I have a weird error showing up in my project when it is open in the VS2012 IDE. Everywhere where I make use of another referenced project it suddenly says "Type or Namespace name could not be found". And by "says", I mean it has the text underlined in red with the error when I hover over it. The intellisense doesn't work for that code. BUT (and here's the weird part), the errors do not show up in the error console and the project builds and runs fine.
I can even debug and step through the code and it works perfectly fine. So at runtime the project is referenced fine but at design time the IDE can't find it. This worked for the past 2 weeks, and only then suddenly went a little bonkers. It's really annoying because I am rubbish at coding without intellisense!
Has anybody ever seen anything like this or have any suggestions?
I have found this is a known problem with VS2012. Check to see how you are building, 64 bit or 32 bit. It won't work with 64 bit but it will with 32 bit. It will say things are missing and design will not work, however the program will run fine. I have heard the new VS update that hasn't been released yet will fix it.
I had this. I referenced assemblies whose "Target Framework" were set to ".Net Framework 4" in the "Application" secion of the projects properties. I changed this to ".Net Framework 4.5" not just in the referenced assembly's project but also the project I was building and it worked. Give this a try.
Manually delete all the references to the libraries of the other projects and re-add. Intellisense rebuilds whatever it needs at that point and doing this has helped me in the past.
Related to a couple other answers here, I had a "Data" project using a "Data" namespace. Built fine, but just started recently showing errors from intellisense (even though it continued building fine.) (VS 2015.)
I fixed this by changing my "Data" project and namespace to "MyCompany.Data".
Oddly, the problem didn't seem to show up until recently, but making the change did fix it. Presumably there was a conflict in namespace with another project or reference, which can build fine, but intellisense can't handle.
You can change this in the Application tab of Project properties. You can also open up an EDMX diagram, right click, choose model browser, choose the second collapsable item in the model browser tab, hit properties, and there you'll find the Namespace option for generated Entity Framework entities and contexts. (Similiarly, if you modify an Entity Framework Model's namespace, you might also need to change the related connection to match [in Web.Config for ASP.Net and MVC.])
Make sure that there is no class with the name same as Project default namespace.
make sure that you don't reference .net framework 4.5 projects in .net 4 projects!
Another source of this problem is a solution with multiple projects containing code for the same namespace. The compiler can handle this. Intellisense won’t.
Examine your .proj files in a text editor and make sure the paths to your references are correct.
This can also happen if you set the Build Action to "None" on the referenced file and forget about it.
I had the same problem, where it would build and run fine, it just would always show that error and I couldn't use intellisense with the class.
I actually used the automatic method of creating the class in a new file to resolve the issue, then just copied the code over from the real class. I deleted the old file, renamed the new file, and now it works.
来源：https://stackoverflow.com/questions/15395042/weird-c-sharp-type-or-namespace-name-could-not-be-found-builds-successfully