Using HttpUtility.UrlEncode() makes assembly's namespace "disappear"?

I have 2 C# projects (let's call them A and B). Project A is compiled to Class library and project B is compiled to Windows Application. Project B is added a reference to project A. Normally, they run well, without any problem.

I added a line that use HttpUtility.UrlEncode() to project A:

string encoded = HttpUtility.UrlEncode(queryString);

and I also added 'using System.Web' and a reference to System.Web.dll. While project A still be built successfully, when I build project B, it gave me erro开发者_运维技巧rs said that the namespace of project A does not exist. Removing the line that use HttpUtility.UrlEncode() solve the problem, but I really need that method.

Please help me! Thanks in advance.

Edit: I've found the problem: I set the "target framework" property of project B as ".net framework 4 client profile", which seems not support System.Web. Just change it to ".net framework 4" and it works fine.

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It may be more helpful to post the actual error message received with all its quirks for more assistance to be given.