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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 DateTime IfModifiedSince { get; set; }

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.
    Uri myUri =new Uri("");			
    // Create a new 'HttpWebRequest' object with the above 'Uri' object.
    HttpWebRequest myHttpWebRequest= (HttpWebRequest)WebRequest.Create(myUri);

    // Create a new 'DateTime' object.
    DateTime targetDate = DateTime.Now;
    // Set a target date of a week ago
    myHttpWebRequest.IfModifiedSince = targetDate;

      // Assign the response object of 'HttpWebRequest' to a 'HttpWebResponse' variable.
      HttpWebResponse myHttpWebResponse=(HttpWebResponse)myHttpWebRequest.GetResponse();
      Console.WriteLine("Response headers for recently modified page\n{0}\n",myHttpWebResponse.Headers);
      Stream streamResponse=myHttpWebResponse.GetResponseStream();
      StreamReader streamRead = new StreamReader( streamResponse );
      Char[] readBuff = new Char[256];
      int count = streamRead.Read( readBuff, 0, 256 );
      Console.WriteLine("\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("\nPress 'Enter' key to continue.................");	
    catch(WebException e)
      if (e.Response != null)
        if ( ((HttpWebResponse)e.Response).StatusCode == HttpStatusCode.NotModified)
          Console.WriteLine("\nThe page has not been modified since "+targetDate);
          Console.WriteLine("\nUnexpected status code = " + ((HttpWebResponse)e.Response).StatusCode);  
        Console.WriteLine("\nUnexpected Web Exception " + e.Message); 

.NET Framework

Supported in: 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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