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HttpRequestHeaderCollection.IfModifiedSince | ifModifiedSince property

Applies to Windows and Windows Phone

Gets or sets the DateTime object that represents the value of an If-Modified-Since HTTP header on an HTTP request.


var ifModifiedSince = httpRequestHeaderCollection.ifModifiedSince;
httpRequestHeaderCollection.ifModifiedSince = ifModifiedSince;

Property value

Type: IReference<DateTime> [JavaScript/C++] | System.Nullable<DateTime> [.NET]

The DateTime object that represents the value of an If-Modified-Since HTTP header on an HTTP request. A null value means that the header is absent.


The IfModifiedSince property represents the value of an If-Modified-Since HTTP header on an HTTP request message. The If-Modified-Since header is the date and time the content was modified since.

Javascript and .NET languages do not use the DateTime object directly. In Javascript a DateTime is projected as a Date object, and in .NET it is projected as a System.DateTimeOffset. Each language transparently handles the conversion to the granularity and date ranges for the respective language.

In C++, a DateTime.UniversalTime value has the same granularity as a FILETIME and supports the date ranges required by Javascript and .NET.

For more detailed information, see the Windows.Foundation.DateTime structure.

The following sample code shows a method to set the If-Modified-Since header on an HttpRequestMessage object using the IfModifiedSince property on the HttpRequestHeaderCollection object.

    public void DemonstrateHeaderRequestIfModifiedSince() {
        var request = new HttpRequestMessage();

        // This is not typically set with a string.

        // Set the header with a strong type.
        var value = DateTimeOffset.Now.AddDays(-1); // Since yesterday.
        request.Headers.IfModifiedSince = value;

        // Get the strong type out
        System.Diagnostics.Debug.WriteLine("IfModifiedSince value in ticks: {0}", request.Headers.IfModifiedSince.Value.Ticks);

        // The ToString() is useful for diagnostics, too.
        System.Diagnostics.Debug.WriteLine("The IfModifiedSince ToString() results: {0}", request.Headers.IfModifiedSince.ToString());


Minimum supported client

Windows 8.1

Minimum supported server

Windows Server 2012 R2

Minimum supported phone

Windows Phone 8.1 [Windows Phone Silverlight 8.1 and Windows Runtime apps]


Windows::Web::Http::Headers [C++]



See also




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