Question: Trying to set the 'If-Modified-Since' header in wp7:

```
request.Headers[HttpRequestHeader.IfModifiedSince] = dateString;

```

Gives the error:

*This header must be modified with the appropriate property.*

Which means that the property .IsModifiedSince should be used on the request class, as described in MSDN: <http://msdn.microsoft.com/en-us/library/system.net.httpwebrequest.ifmodifiedsince.aspx>

But this property does not exist in Silverlight i.e. WP7.

**Has anyone been able to set this header for a http request on Windows Phone 7?**

Shawn Wildermuth posted this problem back in September 2010, with no solution:
<http://social.msdn.microsoft.com/Forums/en/windowsphone7series/thread/8aec7481-2cf3-4047-b0d4-05d4313a9e4c>

Thank you!

Answer: 
This can only be set on the `HTTPWebRequest` object so casting the `WebRequest` should allow you to set the property eg: 

```
((HttpWebRequest)request).IfModifiedSince = modifiedDate;

```

It takes a `DateTime` object so you may need to parse the string first.
The short answer is: It cannot be done, not supported.

Solution would be, as Matt Lacey states, to create a proxy class to handle this.

That proxy would set

```
request.AllowStreamReadBuffering = false;

```

and then parse the response until the header ends or the header value has been found.

Note! This workaround limits the data downloaded to the phone, but not the work needed by the server to process the request.