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: 
You can just use the string that `HttpRequestHeader.IfModifiedSince` represents:

```
request.Headers["If-Modified-Since"] = dateString; 

```

I've had to do this with a number of other headers which WP7 doesn't expose helper methods for setting.

**Update**  

Based on the remarks at <http://msdn.microsoft.com/en-us/library/8y7x3zz2(v=VS.95).aspx> it would appear that it is not possible to set this header in WP7.

As an alternative you could create your own proxy server to handle the caching for your app.
```
request.Headers.Add("If-Modified-Since", datestring);

```