cr + "Unexpected Web Exception " + e.Message) 
  End If 
End Try  

.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1

.NET Framework Client Profile

Supported in: 4, 3.5 SP1
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