cant add reference to asp.net C# web application

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cant add reference to asp.net c# web application[*]im working on visual studio 2010 and .net ver 2 [*]im adding a normal dll such as ajaxcontroltoolkit or sqlserversmo and ...when i add reference to my project get this prompt could not be added.Please make sure that the file is accessible and that it is a valid assembly or COM component.after many search i try the TLIMB.exe and STrongname but get this error my.dll is not a valid type of library