HttpWebRequest.IfModifiedSince Property
Gets or sets the value of the If-Modified-Since HTTP header.
Assembly: System (in System.dll)
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.
Note
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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("http://www.contoso.com");
// 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
targetDate.AddDays(-7.0);
myHttpWebRequest.IfModifiedSince = targetDate;
try
{
// 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);
Console.Write(outputData);
count = streamRead.Read(readBuff, 0, 256);
}
// Close the Stream object.
streamResponse.Close();
streamRead.Close();
// Release the HttpWebResponse Resource.
myHttpWebResponse.Close();
Console.WriteLine("\nPress 'Enter' key to continue.................");
Console.Read();
}
catch(WebException e)
{
if (e.Response != null)
{
if ( ((HttpWebResponse)e.Response).StatusCode == HttpStatusCode.NotModified)
Console.WriteLine("\nThe page has not been modified since "+targetDate);
else
Console.WriteLine("\nUnexpected status code = " + ((HttpWebResponse)e.Response).StatusCode);
}
else
Console.WriteLine("\nUnexpected Web Exception " + e.Message);
}
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.
The IfModifiedSince Property should be set to a date, then the HttpWebRequest's GetResponse (or similar async) method should be called, so that the property is turned into the If-Modified-Since header, and the request is made.
Then the response is returned, or an exception is thrown, depending on whether the URI has been modified or not, respectively.
- 11/21/2010
- Alun Jones
- 7/22/2011
- Steven Baker - MSFT
