System.LINQ name sapce not found

One day while working with LINQ,
After creation of the application, while debugging
It said that the System.LINQ name space is not found....
Then i tried to add assembly by Right Clicking On Project--->Add Reference
and searched for System.LINQ.dll
But, i didn't found any dll with that name.
Then I checked whether i had added System.Core.dll or not.
It was already added.
Hmm...
Confused...
I am using VS 2008..
I checked the properties of the Project By right clicking on the project in solution explorer--->Properties
And saw that the Target Frame Work Is Set To .NetFrameWork 3.0....
Hmm..
Finally I changed the project type to .NETFramework 3.5, it just restarted and worked fine...

Here I am talking about the VS 2008, if u r using VS2005 then u should install some addon, which i will discuss in another post..

Hope this helps for some one...

Thank U dudes..
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