Develop Reference

Failed to load toolbox item. It will be removed from the toolbox

I have a WinForm application. I also have created my own User Control for it. Everything worked fine. Until today that I received the error message when I try to add it back to my program (I never removed it. Visual Studio did).
Failed to load toolbox item #UserControlName. It will be removed from
the toolbox.
I have it in my solution explorer but it vanishes from my toolbox after this error.
I also receive the warning that says:
'#UserControlName' is never assigned to, and will always have its
default value null.
And when I look at my designer code, that is true. But I haven't done anything with my designer code. My user control is defined there but not instantiated. How can I put it back?
Does it have anything to do with the fact that I changed my build platform from x32 to x64? If that is the case I will be surprised because the program was working fine after that change.

I was suffering from the same issue for one of our applications and could not find a solution. So I created the user and custom controls manually. After searching the web again I finally found why the designer is failing for this particular project. The answer is that project is 64 bit and Visual Studio is still does not have a 64 bit version, it is still 32 bit. So the designer could not load the controls in 64 bit applications or in class libraries. Read Microsoft's article about this.
There was an article on Visual Studio web site but it was removed. See the the answer in Visual Studio support forum.
https://social.msdn.microsoft.com/Forums/vstudio/en-US/77e10b58-43cc-4aab-919f-888f14f99571/x64-class-library-of-user-controls?forum=csharpgeneral

I had the same issue lately. As this (nor any other answer on this site and on the internet) actually helped me, I found out the way to fix it.
Just clean the files and rebuild the solution. As simple as that.

I was having same issue but I find the solution:
Click on "Project_Name" with left mouse and click "Build" then you can add the UserControl to your WinForm.

Change to AnyCPU and rebuild the project.
Visual Studio has problems with controls being 64bit.

You don't have to necessarily insert the user-control manually. I had the same situation an there was a reason for it.
In my case, it failed because the EXE assembly was compiled to 'mixed-mode' by using CLR support type /clr. After changing it to 'managed-only' by setting type /clr:pure, it worked.
For details, see my answer here at SO.

After many hours of head scratching with this problem with a large custom control with few changes from a previous working copy in source control I copied all the code into a new control name and file and it all worked.
This was to identify the problem line(s) of code since the debugger would not co-operate. The copied control (along with heaps of support code and modules) worked okay.
So these are the very simple steps that fixed my original code
In solution explorer rename the file (I just added an s at the end)
Rebuild
Test the control can now be added to a plain form
Rename the control back to its original name
Step 4 might be optional for you but if you have source control and it's a in a library you would want to do this.
This brought the control back onto all my forms that weren't working (so far as I can tell). It seems the problem is in VS that somehow remembers it doesn't like it?
Hope this helps and I hope I find this message next time I get this when I have forgotten the fix :)
P.S. Clean, rebuild and/or re-run the solution is the old stand by for this problem but this time it was just one custom control in a whole (DLL) class of them. Hope this helps.

For me, after adding the userControl, I rebuild the application first then refreshed the items on the tool box by going to projects => Refresh project toolbox items

Right click - Rebuild Solution fixed it for me!

Mostly caused by 32 bit / 64 bit architecture. Before Visual Studio 2022 the VS built in 32 bit, therefore cannot show 64 bit components.
Solution 1:
Create a new solution configuration in Configuration Manager with name: "Debug_FormDesign" or anything else.
Set the Configuration to the above name for all projects and set the platform to "AnyCPU".
Now open all projects Compile settings step by step, select the above configuration and change the Compile -> Target CPU option to AnyCPU.
Close all opened windows.
Clean solution.
Restart VS.
Select the "Debug_FormDesign" configuration in toolbar as active.
Rebuild solution.
Open the form designer -> Should work now.
You can easily switch back to the default "Debug" configuration after finish with the GUI.
Solution 2:
Use Visual Studio 2022.

At times like these, you'll be forced to tinker with the designer code. As long as you don't make drastic changes in the designer code, you shouldn't break anything. To play it safe, re-instantiate the object where the other controls are instantiated (near the top of the page). The designer should fill in the blanks for properties and such.
This should also return the control to the toolbox.

Check in your form.designer file if there is a left-behind property with that type.
It happened to me several times.
After I removed that line and rebuilt the project, everything started working.

In my case it helped to manually include the header files of the created user controls at the top of the Form where these controls are to be used.

Visual Studio compiles fine, but it still shows red lines

I am using Visual Studio 2012 and it was working all fine until I started observing some funny behavior. When I open my code it shows red Underlines which we usually see when there is an error in our code. Surprisingly, the code compiles all fine. I have made following observations that are not normal at all.
Red underlines in the code
While cleaning or building the solution no error.
Red underlines go away for some time after I build/clean the solution, but they come back eventually.
Because of this, my IntelliSense stopped working.
I can not right click on any component and go to its definition.
Any ideas?

Visual Studio 2017, Visual Studio 2019, Visual Studio 2022:
Closing Visual Studio and removing the .vs folder located in the solution directory worked for my C# projects.
This folder has a hidden attribute. You may need to change View settings to show hidden files in File Explorer.

Delete the contents of the temporary ASP.NET folder and then rebuild. It'll either be in your user folder (for IIS Express - \AppData\Local\Temp\Temporary ASP.NET Files) or the Windows directory (for IIS - C:\Windows\Microsoft.Net\Framework\vx.xx\Temporary ASP.NET Files)
Paths are off the top of my head and may not be correct

For me, this issue got fixed when I unloaded and reloaded the project again.

I had this issue and it was related to ReSharper.
Solution steps for me:
Disable ReSharper
VisualStudio\Tools\Options\ReSharper Ultimate\General\Suspend Now
Build Solution
(Ctrl + Shift + B)
Re-enable ReSharper
VisualStudio\Tools\Options\ReSharper Ultimate\General\Resume Now

Just had this problem while working with a solution created in Visual Studio 2012 but running in 2013. I closed Visual Studio, deleted all \bin and \obj directories and the problem was gone.

I had this problem after resolving some conflicts from Subversion (SVN). The solution has several projects in it and I resolved some conflicts in a few different projects. I did a menu Build → Clean Solution followed by a men Build → Rebuild Solution and everything was good again.

Do you have any plugins installed, like ReSharper? I had an issues with a bad plugin.
Try running Visual Studio in safe mode, to prevent plugins from running.
devenv /Safemode

If you are using ReSharper like me, you may delete ReSharper cache following by this link: Configure Caches
To specify the location for caches:
Open the Environment → General page of ReSharper options.
Use the Save solution caches in to select the location for cache files:
User local settings folder to store them in the following directory: %LOCALAPPDATA%\JetBrains\Transient
4.System TEMP folder to store them in the following directory: %TEMP%\ReSharperCache
Solution folder to store them in the root folder of the current solution
Custom folder to choose a custom location for ReSharper cache files.
Click Save to apply the modifications and let ReSharper choose where to save them, or save the modifications to a specific settings layer using the Save To drop-down list. For more information, see managing and sharing ReSharper settings.
Reopen your solution for the changes to take effect.

What works for me is deleting the IntelliSense indexfile.
The IntelliSense-file is in the same directory as you solution.
It's filename is SolutionName.sdf
Just delete this file, open you solution again, and IntelliSense will start rebuilding its indexfile. After that the problem will be gone.

In Visual Studio 2013 I solved this problem by deleting all of my obj and bin folders across all projects. The issue was probably due to solution configurations that I had deleted, but I hadn't been cleaned up properly, as doing a menu Build → Clean Solution doesn't remove the old outputs from the obj and bin folders.

This worked for me in Visual Studio Enterprise 2017:
Navigate to Tools > Options > Text Editor > JavaSCript/TypeScript > Linting > General
deselect "Enable ESLint"

I've run into this as well and was able to return Visual Studio to its normal state by doing the following -
Identify the project where the red lined code comes from
Remove the "red line" project from the references where it is being used (ProjectName\References - right click, add references, and uncheck the "red line" project)
Build (you should get errors now)
Readd the project reference that was just removed
Build again
The red lines should be removed and the project should build!

Steps that work
Open the solution and do a rebuild all
Close the solution
Open solution and do a clean
Close solution
Open solution and do a rebuild all
Close and then open the solution. It should be good. This works for me every time
Be careful deleting some of these settings files as you will lose saved debug settings, etc. And it may do more damage than you realize.

I have found recently it is easy to solve this by switching from Debug to Release in the dropdown to left of the Play Button. Then switching back from Release to Debug.

I had the same problem with lots of red lines in several *cpp source files. Though the code compiled perfectly. None of the other solutions worked for me.
Changing the order of #include lines of a *.cpp-file could make the red lines disappear - and reappear with the restored order.
Then I noticed a header file was included twice in a single *.cpp file. I removed the second one and - everything was fine.
Including a header file twice in the same *.cpp file seems to be no problem to the compiler but to the IntelliSense part.

Simply refresh the project/solution. It will get resolved.

I ran into this problem with the latest Visual Studio 2017.
Also the debug version of my program was running painfully slow.
I deleted the Solution file .sln and created a new one.

I had a similar problem when I was seeing lot of red squiggles in a couple of files. I tried all answers proposed previously, but nothing seemed to work.
The moment I started browsing through the classes, structures in other files for which complaining files had references, the problem disappeared. It seemed IntelliSense was not able to resolve dependencies on its own for some reason.

For me, I had at one time enabled fusion logging to debug some assembly dependency errors (fuslogvw from a CMD prompt). That was months ago and I had been experiencing much slower build times (5-7 minutes) since then.
I had also forgotten entirely that I had left them enabled. These logs were my bottleneck and disabling them has made iterating much faster.

In my case with Visual Studio 2017, I have many "red lines" shown below all symbols defined in a third-party library, but my project can actually build without problems. I have tried all suggested solutions (like delete the .VS folder, restart Visual Studio, etc.), but none of them working.
Finally, I fixed it and this is how: I open my application project's property page, then go to C/C++ → General → Additional Include Directories, which is the place I put all needed third-party library header paths.
I delete all the path (but save them somewhere), click "Ok" to confirm. Then I come back to the same setting, paste those paths back, click "Ok" to confirm, and then all those "red lines" disappear.

I have VS2019 with ReSharper, and ran into this issue.
What worked for me was:
Go to the ReSharper >> Options menu
Go to the General tab (should be the default)
Press the "Clear caches" button
Close all instances of Visual Studio (2019)
Restart Visual Studio

Using VS2022 without Resharper when this problem occurred, tried several things, this did help me in the end:
Close Visual Studio
Delete folder .vs in the Solution folder
Go to folder %USERPROFILE%\AppData\Roaming\Microsoft\VisualStudio\
Delete all folders whose names start with 17.
Reopen Visual Studio
More specific subfolders could exist that might be enough to delete, but I had no issues after deleting all of it. AFAIK these only contains user session data, temporary files and/or cache files that can be downloaded again or recreated as needed.

Found this solution:
Close Visual Studio (ensure devenv.exe is not present in the Task Manager).
Delete the %USERPROFILE%\AppData\Local\Microsoft\VisualStudio\xx\ComponentModelCache directory.
Restart Visual Studio.

I have had this problem for months and have finally fixed it. Closing Visual Studio and removing the .vs folder located in the solution directory did not work for me.
There was an assemblyIdentity tag in the web.config file which was referencing a library that was not in my references folder. I removed this tag, cleaned, closed and reopened, and the problem was fixed.
Check each of the assemblyIdentity tags in your web.config file and check them against the references folder in solution explorer
Remove any assemblyIdentity tags, including the parent dependentAssembly tag for any which aren't listed in your references folder.
Clean the solution
Close and reopen the solution

Deleting .vs folder did the trick for me.

for me this works:-
Open the Command Palette ctrl+⇧+p
Then type: reload Window.

Deleting all the folders which start with "asp.xxx" worked for me. You can reach these folders by:
(C:\Windows\Microsoft.Net\Framework\vx.xx\Temporary ASP.NET Files)

Hover over the word that has the red underline squibble. A mini dialog box will appear. Click on 'Quick fix' and then click on 'Disable error squibble'.

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 cannot start debugging because the debug target is missing [duplicate]

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 :-)

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 :-)