I'm trying to use the HTTP functions contained in the System.Web.dll assembly. However, whilst the dll seems to exist in the same directory as every other dll Visual Studio 2010 references in my project, it fails to link in and raises a warning - "The referenced component 'System.Web' could not be found". The dll, however, is definitely there within the same folder as all others referenced by the project and selecting it in 'browse for reference' mode allows me to add it - it then fails to fill in the 'Path' property. Am I doing something wrong? How can I make System.Web available in my project?
Any help is much appreciated!
Edit: I don't have any reference to System.Web in the .Net tab of the 'Add Reference' window.
If you target .NET 3.5 or 4 client profile, you won't see ASP.NET assemblies.
Besides, you should try to use HTTP classes in System.Net.
For people that cant find "System.Web" at .Net References, this might be the answer:
"you need to right-click the project -> properties -> then change the "Target framework" which will probably be ".NET Framework 4 Client Profile" to just ".NET Framework 4".
The answer was given by Tinister (thanks) as a reply to a previuos post. But I thought it was a little bit hidden at his original post. So i put it here to make it easer to be seen. Hope it helps.
System.Web should be referenced from the GAC. This assembly has nothing to do in your application folders.
1 right click on References in Solution Explorer and press add reference...
2 choose the browse tab and go to C:\Windows\assembly\GAC_32\System.Web\System.Web.dll and add the dll file to your references.
Are you trying to add a reference to the System.Web.dll file itself? System.Web is in the GAC on any Windows system, so when you add the reference in your "Add Reference" dialog, choose System.Web from the .NET tab.
JoeR
We've created a tool that will help you to achieve your goal. Muse VSReferences will allow you to add a Global Assembly Cache reference to the project from Add GAC Reference menu item.
Regards
来源：https://stackoverflow.com/questions/3741705/cannot-add-system-web-dll-reference