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 */
public DateTime get_IfModifiedSince ()

/** @property */
public void set_IfModifiedSince (DateTime value)

public function get IfModifiedSince () : DateTime

public function set IfModifiedSince (value : DateTime)

Property Value

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 today As DateTime = DateTime.Now
    If DateTime.Compare(today, myHttpWebRequest.IfModifiedSince) = 0 Then
        ' Assign the response object of 'HttpWebRequest' to a 'HttpWebResponse' variable.
        Dim myHttpWebResponse As HttpWebResponse = CType(myHttpWebRequest.GetResponse(), HttpWebResponse)
        Console.WriteLine("Response headers " + 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.................")
        Console.WriteLine((ControlChars.Cr + "The page has been modified since " + today))
    End If

// 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)
// Create a new 'DateTime' object.
DateTime today = DateTime.get_Now();

if (DateTime.Compare(today, 
    myHttpWebRequest.get_IfModifiedSince()) == 0) {
    // Assign the response object of 'HttpWebRequest' to a 
    //    'HttpWebResponse' variable.
    HttpWebResponse myHttpWebResponse = (HttpWebResponse)
    Console.WriteLine("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("\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"
        + ".................");
else {
    Console.WriteLine("\nThe page has been modified since "
        + today);

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

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

.NET Framework

Supported in: 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0, 1.0