Visual Studio: references in code not recognized?

I would suggest that you clear your Visual Studio temp files - it can often get confused about project structures and require a fresh start.

First, quit out of VS completely and restart it. If the problem is still there, find your VS cache folder and delete it, and then do a rebuild.

For help finding your cache folder, check this post.

Another solution

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If the other answers regarding clearing Visual Studio cache, .NET Cache, and ensuring references are valid don't work, try this one.

Based on the source, and trying this solution, I've had success. Deleting the visual studio solution cache folder

1. Close out of all instances of visual studio
2. Locate the .vs hidden folder within your solution.
3. Delete the entire hidden .vs folder.
4. Rebuild the solution

-- Source

When VS starts acting strangely wonky, and I can't find a logical fix, I kill Visual Studio, and manually do a 'clean' by deleting all of the bin/obj folders.

I have a batch file that does this for me quicker than I could do it manually. I place this in my solution directory, and all my projects are found in subdirectories.

rem "%%~dpa" says: Give me the (d)drive and (p)path of the (a, %%a) file.
rem However, our dir /a:d will result in only listing folders...
rem The final "%%a" is just appending the 'file' (diretory name) to the 
rem drive-and-path string
for /f %%a in ('dir /b /a:d *.*') do call :process "%%~dpa%%a"
pause
goto :eof

:process
echo Looking in %1
cd "%1"
if EXIST "%1\bin" (
   echo    Found 'bin' - it's gone now.
   rd /s /q "%1\bin"
)
if EXIST "%1\obj" (
   echo    Found 'obj' - it's gone now.
   rd /s /q "%1\obj"
)
cd ..