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.
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.