Develop Reference

100s of errors that all go away when I view them?

I have hundreds of errors saying type or namespace doesn't exist (some are types and some are namespaces). When I click to view the error it initially opens the file and shows the squiggly line under code in question, however within a second or two the squiggly line disappears and so does the error.
After I finish opening up all the files with "errors", all the errors disappear and it compiles fine. However, if I close any the files then the error reappears for that file.
This happened after I tried to merge branches. Not really sure how to proceed to find the root of the issue, or whether there even is a real issue... maybe this is just some bug in Visual Studio and I need to do something to clear some kind of cache to fix the problem?
Please advise.
Update:
I have tried all of tymtam's suggestions but to no avail.
However, after deleting the .vs folder, now the errors don't even go away when I open the files so I can no longer build by doing that either.
I am thinking the error may have something to do with dll versions or something like that but I have tried to rebuild and replace all dlls with latest ones and that hasn't helped either.

The problem for me was an outdated version of the .net framework. The new assemblies I was referencing were 4.8 based but my projects were targetting the 4.6.1 framework. Updating the project files to target 4.8 fixed it.

IntelliSense, which is likely the source of these errors, sometimes takes time to adjust to changes caused by switching branches (or more generally code changes that happen suddenly).
Here are the steps that should help, in order:
(Re)building the solution
Restarting Visual Studio
Closing Visual Studio and deleting .vs folder in the root of the solution folder.
Finally you can see if the errors are 'real' by changing what the Error List window shows to Build Only. I would recommend to have it as 'Build + IntelliSense' normally, and using Build Only only on special occasions.
Focus on the 1st error
More generally, when dealing with a large number of errors it's not a bad idea to look at the first one only, solve it, and build the code. A single syntax error can result in many errors and trying to solve the 2nd and subsequent errors would be futile.

In addition to the above, make sure you also look at the warnings (possibly information messages).
In my case, I had them turned off to focus on the errors only and missed the fact that my two projects were set to different versions of the .NET framework.

I did these two things to fix my problem:
I used WinMerge to copy files from the old branch over everything in the new branch for the application in question.
I updated the framework on the project to the latest version.
Originally, I did those two steps except I used TFS to merge the old branch to the new (instead of using WinMerge to copy/overwrite files) and accepted the old version for all conflicts.
This was the nuclear option as I lost any changes that were made in the new branch :(. I have a copy that I can use WinMerge to try to manually merge changes back in but it's definitely frustrating.

Did not find new element in the hierarchy for item

I am trying to create a simple page for a Xamarin app but am completely unable to proceed with the most basic of steps. I check out the branch from the project repo, and attempt to add a new file. When I do so (Right clicking on folder, Add-> New Item->Content Page) I get this error:
It does actually create the .xaml file but not the .xaml.cs file to go with it. I first tried to manually create the cs file and associate it but ran into increasingly mysterious errors. Today I created them outside of VS before booting it up and both files are now present but with the dotted icon instead of a regular one and the cs file is not nested beneath the xaml file.
If I try to open my projitems file inside VS to make the association I am given the error "Specified method is not supported". If I choose to 'Open With', no screen ever pops up allowing me to choose. After this I closed down VS and edited the projitems file in notepad again and linked the xaml and cs file. This looked as though it worked but when I add it to the main TabbedPage with a simple Children.Add(new MyReviews()); it is not recognized -> The type or namespace name 'MyReviews' could not be found.
These error happen if I make my own branch based on the master or try to add a new file to the master directly. No one else in the project is having these errors.
The only thing I really did differently from the rest is try to get the emulator working for android which apparently has a lot of compatibility issues and requires older versions of the xamarin plugins. I was not able to do this and ended up uninstalling and reinstalling everything twice since I didn't touch the installer software the first time and everything stuck.
Is anyone able to help me debug these issues or know why I am encountering so many at every turn?

I've been in the exact same situation. As it turned out, the rest of my team had added Universal Windows Platform (UWP) support to their VS2017 install, and I hadn't. Running the Visual Studio Installer and adding UWP fixed this issue completely.
Of course, you might not need or want to include UWP support in any of your Xamarin apps (I didn't). It seems most likely that it's just one or two support assemblies that are mssing or wrong without the UWP support, but I haven't found the time or energy to figure out any more details.

TFS on VS2010, merging code issues

Recently we have been having problems with TFS and our code base. We have a section of tests that use Webdriver, and a section that use Coded UI. Lately, we've been having problems merging our code after a new build has been pushed out. Some folders show up as a white outline of a folder, which we can right click and select "include in project". We also notice some files missing, and we can't "get latest" to grab them. We have to go some round about way to do it (shelve code, delete local files, get latest, merge code). Even when we do that we still have some issues.
But the main issue we are having is that our Coded UI maps are breaking. Instead of being the normal structure such as:
.uitest
----.cs
----.designer.cs
When we merge in the new code, the UI maps break out like:
.cs
----.designer.cs
.uitest
I opened the .csproj in notepad++ and noticed that both the .cs and .designer.cs files no longer have a dependancy of the .uitest file. Now the .cs file has no dependancy, and the .designer is dependant on the .cs file. We can fix it with relative ease, but it keeps breaking in every build and we have several maps we need to fix. What exactly could the problem be here? Also, referring to my frist problems of files not being included in the project, what could the issue be there?
I asked the same question on the MSDN forums, but the suggestions I have received I have already tried, like creating a new workspace since the other might be corrupt. Still didn't work.
Thanks in advance
Edit: Had a suggestion on the MSDN forums to run VS as administrator. That didn't work either. Not really sure what could be causing this issue. It doesn't seem to be causing problems for everybody because some new builds cause issues for some people but not others. Some more info would be I run on Windows 7 64bit and Visual Studio Ultimate 2010.

In the past I've had issues with TFS that sound similar, 2 of our developers seemed to periodically create problems when merging files into the data store, folders and files would show as not included while the project file would make it in (breaking everything after get latest). Eventually the problem went away but we never truly found the source. I believe it had something to do with our network - maybe a switch issue in combo with the way we mapped working folders to network drives pointing to a shared dev server. Also one of the workstations seemed to have a problem with windows explorer not seeing file updates on the dev server share... developer would have to hit \servername\C$\sharefolder then hit his mapped network drive again before updates would refresh. (And his check in's created the most problems) ... we ran VS2008, TFS2010
Just some ideas...

Visual Studio. Code changes don't do anything

Any changes I make to my code aren't doing anything. I've even tried putting lines in that I know will crash my program, and nothing. It just keep running the old version. It's even loading old versions of files I've edited and saved.
There a 3 projects in my solution. 2 are pure C#. 1 is a WinForms application.

It sometimes happens that some files "are being used by another process".
Close your solution and delete all "bin" and "obj" subfolders of all your projects that are included in the solution.
Then open your solution again, execute "Clean solution" and build it again.

Check this setting in Tools/Options, then under Projects and Solutions>Build and Run.
On Run, when build or deployment errors occur:
My personal preference is for this value to be 'Do not launch'. It will prevent Visual Studio from launching the "last successful build".

Try Ctrl+Shift+B. This will build your solution (not just a particular project). Other environments might build when you save but this needs to be explicit in Visual Studio.

Had issue with saving code changes on VS2017 (mac) Version 7.3 Preview (7.3 build 740)
Only way to get file changes to save was to do File > Save All
Doing File>Save would act like the file saved. But, when I opened the file again and no changes were saved.

Ok, you need to add more detail, but you may want to have a look at this answer:
How to enable/disable compile errors warning in Visual Studio

delete Published DLL From Bin
file names -
projectName.ddl
projectName.pdb
delete these two files and run project

I had the same problem (Visual studio 2015 is not noticing my modifications in C# XAML files).
Problem solved by deleting 2 'debug' subfolders. One in the bin folder and one in the obj folder.

I met the same problem. the code changes is not saved.(you can see from the edit it is in Yellow color. e.g. indicating changes). SAVE and BUILD will not let it save the changes. i have to wait sometime, then it could possibly save success..

Deleting the whole project and the corresponding folder from local and check out the solution again. This worked for me=> VS2017

I solved the problem like this:
1. I closed the program
2. I opened the folder that contains the file (.csproj) and opened it with Visual Studio.
Next, I chose the folder to run the command as in the image

Had the same problem. Turns out I had moved a project folder. Even when opening the solution from the moved folder and editing and saving this source code, VS 2019 was building and running a different instance of the program with the same name. Go to File-> Save yourproject.cs as..and manually walk back through the directory structure to find the instance VS is actually running. Hope this helps.

also check another option in Tools>Build and Run>Only build startup project & dependencies if you have more than 1 nested dependency, especially when using Prism or any other reflection-based frameworks that prevent "dependency detection". Also, make sure you're in the editor when using shortcuts like Ctrl+S (Save) or Ctrl+Shift+S (Save all)

Go to this location:
C:\Users\<your_system>\AppData\Local\Microsoft\VisualStudio\<studio_version>
Then remove all files and start Visual Studio.

this was the error , no changes made after build.

SVN keeps corrupting files with "<<<<<<< .mine", how to fix?

I've got a Visual Studio C# project which is under version control (SVN).
I've always commited and updated the project without any problems. But a couple of hours ago Visual Studio throws the following error when I try to launch/rebuild the project:
Files has invalid value "<<<<<<<
.mine". Illegal characters in path.
I don't know how to fix this problem. What should I do?

That happens when svn encounters a conflict: You changed a file, the file on the server was changed and it cannot (easily) be merged automatically. You need to decide what is the correct solution now.
Subversion just adds the diff into your source file (and creates files next to it, called OriginalName.mine (unchanged) and OriginalName.rsomething (unchanged, server version)).
Fix the conflict and tell subversion that this is resolved.

just delete the obj folder and it will worked fine.

Remove the code that shouldn't be in the file throwing the error and remove the the three files with extensions .mine, .<somerevision> and .<some_other_revision>. svn updated files that now contain 'conflicts' and you need to resolve these conflicts by hand. Usually this means you edited a file, someone else edited the same file and checked in changes and you didn't pay attention when checking out the changed file.

Delete every thing you have in obj folder .
Remove your obj folder from svn version control . Because on every build it get updated and when other developer commit changes to solution SVN is unable to marge obj folder files and raise error
Files has invalid value "<<<<<<< .mine". Illegal characters in path.

Please read the Basic Usage chapter in the subversion book. It has a section about Merging conflicts by hand which explains the conflict markers you're seeing.

Removing the debug folders worked for me (see comment-not answer above).
I got this after moving 12 folders from one section of svn to a new section. So if you get this after moving a project and the error does not point to an actual file, this is likely your issue.

If you have AnkhSVN or VisualSVN installed resolving this is most likely as easy as right clicking the file in the solution explorer and selecting edit conflict.
This will open the changed file in your merge editor. (See Tools->Options->Source Control->Subversion User tools for AnkhSVN). With a good merge tool like the free to use SourceGear DiffMerge or TortoiseMerge, resolving the conflict is just a few mouseclicks away.

I have had this happen on a large scale where the files get marked resolved but the conflict metadata is still there. I wrote a regular expression for visual studio to find these, for instances where it is not feasible to simply revert the changed files.
http://www.codetunnel.com/blog/post/90/ever-merge-with-svn-and-mess-up-when-resolving-conflicts-read-on

I closed the IDE, then deleted the obj folder and restarted the IDE and rebuilt my Code. This worked for me.

Inside your project :
odj folder -> Debug -> project name.csproj.Filelistabsolute.txt(snb.csproj.Filelistabsolute.txt)
Inside the text file
>>>>>>>.mine and >>>>>>>.r150 occurs
to remove the things the program works

1)Just save your local changes whatever u have edited in the file
2)revert the file
3)update it from SVN
4)Paste your local changes

If you are using TortoiseSVN you should have a right click option on the file called Edit Conflicts. This should bring up TortoiseMerge which is able to read those obnoxious notations stuck into the file (really, to break your code so you KNOW there's an issue and don't blindly check it in).
TortoiseMerge will read it properly and present you with a 3-way merge. This was what I was looking for. Although it is true that it does also create the separate .mine and .rxxx and .ryyy files, and there are various manual and command-line ways to deal with all this.

Have a simple solution. just delete all file from debug folder and rebuild the solution, an error display on the screen "There were build errors. Would you like to continue and run the last successful build? click on "Yes" button. Now stop the program and run normally.