Develop Reference

Remove and Sort Usings not working in Visual Studio 2019

On a certain solution the "Remove and Sort Usings" option does not work.
It sorts the Usings correctly, but does not remove any unnecessary ones.
The IDE is flagging the unused ones correctly, but it can't seem to remove them.
No errors or messages are displayed anywhere, it sorts and behaves like it has completed successfully, but the unnecessary Usings remain and are still flagged by Intellisense as such.
I have verified that in a different solution it does work, so it is not my VS2019 install.
And I don't have any plugins installed (like Resharper) that could conflict.
Have also tried clean and rebuild in case it needed an error-free compilation to work with.
Any ideas what could be different/special about a certain solution that would prevent the remove functionality?

I have found another thing which causes this issue.
For me, I discovered it was only happening in 1 particular project in a solution.
After comparing the non-working csproj to a working one, I determined that the difference was the Warning Level.
The non-working one was set to 1.
Resetting this to the default 4 allows the Remove and Sort Usings function to work as expected.

I found the cause, and it's an annoying one!
There is a custom ruleset specified for static analysis of the projects, and that had both CS8019 and IDE0065 (Unnecessary using directive) unselected.
On selecting these the remove unused usings command worked again.
Thanks to other answers for suggestions.

I had the same problem and figured out what was wrong. In your Tools|Options|Text Editor|C#|Advanced look for theses settings:
HTH.

Any ideas what could be different/special about a certain solution
that would prevent the remove functionality?
This is quite a strange behavior. You could try these suggestions:
1) close VS Instance, delete .vs hidden folder, bin, obj folder.
2) clean vs component caches under C:\Users\xxx\AppData\Local\Microsoft\VisualStudio\16.0_xxxx(every this folder)\ComponentModelCache
3) use devenv /safemode to start VS IDE, open your project and then test again.
4) If your project is an old project which means that the project structure is a bit different from VS2019, please try to create a new vs2019 project and then migrate its content into the new project.
In addition, if these do not work, you can try this link's function to run Code Cleanup command with remove unused usings.

Visual studio - getting error "Metadata file 'XYZ' could not be found" after edit continue

I have stumbled into an issue that is really annoying.
When I debug my software, everything runs OK, but if I hit a breakpoint and edit the code, when I try to continue running I get an error:
Metadata file 'XYZ' could not be found
After looking around for a while, I found some a similar issues, but they were all regarding a build failure, which is not my case (this happens only after edit-continue).
What I have tried so far:
My code is compiling and running.
I cleaned the solution and restarted VS.
I made sure that the missing file's project is being build for the configuration I am running (in configuration manager).
I manually built the missing file's project.
Some extra info:
It does not matter what I change, still get the same error (the change is not related to the missing file).
This happens also when I pause and continue (not only breakpoints)
I am running the project using a custom configuration (configuration manager...). When I run it using the default Debug configuration the error does not occur.
Any ideas?

Eventually what solved the issue was:
Clean every project individually (Right click> Clean).
Rebuild every project individually (Right click> Rebuild).
Rebuild the startup project.
I guess for some reason, just cleaning the solution had a different effect than specifically cleaning every project individually.
Edit:
As per #maplemale comment, It seems that sometimes removing and re-adding each reference is also required.
Update 2019:
This question got a lot of traffic in the past, but it seems that since VS 2017 was released, it got much less attention.
So another suggestion would be - Update to a newer version of VS (>= 2017) and among other new features this issue will also be solved

As far as I can tell, this happens when the project dependencies gets messed up for whatever reason (whilst all the inter-project references are still intact). For many cases, it is NOT a code issue. And for those who have more than a few projects, going through them one at a time is NOT acceptable.
It's easy to reset project dependencies -
Select all projects and right click unload
Select all projects and right click reload
Rebuild solution
For those who have an issue in their code or some other issue that's causing this problem you'll obviously have to solve that issue first.

One possible reason could be you have upgraded the some of your projects (in the solution) to higher version e.g. from .NET 4.0 to 4.5 This happened in my case when I opened the solution in VS 2013 (originally created using VS 2010 and .NET 4.0). When I opened in VS 2013 my C++ project got updated to .NET 4.5 and I started to see the problem.

Generally this kind of error comes with human mistakes like if we change the namespace in some improper way, or changing folder names from explorer for current project etc, where compiler is unable to detect sometimes.
I came across the same error, to resolve which I tried few steps. Please follow all the steps :
Clean whole Solution
Right Click on every Project in your solution , Go to Properties and make your Default namespace as well as Default assembly name same as in your code (i.e namespace before class name)
Check Folder names for each project by going through the explorer(Where your project solution is). If not matching with your project names, make it similar (Like step 2) to them.
Remove all your references from each project relevant to another of same solution, and add it again.
In Your Project Solution folder, you will find Visual c# Project file. Right click and open with Notepad. In your initial lines you would find for lines for every project like below:
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "**Client**", "**Client** \ **Client**.csproj", "{4503E259-0E3B-414A-9074-F251684322A5}"
EndProject
Check again Foldernames (I have highlighted in BOLD) and make it similar to what you did in step 2.
Clean the whole solution again
Build The Solution (If doesn't work try building individual after cleaning again)

Make sure all your dependent projects are using the same .Net Framework version. I had the same issue caused by a dependent project using 4.5.1, while all others were using 4.5. Changing the project from 4.5.1 to 4.5 and rebuilding my solution fixed this issue for me.

XYZ couldn't be found because is not built yet....
Right click on the solution and check Project Dependencies, the Project Build Order should also change according to the dependencies that have been set.

The only thing that worked for me was to delete the Solution User Options (.suo) file. Note that, this is a hidden file.
To locate this file, close your Virsual studio and search for .suo from the file explorer within your project.
PS: a new .suo file will be created again when you rebuild your project and hopefully this newly created one wont give you issues.
I hope that helps someone get rid of this anoying error :).

I had this problem for days! I tried all the stuff above, but the problem kept coming back. When this message is shown it can have the meaning of "one or more projects in your solution did not compile cleanly" thus the metadata for the file was never written. But in my case, I didn't see any of the other compiler errors!!! I kept working at trying to compile each solution manually, and only after getting VS2012 to actually reveal some compiler errors I hadn't seen previously, this problem vanished.
I fooled around with build orders, no build orders, referencing debug dlls (which were manually compiled)... NOTHING seemed to work, until I found these errors which did not show up when compiling the entire solution!!!!
Sometimes, it seems, when compiling, that the compiler will exit on some errors... I've seen this in the past where after fixing issues, subsequent compiles show NEW errors. I don't know why it happens and it's somewhat rare for me to have these issues. However, when you do have them like this, it's a real pain in trying to find out what's going on. Good Luck!

Well, my answer is not just the summary of all the solutions, but it offers more than that.
Section (1):
In general solutions:
I had 4 errors of this kind (‘metadata file could not be found’) along with 1 error saying 'Source File Could Not Be Opened (‘Unspecified error ‘)'.
I tried to get rid of ‘metadata file could not be found’ error. For that, I read many posts, blogs etc and found these solutions may be effective (summarizing them over here):
Restart VS and try building again.
Go to 'Solution Explorer'. Right click on Solution. Go to Properties. Go to 'Configuration Manager'. Check if the checkboxes under 'Build' are checked or not. If any or all of them are unchecked, then check them and try building again.
If the above solution(s) do not work, then follow sequence mentioned in step 2 above, and even if all the checkboxes are checked, uncheck them, check again and try to build again.
Build Order and Project Dependencies:
Go to 'Solution Explorer'. Right click on Solution. Go to 'Project Dependencies...'. You will see 2 tabs: 'Dependencies' and 'Build Order'. This build order is the one in which solution builds. Check the project dependencies and the build order to verify if some project (say 'project1') which is dependent on other (say 'project2') is trying to build before that one (project2). This might be the cause for the error.
Check the path of the missing .dll:
Check the path of the missing .dll. If the path contains space or any other invalid path character, remove it and try building again.
If this is the cause, then adjust the build order.

Are you using a database code generation tool like SQLMETAL in your project?
If so, you may be facing a pluralized to unpluralized transition issue.
In my case, I have noted that some old pluralized (*) table names (upon which SQLMETAL adds, by default, an "s" letter at the end) table references to classes generated by SQLMETAL.
Since, I have recently disabled Pluralization of names, after regerating some database related classes, some of them lost their "s" prefix. Therefore, all references to affected table classes became invalid. For this reason, I have several compilation errors like the following:
'xxxx' does not contain a definition for 'TableNames' and no extension method 'TableNames' accepting a first argument of type 'yyyy' could be found (are you missing a using directive or an assembly reference?)
As you know, I takes only on error to prevent an assembly from compiling. And that is the missing assemply is linkable to dependent assemblies, causing the original "Metadata file 'XYZ' could not be found"
After fixing affected class tables references manually to their current names (unpluralized), I was finnaly able to get my project back to life!
(*) If option Visual Studio > Tools menu > Options > Database Tools > O/R Designer > Pluralization of names is enabled, some SQLMETALl code generator will add an "s" letter at the end of some generated table classes, although table has no "s" suffix on target database. For further information, please refer to http://msdn.microsoft.com/en-us/library/bb386987(v=vs.110).aspx
Hope it helps!

I had this error come up. I followed all of the solutions here but nothing worked. I was using Visual Studio 2013 Professional. I couldn't get the individual project rebuilds to work and I finally figured out there was a circular dependency in my references. Visual Studio does a pretty good job normally of warning you if you are adding a reference to something that references back, but for some reason it didn't in this instance. I added a reference to a project that referenced the project I was working on - and it accepted it. VS bug perhaps?

My 5 cents.
This problem started after a solution wide clean.
I managed to get the problem to go away by setting the Active Solution configuration in: Build -> Configuration manager to release. Then build and set it back to debug again. The build succeeded after that.

Close VS, locate and remove the 'packages' folder from outside of visual studio. Restart VS and build -> all dependencies are reinstalled

Visual Studio 2019 Community 16.3.10
I had similar issue with Release build. Debug build was compiling without any issues.
Turns out that the problem was caused by OneDrive. Most likely one could experience similar issues with any backed-up drive or cloud service.
I cleaned everything as per Avi Turner's great answer.
In addition, I manually deleted the \obj\Release -folder from my OneDrive folder and also logged to OneDrive with a browser and deleted the folder there also to prevent OneDrive from loading the cloud version back when compiling.
After that rebuilt and everything worked as should.

this happens because of the difference of names in the folder name and namespace name. If u create a namespace in a certain name , and later you rename it the namespace will have the old name itself. And the compilation will take the old path to find the .dll and .exe file . To avoid this open the .csproj file of each namespace with a text file , and find the old path in the file.
remove this, clean and rebuild the solution. This worked for me. I spent an entire day working on this problem.

I had this and managed to fix it using this SO answer:
Metadata file '.dll' could not be found
I had to uncheck all of the boxes, click Apply, reenable all of the checkboxes and then click apply again, but it fixed the problem.

I just ran into this issue and after an hour of screwing around realized I had added an aspx file to my product that had the same name as one of my Linq-To-Sql classes.
Class and Page where "Queue".
Changed the page to QueueMgr.aspx and everything built just fine.

For a new build, it could be that some dependencies aren't installed. For me it was Crystal Reports.

It happens when one project dll is failing and that is referenced by number of projects. So first fix it and then Build individuals.

I ve had this problem and it has started after importing our solution to TFS as a new project.I came across this topic and found a quick solution with some inspiration from your answers.
All i needed to do is to rebuild the project thats supposedly lost its metadata file and voila , problem solved.

There's also one another silly reason which you should check with patience... as it occurred to me after wasting 4hours searching for answers:
The story to me was that I accidentally changed a small line of code among thousands of c# class files and then trying to rebuild the solution. As you could imagine, I ended up with 40+ meta data file missing errors and with 1 compilation error among them -- which I didn't check carefully, purely thinking all errors were the same!
after 4 hours searching and then accidentally double checking my error list, I found that silly code error, fixed it, compiled, and then error disappeared.
Not a good answer to your problem, but do hope my case wasn't same to yours.

I had the same problem. In my case I had by mistake I had set all the projects apart from the project with the main method as console application.
To resolve I went to every project other than the one with main function and right click> properites > output type > class library

it was happened to me because I've a strange clash in the namespaces:
I had
AssemblyA
with namespace
AssemblyA.ParentNamespace
witch defines ClassA
and in the same assembly another namespace with name
AssemblyA.ParentNamespace.ChildNamespace
witch defines a different ClassA (but with the same name)
I had then in AssemblyA.ParentNamespace IInterfaceB witch had a method that in the beginning returns IEnumerable and a ClassB witch implements IInterfaceB
I had later modified the method in ClassB to return IEnumerable but I've forgot to update the IInterfaceB definition, so the method there was still returning IEnumerable
the fun fact was that the solution still complile if I did a rebuild all, but the tests witch refers AssemblyA didsn't work and returns the "Metadata file could not be found"error.
updating InterfaceB to correctly return IEnumerable as its implementor ClassB did solved the problem, unfortunately the error message was vague and also the fact that the compilation worked makes me suppose that maybe there is something to fix in the compiler

A coworker was running into this problem and the cause was eluding us. Eventually we realized that the project directory (and therefore the path to the NuGet packages) contained %20 (thanks, some Git gui tool which shall not be named) and the error messages showed that the compiler was looking for an very similar-looking path but one which had to %20, rather a space. Apparently something in the build system somewhere performs HTML-decoding on local filesystem paths.
Renamed the working copy directory and everything started working.

I had this issue too.
It started after I did a little folder tidying in my project.
I then tried to compile and got many duplicate class errors. (despite them not being duplicated. I think the linking was just out of wack)
Upon checking these, the errors would all disappear leaving only the "Metadata file ...debug\application.exe could not be found" error.
I solved this by looking in the build output window to find which classes were duplicated.
I would then right click the class name and "go to definition".
there will be two definitions to select from, open them both, the second definition will seem to open the same file again, however the second one will identify as the error source(red underline).
Delete all the code out of the file and save(This will not effect your actual file).
This should now compile correctly.

Ensure that there are no spaces in the path to your project...
I am using Windows 10 with Visual Studio Community 2019 and I was cloning a multi project solution as it was from a GIT repo. I was having this error with all other dependencies in the solution along with a E_POINTER error. Its path, inherited from GIT, had spaces like C:/repos/MY PROJECT NAME/ ...
I deleted it, cloned it again and make sure that its path contained no spaces like C:/repos/MY_PROJECT_NAME/ ...
That fixed my problem.

I had same issue too.
In my case, I recently add an internal class to somewhere in project. One of the dependencies in solution has same class name and both of them are added correctly to references.
I changed my last activity and rebuild, it works.
Be sure that your compiler messages are valid. In my case I catch reference error from there, not listed as an error in Error List.

Visual Studio "Rebuild all failed"

Why does Rebuild fail with no errors?
Since this morning, this error keeps showing up. I build the entire solution (25 C# managed projects) and a "Rebuild All failed" appears, but without any errors! (I have 13 warnings about COM not supporting Generics, but it's "normal" because one dll is exposed as COM.)

Not an answer per se - but you're better off looking at the output window and seeing what it says there.
Also, to help with that you might want to look at your MSBuild verbosity - as shown on this screenshot (last two options):
Beware - the highest level generates a MASSIVE amount of information.
Finally - running msbuild from the solution folder in a command prompt will really nail the issue - because error messages and warnings come up in red and yellow respectively.

I found my own solution and it is simple:
When this error occurs, save the project and close VS 2013. After that, re-open VS2013 and open the last project.
It works like a charm. But it is very annoying every time!
Many people reported this problem in VS2010, VS2012 and VS2013.

Could be a corrupt Solution User Options file.
Close the solution, delete its .suo (.v12.suo for VS2012+), reopen the solution, and Visual Studio will build a new one. You will lose the StartUp Project, breakpoints, bookmarks, which files are open, which projects/folders are expanded, etc. But that's all minor compared to the solution not building!

I had the same problem. I was trying to refrence a higher .net framework version(4.5.2) to lower .net framework version(4.5) which was causing build error. I made the version same in both projects and it worked.

Check the Output Window (View -> Output) as that will tell you what's going wrong. Sometimes a reference might be missing or there is an issue with the targeted version of .NET for one project in a solution.

Have you tried to clean the solution befor rebiuld it?

This is the list of checks & things I would do if I were you (try to build after each step):
Is error list activated? (Sometimes I forgot to activate and I can see only warnings & messages)
Check output window for error messages..
Clean solution.
Double check after clean that everything is deleted from debug folders.
Build it in release mode.
Build solution project to project until you isolate problematic project.
Remove COM and comment code to see if is this the source of problem.
Restart VS2010.
Restart windows.

Few moments ago I fix it with repair of .NET Framework installation (.NET Framework v4.0 Extended in my case).

I had the same issue in VS 2015. I tried the following with no success:
Close VS project and reopen
Close all open VS projects and reopen just the project that had the issue
Clean solution
Rebuild solution
Delete all files in bin\debug and bin\release
Lastly I tried Keith Robertson's answer, delete .suo (\Visual Studio 2015\Projects\[ProjectName]\.vs\[ProjectName]\v14\.suo). Although this didn't get me a good build, it did finally give me an error message stating that I had two entry points to my application. I went to application properties (Alt + Enter) and select a Startup object from the drop down.

This error seems a bit generic to me. I also went through this situation, but I managed to solve it differently than any of the ones mentioned here.
I have a project and several dependencies. And one of these dependencies has undergone a change.
When compiling the main project in debug mode, I verified that everything was ok.
However, switching to release mode and recompiling the problem occurred.Rebuild all failed and 0 Errors
By analyzing the debug output, I encountered an error:
Although the build dependencies are configured correctly. When compiling in release mode, the main project did not find the new method created in the secondary project.
So I had to recompile each secondary project one by one in release mode. After that, I recompiled the main project and everything worked.
Hope it helps someone!

I just had the same thing. For me, it helped to restart VS and run it as Administrator.

Select the appropriate target framework
- Right click on project
- Properties
- In application tab, Select the target framework

clean the solution
Try and build each project and see where the issue is.
Check each of the references (of each project) to make sure not have the yellow warning sign
Has the solution ever built?

I just had this happen to me, and realized that I left a '#error' line in my code and forgot about it. When I tried to build, the build failed but the #error line didn't show up in my errors.
Try searching all for '#error'

I fixed it on my new implementation of Visual Studio 2013 by going to the database project / Project Settings and noticing that the Target Platform was SQL Server 2014 instead of 2012 like it should be.

Once chance of getting this error is when we try re naming the service reference name, we give some other name in the service reference, but in the namespace some where it will be referring the old name, so if you delete and add a service reference then keep the same name, else we may face this error, but we can see the error in the Output window.

There are apparently many causes of this. I just found the cause of my issue: the .NET version of a new project I created was higher than the version of the top-level project. (4.5.2 vs 4.0)

I got a similar issue today, and fixed it with repair.
Start
Run…
Appwiz.cpl
(Find your installed Visual Studio version)
Right click
Change
Repair

In my case it was the wrong date and time of computer.

I was getting no feedback/messages/errors. Just that all projects failed to build.
I closed and tried again--I noticed an error saying "you are not authorized to access..."
I clicked on my account, re-entered my credentials, and rebuilt the solution.
Voila! I got what I am used to seeing when I build a solution -- plenty of errors in all their glory.
Hope this helps someone.

Here's yet another reason which may sound familiar to some. I had integrated some code into my solution that wrapped a DLL. The C# code file that came with it offered a nice managed API and handled the low-level LoadLibrary stuff to access the DLL. Both had the same base name, so I had SomeName.cs and SomeName.dll. I could just drop it into any project and it would work.
This wasn't so nice after a while as I started using it in different projects. I got copies of both the DLL and the wrapper code in multiple projects. So I figured it would be better to drop the wrapper code and the DLL into a new class library project and then reference that new project from other projects.
After I had done that, I started to get this issue. The build went well up until the very last stage and then failed without error. Output showed nothing but successes.
The problem was the name of the wrapping class library project. I used the same base name (SomeName) for this. By default the assembly name would be SomeName.dll and I already had one such file (the DLL to be wrapped), thus I had a conflict with output files.
After renaming the wrapping project and its output assembly to SomeNameWrapper, the problem went away.
This may not be your exact cause but it seems likely you have some name clash or deployment issue as well. And it is not surprising the compiler won't give you an error because there is no problem in the compilation phase, the trouble starts with deployment and apparently this does not come out in an obvious way.

I had the same problem the original poster was displaying with 0 errors and Rebuild all succeeded. The Output tab showed a message that a referenced dll was built with a higher version of the .NET Framework.
Changing the .NET framework to match resolved the issue I was having with 0 Errors and Rebuild All succeeded.

The solution:
Because Prerequisites not set for debug set only for release
01-Change solution configuration ( in main screen )
set (debug to release)
set solution platform to (Any CPU)
02-Set Prerequisites for debug ( If you want to continue in debug mode )
03-set target platform version for all Projects

Some of the files included in your solution are not in the correct directories, or you have changed the name of one or more directories in your application. In the solution explorer under Setup review the list of all files and remove those that are not properly listed in the SourcePath Property.

One of my dependency in View file caused this. Check your view files for any dependencies which is not injected yet.

Changes are not happening until a rebuild is performed

I have a large windows application.
Recently when I make any change in the source code and run the project normally with or without debug, the changes are not made.
I tried to rebuild it, and everything goes fine,
now every change I made to the source code I have to rebuild the project then run it so that the changes are taking effect.
also I can't debug until I rebuild the solution.
my project is attached to source safe control.
any help is appreciated.

You frequently see this happen when a project gets deselected from the "Current Build Configuration" Dialog in visual studio. Try checking there to see if your project is unchecked. I think you can get to that menu by clicking the dropdown that says Debug/Release, and clicking manage/edit.

This is an old question but I happened to face a similar challenge. Well, for the benefit of someone who may experience the same here is how my problem was solved. I had a multi-project solution where some project depended on the source code from the other projects. Just before giving up and continue with the rebuild thing, I decided to check on the Build Dependencies=>Project Dependencies(Right-Click on the project). I found that the problematic code was from a Class Library that was not part of the dependencies(Unchecked). When I Included the project in the dependencies my problem vanished into thin air.

If you are referencing a .dll whose code is in another project you will need to build the project before the changes are reflected even if both projects are in same solution. sadly there is no running away from it.

Go to your configuration Manager and make sure build checkbox is selected for your project.
other thing you can try is:
Go to tools > Options>> A window will pop up> Under Project and Solutions > select Build and Run
Make sure Before building is set to Save all changes
(Copied from answer by amit dayama at https://stackoverflow.com/a/32820701/3195477)

I have faced the same problem because I have done some unsafe modifications upon the project names and have used find/replace upon the solution. Which resulted in a hidden exception in the android manifest file and exactly at the package name.
I avoid all types of headaches, I have created a new blank project with the desired name and namespaces and then moved the functional files, resource files and then modified static files like manifest and so on.
Never use unsafe method for project data configuration & modification

Debug Target Is Missing?

When I try to build my solution, I get the following error:
Visual Studio cannot start debugging because the debug target 'c:\target' is missing. Please >build the project and retry, or set the OutputPath and AssemblyName properties appropriately >to point at the correct location for the target assembly.
My output path is set correctly to bin\Debug, but the exe is never created in that folder. Instead, all I get are the exe.config, vshost.exe, and vshost.exe.config files.
Any idea what's going on?

Make sure that output path of project is correct (Project > Properties > Build > Output path)
Go in menu to Build > Configuration Manager, and check if your main/entry project has checked Build. If not, check it.
Go to properties > Application , and select the output type of your project

I've had the same problem;
Here are solutions that didn't work for me:
Building/rebuilding entire solution
Making sure the output path was correct (MyProject > Properties > Build > Output > Output path)
Here's the solution that did work for me:
Rebuilding just the project

You could open the project file with a text editor and replace 'c:\target' by 'bin\Debug'
EDIT
There are other more helpful answers but I can't delete mine since it's the accepted one.
#CZFox Answer
#Yehuda Shapira Answer
steps for changing target path is
Go to Properties
Then go to Debug
Browse the Start external program and select the bin/Debug/.exe file

I have solve this type of problem follow this step
1.VS2010 right click on the solution explorer and select the Build.
Again press Ctrl+F5 or F5

You can try the following steps to resolve the problem.
Step 1:
Right click on the solution and select the property
Step 2:
In Configureation property select the Build option button

I just stumbled across this problem, but I'm using Visual Web Developer Express 2010 and couldn't find any wrong path either within IDE or in the project file. Rebuilding or deleting build folders didn't help.
But after examining the projects .user file, which I've never done before, I discovered that the bad path was in there. Very simple if one knows where to look.

I had the same problem and the real solution was embarrassingly easy:
If, in your project, Visual Studio has never successfully compiled the program (before finding the first bug), you will get this error. What I did was remove all offending code (in my case, leaving just a simple button1_Click with no code). Run/Compile the code one time; exit the running program, and this message goes away.
The Compiler builds various directories and files on a first successful compile and these are used by the debugger. I am now recommending with all new projects, define the form, compile, close, and then begin coding.

I've found that this can happen if all the files are deleted from the bin folder. ReBuild the app to force a full build: right click on the project in solution explorer and select ReBuild.

Please follow the below steps to overcome this problem:
If you are working with VS2010, change platform target to x64
Select .net framework as 3.5
If you are using any custom code for Sharepoint and like to debug or deploy the use the above....and my bad sake i dont abt the .net applications

I had this error too (in VS2010), and in my case (two projects in one solution, with one being for unit tests) the answer was to go into the solution's (not the project's) properties and set a single startup project. I would've thought it also necessary, in that project's settings, under Application, to specify the "Startup Object", but it's working for me with or without that.

Although this has already been answered, I found that my own solution was none of the above. Admittedly a rookie mistake, within my solution I had multiple projects, and thus when trying to run solution, the wrong project was set as the Startup Project.
So in my own case, not to say others, the solution was to right click the project and select Set as Startup Project

I tried everything mentioned in this thread but none worked.
Then, i tried the simplest thing and it worked.
Close visual studio and open it back up again.

This was a really annoying error!
I kept trying to start a debug instance but it just wouldn't make an exe! Though there were errors in my ConnectionString (while trying to make an SQL connection). There were two backslashes that were supposed to be a part of a path and the tutorial I was following told me to ignore it.
Well, turns out that was the error. A backslash marks the beginning of some escaping that you want to do, and the way to have a backslash displayed is \\ instead of \.
Got rid of that, and it worked for me.
EDIT: It would seem that you have to get rid of the tiny errors that you have made while writing your code to let it compile properly.

I have solved this problem by changing the Platform Target to "any CPU".

If the above explanation does not help you, then you could have error in the program. I have the same issue and I solved it as I cut the functions used in the same class and one of the functions were the cause of it.

I had a very very similar problem, but almost non of the solutions worked for me, finally when i reset the VS setting, it fixed...
To reset settings:
Tools Menu >
Import and Export Settings >
Select Reset all settings radio >
Next >
Next (You can backup your current settings in this step) >
Finish

Problem:
The problem was I had bad nuget source configuration, so the solution could not start properly despite the fact it was build correctly because it still saw old dll references.
Solution:
It was not enough to change nugget source url, I just had to remove entire nugget source and add it again with proper url.
Clean solution and rebuild it.

There are many issues that can lead to this problem, after losing 2 days to this issue I think I have the root cause of this issue and also the problem of the Form Designer throwing an error when switching to the Design view (also seems to effect the DataSet Designer):
A language syntax error that Intellisense doesn't catch.
Once I went through my code with a fine tooth comb I found a couple of really boneheaded mistakes that I kept overlooking, once those were resolved the solution compiled just fine and the output was in the correct place.

Here is the solution for this problem, no need to change anything for this problem.
You all know C# is case sensitive language and we have to write all methods and statements in correct case.
We all are just missing this thing and we just have to change method 'main() --> Main()'
This thing solved my problem please let me know if you still find any :-)