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ASP.Net Core: Hundreds of Build Errors Suddenly

What might cause an ASP.Net Core web application to display hundreds of build errors suddenly without explanation? I started working on an app I had not worked on for awhile and when I clicked publish I got hundreds of build errors that never existed before.
Some were quite simple. Like this block of code used for a Blazor component (file extention .razor) that never had problems before. This block displays red squiggly lined underneath both instances of the string "crumbicon" I can think of no reason why there would be a problem with this:
#using PostAlmostAnything.Models
#crumbiconHome
#code {
string crumbicon = "<<";
}
The exact errors say that there is "ambiguity between filename.crumbicon and filename.crumbicon "the type [blazor file name] already contains a definition for crumbicon". This error has never been encountered before, but now exists on every .razor file that was designed this way.
The other error appear related to various assemblies or variables on pages. They typically say that something has already been defined someplace else, but if the code were the problem then it would have been an error long ago and not suddenly.
When I try to update Nuget packages they say the updates are not compatible with version 3.1 of .Net Core.

Try to close Visual Studio and reopen. That always works for me to get the libraries back

Short version: jiggle it until it works.
This happens a lot. I think it's a conflict between an old assembly that didn't get cleared out of Intellisense and a new one or something like that. You can try any / some of:
clean and rebuild
close all open files and re-open them
just ignore the errors and run the app
delete the bin folder from your project
restart Visual Studio
go get yourself a coffee and wait for Visual Studio to figure it out
on its own

Visual Studio is never stable nowadays.
Try cleaning and rebuilding or cleaning obj and bin directories. this may help too

Of course, try the other suggestions mentioned in the other great answers here.
Another option is to comment out the offending lines that are giving you problems, then rebuild. Keep commenting/building until it works. Once it builds and starts again, uncomment sections one at a time until you are back up and running.
As an extra bonus you may find an errant "}" or missing tag that helped cause the issue in the first place.

How to fix "Edited in both" Conflict on Microsoft's git?

I have a project and recently one partner joined to the adventure. The thing is, he has created some files on his own branch while me, on the master branch, created some others and edited several others. Now while creating a Pull request there are two files with conflict:
someProj.csproj Edited in both
Web.config Edited in both
Next steps: Manually resolve these conflicts and push new changes to the source branch.
How can I fix that?
All I get from Google are Git's command lines to resolve this issue but it is over MS's visualstudio.com; I already tried to run the commands on my Visual Studio (2017) but it prompts "git" command is not valid.
Other thing I tried to do is manually change these files and now my VS gives me an error:
1>CSC : error CS2001: Source file 'Blahblah.cs' could not be found.
I don't know what to do since there are several new files on both branches.

I'm sure you've solved this by now, but I hit this question when researching the same error message so here is how I resolved it.
Your colleague needs to Merge into his local branch. Merge from origin/ (so I guess origin/master in your scenario).
When this merge happens VS2017 will highlight the conflicts and ask your colleague to resolve manually. Once they have done this, the merge can be committed up to master (your code).
As you probably have realised, working directly on master is a bad idea - you should have branched as well. But to resolve this your colleague needs to get the code from master, merge it locally, and push the changes.

Error allowDefinition='MachineToApplication' beyond application level in asp.net

recently while working on asp.net application, i got below error:
It is an error to use a section registered as allowDefinition='MachineToApplication' beyond application level. This error can be caused by a virtual directory not being configured as an application in IIS.
i am sure the previous day it was working absolutely fine, however i guess this error occurs usually because of multiple web.config files, but i checked in my project, whole project contains single web.config files, can anyone please tell me what could be the reason for this error.

As the error suggests, You need to set up the directory you've placed the website in as a web application within IIS.

Thanks Nawaaz, i got the resolution, actually that was because i was opening website (using file-->open-->website) and not the solution, so i opened the solution and cleaned the solution and build it, and worked properly, thanks a lot for taking time to revert.

Parser Error Message: Could not load type

We are working on a Web Application.We are using ASP.net with c# (3.5 framework). We are Continue getting this error on a specific page
Server Error in '/' Application.
Parser Error
Description: An error occurred during the parsing of a resource required to service this request. Please review the following specific parse error details and modify your source file appropriately.
Parser Error Message: Could not load type 'Page Name'.
Each time we build the solution & upload the file,it will work and then suddenly after 2 to 3 days same problem again.Please help me resolve the issue.Thanks in Advance.

We found 2 things,may be that will help in future who will face an issue.
a)The file in which we have changed it was not updated properly,so
that's why It was causing an error,so project .dll has updated but
that page was not updated properly.
b)That page Excluded from the project.
so Please keep these thing in your mind.Most of the cases files was not updated.
Thanks.

This happened to me after creating a new page. It turns out I was running under x86 CPU instead of Any CPU mode under the Configuration Manager. When testing in debug mode the application was running against the local bin, but the file was actually being placed in the bin/x86 directory until I reconfigured it.

It sounds to me like there is some other software on the machine (anti-virus, backup, indexing software, ???) that is modifying the page source.
You could try making the directory or file readonly after uploading it to see if that resolves the problem.

Check the Inherits property of your page directive. It may have a number of attributes, but I suspect it will contain at least the following:
<%# Page Language="C#" Inherits="Page Name" %>
Change the value of Inherits to the fully qualified name of the class outlined in the page's code behind file, i.e. something like: MyProject.MyFaultyPage. If you don't have a code behind file for that page, you can inherit System.Web.UI.Page.

Close Visual Studio and select the solution folder and uncheck Readonly that will solve your issue!

Unable to automatically step into the server. The debugger failed to stop in the server process

I receive the following error when attempting to Step Into a process using F11 in Visual Studio 2008 Team System:
"Unable to automatically step into the server. The debugger failed to stop in the server process."
I have searched pretty long and hard on the internet and the most relevant message I found was http://msdn.microsoft.com/en-us/library/65004e38%28v=VS.90%29.aspx. Unfortunately, this did not help. All signs are pointing that my ASP.NET setup is incorrect, but it's the IIS environment that ships with VS2008 Team System so I didn't make any figuration changes.
The answer to this question, which sounds like it might have fixed my solution is no longer an active link: Unable to automatically step into the server when debugging WCF. Does anybody know how I can jump into and fix this problem?

This took me 3 days and no less than 5 people looking at the problem to solve; although I'd like to report the exact problem I'm still left to speculate. However, I am no longer getting this error when I do the following:
I opened the .cs file of the base class my service derived from within my Testing solution. I set a break-point in the constructor of this base class. Because the base class wasn't within the solution of either my web service nor my test solution (it was simply referenced), VS2008 was unable to break into this .cs file.
The solution was rather moronic. I opened the base class.cs file within my Testing solution (the file only, not the solution). I set a break-point in the constructor class, and I was then able to step-into all other files from there. Once I was able to successfully set a break-point and stop the debugger in the base class within my actual Test solution, I was able to continue debugging as I would normally expect.
My only conclusion to the root cause of this problem is that VS2008 is retarded. I'm sorry it's not much of an answer, but this is how I was able to resolve the issue which sunk over 30 hours of time.

This could be because the debug not reaching to your break point there may be some error occurring before it.Try add break point in the base class constructor.

I know this is an old question, but I encountered a similar issue and the problem was that I was in Release mode rather than Debug mode. Changing to Debug mode solved the problem. I'm using VS 2010.

this problem can occur if you have another solution opened and running (another instance of visual studio).

I got this error once when I had the wrong project set as the startup project in the solution.

This same thing happened when I had the service open in a browser. I closed it out and it fixed the issue.
Guessing you have it open multiple times. (basically repeating what Avicena00 said)

I got this error when trying to import a file that was too big. I had to increase the maxAllowedContentLength value in my requestLimits area in my web.config to fix it.

As stated this is an old problem, but I ran into it running later versions of Visual Studio, and have not seen the solution that I found listed. This problem in my experience happened when the service was using a lower version of .Net (in my case 3.0 vs 4.5). Changing the .Net version int he project properties to 4.5 (which matched the project referencing the service), solved my problem.

This worked for me: Tool > Options > Debugging > Use Managed Compatibility Mode
Visual Studio 2018 Options

In my case the problem was that I had open the same project and its copy (in different directory) in two Visual Studio instances. Solution than in final was simple as closing all IIS instances of Web projects and closing all instances of Visual studio. After opening just one project at a time debugging started to work like a charm.

I get this error when I debugged one and another solution.
The answer is very simple
Click on the solution->Properties->Multiple startup projects
Then select projects and click f11.