Using files in website.

Hello,
I am using files on web-site,
written in C#, VS 2008, framework 3.5.
 
I have the following exception :
request for the permission of type 'system.security.permissions.fileiopermission,mscorlib, version = 2.0.0.0, Culture = neutral,PublicTokenKey=b77a5c561934e089' failed

How can I overcome this exception (writing something in web.config, my code, assemblyInfo.cs) ?
 
Thanks :)
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