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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
Dim instance As HttpWebRequest 
Dim value As DateTime 

value = instance.IfModifiedSince

instance.IfModifiedSince = value

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 assumed to be local time.


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  
// Create a new 'Uri' object with the mentioned string.
Uri* myUri = new Uri(S"");
// Create a new 'HttpWebRequest' object with the above 'Uri' object.
HttpWebRequest* myHttpWebRequest =
// Create a new 'DateTime' object.
DateTime today = DateTime::Now;
if (DateTime::Compare(today, myHttpWebRequest->IfModifiedSince)==0) {
   // Assign the response object of 'HttpWebRequest' to a 'HttpWebResponse' variable.
   HttpWebResponse* myHttpWebResponse =
   Console::WriteLine(S"Response headers \n {0}\n",
   Stream* streamResponse = myHttpWebResponse->GetResponseStream();
   StreamReader* streamRead = new StreamReader(streamResponse);
   Char readBuff[] = new Char[256];
   int count = streamRead->Read(readBuff, 0, 256);
   Console::WriteLine(S"\nThe contents of Html Page are :  \n");
   while (count > 0) {
      String* outputData = new String(readBuff, 0, count);
      count = streamRead->Read(readBuff, 0, 256);
   // Close the Stream object.
   // Release the HttpWebResponse Resource.
   Console::WriteLine(S"\nPress 'Enter' key to continue.................");
} else {
   Console::WriteLine(S"\nThe page has been modified since {0}", __box( today));

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0