Most likely, it happens bacause it is impossible to "Create a Web Part that interacts with a Web service or a Windows Communication Foundation (WCF) service." in sandboxed solutions.

As a workaround you can create full-trust proxy.

Some links which could be usefull:

• Walkthrough: Creating and Calling a Full-Trust Proxy Operation
• SharePoint 2010 Sandboxed soulutions: Full Trust Proxies

Hope it helps. If so, please press upvote and mark as answer :-).

Regards, Michael

Most likely, it happens bacause it is impossible to "Create a Web Part that interacts with a Web service or a Windows Communication Foundation (WCF) service." [in sandboxed solutions][1].

As a workaround you can create [full-trust proxy][2].

Some links which could be usefull:

• [Walkthrough: Creating and Calling a Full-Trust Proxy Operation][3]
• [SharePoint 2010 Sandboxed soulutions: Full Trust Proxies][4] [1]: http://msdn.microsoft.com/en-us/library/ff798382.aspx [2]: http://msdn.microsoft.com/ru-ru/library/gg622616%28v=office.14%29.aspx [3]: http://msdn.microsoft.com/ru-ru/library/gg622617%28v=office.14%29.aspx [4]: http://blah.winsmarts.com/2009-12-SharePoint_2010_Sandboxed_Solutions__Full_Trust__Proxies.aspx