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Dependent DLL is not getting copied to the build output folder in Visual Studio

I have a visual studio solution.
I have many projects in the solution.
There is one main project which acts as the start up and uses other projects.
There is one project say "ProjectX". Its reference is added to main project.
The ProjectX references another .NET dll (say abc.dll) that isn't part of the solution.
Now this abc.dll should be copied to bin/debug folder of main project, but it isn't getting copied there. Why is it not getting copied, any known reasons ?

I found that if ProjectX referenced the abc.dll but didn't directly use any of the types DEFINED in abc.dll, then abc.dll would NOT be copied to the main output folder. (It would be copied to the ProjectX output folder, to make it extra-confusing.)
So, if you're not explicitly using any of the types from abc.dll anywhere in ProjectX, then put a dummy declaration somewhere in one of the files in ProjectX.
AbcDll.AnyClass dummy006; // this will be enough to cause the DLL to be copied
You don't need to do this for every class -- just once will be enough to make the DLL copy and everything work as expected.
Addendum: Note that this may work for debug mode, but NOT for release. See #nvirth's answer for details.

Just a sidenote to Overlord Zurg's answer.
I've added the dummy reference this way, and it worked in Debug mode:
public class DummyClass
{
private static void Dummy()
{
var dummy = typeof(AbcDll.AnyClass);
}
}
But in Release mode, the dependent dll still did not get copied.
This worked however:
public class DummyClass
{
private static void Dummy()
{
Action<Type> noop = _ => {};
var dummy = typeof(AbcDll.AnyClass);
noop(dummy);
}
}
This infomation actually costed me hours to figure out, so I thought I share it.

Yes, you'll need to set Copy Local to true. However, I'm pretty sure you'll also need to reference that assembly from the main project and set Copy Local to true as well - it doesn't just get copied from a dependent assembly.
You can get to the Copy Local property by clicking on the assembly under References and pressing F4.

It looks slick when you make it an assembly attribute
[AttributeUsage(AttributeTargets.Assembly)]
public class ForceAssemblyReference: Attribute
{
public ForceAssemblyReference(Type forcedType)
{
//not sure if these two lines are required since
//the type is passed to constructor as parameter,
//thus effectively being used
Action<Type> noop = _ => { };
noop(forcedType);
}
}
The usage will be:
[assembly: ForceAssemblyReference(typeof(AbcDll.AnyClass))]

Ran into this same issue. Background info: before building, I had added a new Project X to the solution. Project Y depended on Project X and Project A, B, C depended on Project Y.
Build errors were that Project A, B, C, Y, and X dlls could not be found.
Root cause was that newly created Project X targeted .NET 4.5 while the rest of the solution projects targeted .NET 4.5.1. Project X didn't build causing the rest of the Projects to not build either.
Make sure any newly added Projects target the same .NET version as the rest of the solution.

Not sure if this helps but for me, many times I reference a DLL (which automatically adds it to the bin folder of course). However that DLL might need additional DLLs (depending on what functions I'm using). I do NOT want to reference those in my Project because they just simply need to end up in the same folder as the DLL I am actually using.
I accomplish this in Visual Studio by "Adding an existing file". You should be able to add it anywhere except the Add_data folder. personally I just add it to the root.
Then change the properties of that file to ...
Build Action = None (having this set to something like Content actually copies the "root" version to the root, plus a copy in the Bin).
Copy to output folder = Copy if Newer (Basically puts it in the BIN folder only if it is missing, but doesn't do it after that)
When I publish.. my added DLL's only exists in the BIN folder and nowhere else in the Publish location (which is what I want).

You could also check to make sure the DLLs you're looking for aren't included in the GAC. I believe Visual Studio is being smart about not copying those files if it already exists in the GAC on the build machine.
I recently ran in this situation where I'd been testing an SSIS package that needed assemblies to exist in the GAC. I'd since forgotten that and was wondering why those DLLs weren't coming out during a build.
To check what's in the GAC (from a Visual Studio Developer Command Prompt):
gacutil -l
Or output to a file to make it easier to read:
gacutil -l > output.txt
notepad.exe output.txt
To remove an assembly:
gacutil -u MyProjectAssemblyName
I should also note, that once I removed the files from the GAC they were correctly output in the \bin directory after a build (Even for assemblies that were not directly referenced in the root project). This was on Visual Studio 2013 Update 5.

If you right Click the referenced assembly, you will see a property called Copy Local. If Copy Local is set to true, then the assembly should be included in the bin. However, there seams to be a problem with Visual studio, that sometimes it does not include the referenced dll in the bin folder... this is the workaround that worked for me:

In my case, it was the stupidest thing, caused by a default behavior of TFS/VS that I disagree with.
Since adding the dll as a reference to the main project did not work, I decided to add it as an "Existing Item", with Copy Local = Always. Even then the file was not there.
Turns out that, even though the file is present on the VS Solution and everything compiled both locally and on the server, VS/TFS did not add actually add the file to source control. It was not included on the "Pending Changes" at all. I had to manually go to the Source Control Explorer and explicitly click on the "Add items to folder" icon.
Stupid because I've been developing for 15 years in VS. I've run into this before, I just did not remember and somehow I missed it because everything still compiled because of the file being a regular reference, but the file that was added as Existing Item was not being copied because it did not exist on the source control server.
I hope this saves someone some time, since I lost 2 days of my life to this.

Issue:
Encountered with a similar issue for a NuGet package DLL (Newtonsoft.json.dll) where the build output doesn't include the referenced DLL. But the compilation goes thru fine.
Fix:
Go through your projects in a text editor and look for references with "Private" tags in them. Like True or False. “Private” is a synonym for “Copy Local.” Somewhere in the actions, MSBuild is taking to locate dependencies, it’s finding your dependency somewhere else and deciding not to copy it.
So, go through each .csproj/.vbproj file and remove the tags manually. Rebuild, and everything works in both Visual Studio and MSBuild. Once you’ve got that working, you can go back in and update the to where you think they need to be.
Reference:
https://www.paraesthesia.com/archive/2008/02/13/what-to-do-if-copy-local-works-in-vs-but.aspx/

Make sure that the dependent DLL used by you does not have target .NET Framework higher than the target .NET framework of your project's Application.
You can check this by selecting your project, then press ALT+ENTER, then select Application from left side and then select Target Framework of your project.
Suppose,
dependent DLL Target Framework = 4.0 and
Application DLL Target Framework = 3.5 then change this to 4.0
Thank you!

This is a slight tweak on nvirth's example
internal class DummyClass
{
private static void Dummy()
{
Noop(typeof(AbcDll.AnyClass));
}
private static void Noop(Type _) { }
}

I would do add it to Postbuild events to copy necessary libraries to the output directories. Something like XCopy pathtolibraries targetdirectory
You can find them on project properties -> Build Events.

TLDR; Visual Studio 2019 may simply need a restart.
I encountered this situation using projects based on Microsoft.NET.Sdk project.
<Project Sdk="Microsoft.NET.Sdk">
Specifically:
Project1: targets .netstandard2.1
references Microsoft.Extensions.Logging.Console via Nuget
Project2: targets .netstandard2.1
references Project1 via a Project reference
Project2Tests: targets .netcoreapp3.1
references Project2 via a Project reference
At test execution, I received the error messaging indicating that Microsoft.Extensions.Logging.Console could not be found, and it was indeed not in the output directory.
I decided to work around the issue by adding Microsoft.Extensions.Logging.Console to Project2, only to discover that Visual Studio's Nuget Manager did not list Microsoft.Extensions.Logging.Console as installed in Project1, despite it's presence in the Project1.csproj file.
A simple shut down and restart of Visual Studio resolved the problem without the need to add an extra reference. Perhaps this will save someone 45 minutes of lost productivity :-)

You may set both the main project and ProjectX's build output path to the same folder, then you can get all the dlls you need in that folder.

NO NEED FOR DUMMY IN CODE
Just :
add a Reference to the Executeable Project
or/and ensure that the reference in the executeable project has "Copy Local" set to TRUE (which was my "fault") is seems that this "overwrote" the setting in the base referenced library-project...

Other than the common ones above, I had a multi-project solution to publish. Apparently some files target different frameworks.
So my solution: Properties > Specific Version (False)

Add the DLL as an existing item to one of the projects and it should be sorted

VS2019 V16.6.3
For me the problem was somehow the main .proj file ended up with an entry like this for the project whose DLL wasn't getting copied to the parent project bin folder:
<ProjectReference Include="Project B.csproj">
<Project>{blah blah}</Project>
<Name>Project B</Name>
<Private>True</Private>
</ProjectReference>
I manually deleted the line <Private>True</Private> and the DLL was then copied to the main project bin folder on every build of the main project.
If you go to the reference of the problem project in the references folder of the main project, click it and view properties there is a "Copy Local" setting. The private tag equates to this setting, but for me for some reason changing copy local had no effect on the private tag in the .proj file.
Annoyingly I didn't change the copy local value for the reference, no idea how it got set that way and another day wasted tracking down a stupid problem with VS.
Thanks to all the other answers that helped zone me in on the cause.
HTH

I had a similar issue in which a DLL I had included in the project as content and 'Copy always' set, wasn't being copied to the bin folder. I solved this by adding a dependentAssembly reference to the DLL in the app.config.

Visual Studio Long wait before Starting to build

We have a moderately sized solution, with about 20 projects. In one of them I have my business entities. On compiling any project, visual studio waits and hangs about one and a half minutes on this BusinessEntities project.
I tried our solution in SharpDevelop and it compiles our complete solution, in 18 seconds. Similar timing with MSBuild.
My guess is that VS is trying to find out if the project needs a compile, but this process is about 15 times slower than actually performing the compile!!
I can't switch to the great sharpdevelop, it lacks some small, but essential requirements for our debugging scenarios.
Can I prevent VS from checking this project, And have it compile the projects without such a check, just like sharpdevelop?
I already know about unchecking projects in configuration management to prevent building some projects, but my developers will forget they need to compile this project after updating to latest sources and they face problems that seem strange to them.
Edit: Interesting results of an investigation: The delay happens to one of the projects only. In configuration manager I unchecked all projects, then compiled each of them individually. All projects compile in a few seconds!! The point is this: if that special project is built directly, compiles in a few seconds, if it is being built (or skipped, because it is up-to-date) as a result of building another project that depends on it, VS hangs for about a minute and half, and then decides to compile it (or skip it). My conclusion: Visual studio is checking to know if any files are changed, but for some reasons, for this special project it is extremely inefficient!!

I'd go to Tools -> Options -> Projects and Solutions -> Build and Run and then change the "MSBuild project build [output|build log] verbosity" to Diagnostic. At that level it will include timings which should help you track down the issue.

We had the same problem with an ASP.NET MVC web project running in Visual Studio 2013. We build the project and nothing happens for about a minute or so and then the output window shows that we are compiling.
Here's what fixed it... open the .csproj file in a text editor and set MvcBuildViews to false:
<MvcBuildViews>false</MvcBuildViews>
I had to use sysinternals process monitor to figure this out but it's clearly the cause for my situation. The site compiles in less than 5 seconds now and previously took over a minute. During that minute the Asp.net compilation process was putting files and directories into the Temporary Asp.net Files folder.
Warning: If you set this, you'll no longer precompile your views so you will lose the ability to see syntax errors in your views at build time.

There is the possibility that you are suffering from VS inspecting other freshly built assemblies for the benefit of the currently compiling project.
When an assembly is built, VS will inspect the references of the target assembly, which if they are feshly built or new versions, may include actually loading them in a .Net domain, which bears all the burdens of loading an assembly as though you were going to run it. The build can get progressively slower as it rebuilds more and more projects. When one assembly becomes newer the others do a lot more work. This is one possible explanation for why building by itself, versus already built, versus building clean, all have seemingly relevantly differing results. Its really tht the others changed and not about the one being compiled.
VS will 'mark down' the last 'internal' build number of the referenced assembly and look to see if the referenced assembly actually changed as it rolls through its build process. If its not differnt, a ton of work gets skipped. And yes, there are internal assembly build numbers that you dont control. This is probalby not in any way due to the actual c# compiler or its work or anything post-compile, but pre-compile steps necessary for the most general cases.
There are several reference oriented settings you can play with, and depending on your dev, test, or deployments needs, the functional differences may be irrelevant, however may profoundly impact how VS behaves and how long it takes during build.
Go to the references of one of the projects in Solution Explorer:
1) click on a reference
2) open the properties pane if its not (not the Property Pages or the Property Manager)
3) look at 'Copy Local', 'Embed Interop Types', 'Reference Output Assembly'; those may be very applicable and probably something good to know about regardless. I strongly suggest looking up what they do on MSDN. 'Reference Output Assembly' may or may not show in the list.
4) unload the project, and edit the .proj file in VS as text. look for the assembly reference in the XML and look for 'Private'. This means whether the assembly referenced is to be treated as though its going to be a private assembly from the referencing assemblies perspective, vs a shared one. Which is sort of a wordy way of saying, will that assembly be deployed as a unit with the other assemblies together. This is very important toward unburdening things. Background: http://msdn.microsoft.com/en-us/magazine/cc164080.aspx
So the basic idea here is that you want to configure all of these to be the least expensive, both during build and after deployment. If you are building them together, then for example you probably really don't need 'Copy Local'. Id hate to say more about how you should configure them without knowing more about your needs, but its a very fine thing to go read a few good paragraphs about each. This gets very tricky however, because you also influence whether VS will use the the stale old one when resolving before the referenced one is rebuilt. As a further example explaiing that its good to go read about these, Copy Local can use the local copy, even though its stale, so having this set can be double bad. Just remember the goal at the moment is to lower the burden of VS loading newly built assemblies jsut to compile the others.
Lastly, for now, I can easily say that hanging for only 1.5 mins is getting off very lucky. There are people with much much worse build times due to things like this ;)

Some troubleshooting idea's that have not been mentioned:
Clean solution?
Delete Obj and Bin folders plus the .suo file? FYI, neither Clean nor Rebuild will delete non-build files, eg files copied during a pre-build command.
Turn off VS scanning outside files. Options > tools > environment > document > detect when file is changed outside the environment?
Rollback SVN history to confirm when it started to occur? What changed? If the project file on day 1 takes the same time, recreate the project, add all the files and build.
Otherwise could you please run Process Monitor and let us know what Visual Studio is doing in the prep-build stage?

Sounds silly, but remove all breakpoints first. It sped up my pre-build checks massively - still don't know why though.

Based on the (limited) information provided one possibility is that there could be a pre-build action specified in the project file that is slow to compile.

Try disabling platform verification task as described here.

If your individual projects are compiling correctly then all you can do is change order of compilation by setting dependent projects explicitly in configuration.
Try to visualize your project dependency hierarchy and set dependent projects. For example, if your business entities project is referenced in each project, then in configuration of each project, this project must be selected as dependent.
When an explicit build order is not set, visual studio is analyzing projects to create an order of building project. Setting explicit dependent projects wiki make visual studio skip this step and use the order provided by you.

With such an extreme delay on a single project and no other avenue seeming to provide a reason I would attempt to build that specific project while running procmon from sysinternals and filter out all the success messages. You could probably also narrow it down to just the file system actions as well. From your description I might guess that the files are being locked by an external source like the event collection or workflow management process services.
Other things to consider would be whether or not this is a totally clean build machine or if it has been used to perhaps test the builds as well? If so, is there a chance that someone mapped an IIS application path to the project directly or registered it as a service location?
If you run procmon and see no obvious locks or conflicts I would create a totally new solution and project and copy the files over to see if that project also has the same delay. If it does have the same delay I would create a sample project of the same type but generic data (essentially empty) and see if that too is slow. If the new project with the same files builds fine you can then diff the directories to see what the variance is that causes the problem (perhaps a config or project setting).

For me, thoroughly disabling code analyzers helped per instructions here:
https://learn.microsoft.com/en-us/visualstudio/code-quality/disable-code-analysis?view=vs-2019#net-framework-projects.
I thought my code analyzers were already off, but adding the extra xml helped.
Thanks Kaleb's for the suggestion to set "MSBuild project build [output|build log] verbosity" to Diagnostic. The first message took more than 10 seconds to display:
Property reassignment: $(Features)=";flow-analysis;flow-analysis" (previous value: ";flow-analysis") at C:\myProjectDirectory\packages\Microsoft.NetFramework.Analyzers.2.9.3\build\Microsoft.NetFramework.Analyzers.props (32,5)
Which led me to the code analyzers.

Just in case someone else trips into this issue:
In my case the delay was being caused by an invalid path entry in "additional include directories" that referred to a non accessible UNC location.
Once this was corrected, the delay disappeared.

DLL size differs when built in separate solutions

I have two applications developed in VS2010 (C#, .NET 4.0) - each is a separate solution with 2 projects, one for the app-specific elements, plus the project for a shared library which builds a DLL.
My problem is when this DLL is built, under one of the builds the DLL is 16384 bytes (0x4000) larger than it should be. This size difference is the same for both Debug and Release builds. WHile the proper version is within ClearCase, I get exactly the same result with a copy on the C: drive, so it doesn't seem to be a config-spec or view-related issue.
I've trawled through the VS settings, completely removed the bin/obj folders to force a complete rebuild, used WinMerge to diff all the files/folders, all with no success.
Using dumpbin, I can see in the clr header a 0x3F20 size difference for the Resources Directory. I suspect the remaining 0xE0 is attributable to section alignment within the DLL.
At this point I'm stumped. Any suggestions on what could be causing this or what steps to investigate further???
EDIT:
Maybe I didn't describe the VS solution very well - The solution has 2 projects. The shared library is simply added to the solution via Add->Existing Project. Both projects are then built when selecting Build->Rebuild Solution.
Re. Hans' comment, it's a problem from a configuration management/confidence standpoint. If no source code/project files have changed, the only thing I'd expect to change in the DLL is any build time/version details - not a 15K delta like I have.

The problem was in the .suo (Solution User Options) file, which must have been corrupted (and unfortunately placed and maintained under version control by previous users). Thankfully it appears nothing significant is lost by the removal of this file.
A little more info on .suo files can be found in the SO question Why are there sln, suo and csproj files?
A quote from Debugger Settings Visual Studio Add In– Easily Copy Breakpoints between Machines sums it up well:
The .SUO file is the bane of your existence. Nearly all the problems you encounter with Visual Studio are the result of a corrupt .SUO file. Sadly, it seems all it takes to corrupt the .SUO file is your heart beating. In other words, whenever you have Visual Studio crash, refuse to debug, or behave strangely it's the .SUO file's fault. Whenever anyone asks me about strange Visual Studio behavior, my instantaneous response is "Delete the .SUO!" When it comes to Visual Studio hygiene, deleting the .SUO file is right up there with cancer screenings.

Project's referenced DLL's path changes "by magic" when DLL is deleted

Hereis the situation:
I have a folder that contains library DLLs, which are not built as part of my solution - lets say it is .\libs.
I add references to these DLLs. I then build. Everything is fine.
If I then delete the libs folder and rebuild my solution, the compilation still succeeds! Weird - I would have expected compilation errors since the library dlls are not there!
But looking at the reference properties in Visual studio, I see that the reference path has been changed from .\libs\foo.dll to myproject\bin\Debug\foo.dll. So it is picking up the referenced DLL from its old build output.
If I open myproject.csproj in a text editor, I see that the HintPath of the reference is still .\libs\foo.dll. If I re-create the libs folder, visual studio still uses myproject\bin\Debug\foo.dll (it does not revert to the actual DLL I want!)
Is this expected behaviour?
Is there a way to stop this behaviour because it is causing me problems - especially when I want to rebuild myproject with different versions of the libs: half the time I find that I am using a different version than what I wanted.

This is not magic. Your DLL's Copy Local Property is probably set to true, that's all.
Setting it to false will get you the desired behaviour.

the source file is different from when the module was built

This is driving me crazy.
I have a rather large project that I am trying to modify. I noticed earlier that when I typed DbCommand, visual studio did not do any syntax highlighting on it, and I am using using System.Data.Common.
Even though nothing was highlighted, the project seemed to be running fine in my browser. So I decided to run the debugger to see if things were really working as they should be.
Every time the class that didn't do the highlighting is called I get the "the source file is different from when the module was built" message.
I cleaned the solution and rebuilt it several times, deleted tmp files, followed all the directions here Getting "The source file is different from when the module was built.", restarted the web server and still it tells me the source files are different when they clearly are not.
I cannot test any of the code I have written today because of this.
How can the source be different than the binary when I just complied
it?
Is there any way to knock some sense into visual studio, or am
I just missing something?

I got this issue running a console app where the source that was different was the source that had the entry-point (static void Main). Deleting the bin and obj directories and doing a full rebuild seemed to correct this, but every time I made a code change, it would go out-of-date again.
The reason I found for this was:
I had checked "Only build startup projects and dependencies on Run" (Tools -> Options -> Projects and Solutions -> Build and Run)
In Configuration Manager, my start-up project didn't have "Build" checked
(For #2 -> accessible via the toolbar under the 'Debug/Release' drop down list.)

I was just having this same problem, my projects were all in the same solution so they were using Project to Project references, so as one changed the others should have been updated. However it was not the case, I tried to build, rebuild, close VS2010, pulled a new copy from our source control. None of this worked, what I finally ended up trying was right clicking on the project and rebuilding each project individually. That updated the .dlls and .pdb files so I could debug through.
The issue here is that your dll and or your pdb files are not in sync.

Follow these steps
Just delete the bin directory from the project where the DLL is generated.
Re-build the project.
Remove reference from the project that make reference to the DLL.
Include again the reference.
Enjoy.

In addition to these answers I had the same issue while replacing new DLLs with old ones because of the wrong path. If you are still getting this error you may not refer the wrong path for the DLLs. Go to IIS manager and click the website which uses your DLLs. On the right window click Advanced Settings and go to path of the Physical Path folder on File Explorer and be sure that you are using this folder to replace your DLLs.

Some things for you to check:
Have you double checked your project references?
Do you have a Visual Studio started web server still running? Check the system tray and look for a page with a cog icon (you may have more than one):
(source: msdn.com)
Right click and close/exit it. You may have more than one. Can you debug your changes now?
Are you running the debug version but have only built the release version (or vice versa)?
Did the compile actually succeed? I know I've clicked through the "there were errors, do you want to continue anyway?" message a couple of times without realising.

With web services, the problem can be caused by using the Visual Studio "View in Browser" command. This places the service's DLL and PDB files in the bin and obj folders. When stepping into the web service from a client, somehow Visual Studio uses the PDB in the bin (or obj) folder, but it uses the DLL in the project's output build folder. There are a couple workarounds:
Try deleting the DLL and PDB files in the web service bin and obj files.
Try clicking "View in Browser" in Visual Studio.
If you previously got the source file mismatch error, Visual Studio might have added the filename to a black list. Check your solution properties. Choose "Common Properties -> Debug Source Files" on the left side of the dialog box. If your web service source files appear in the field "Do not look for these source files", delete them.

Unload the project that has the file that is causing the error.
Reload the project.
Fixed

I just had this issue.
I tried all the above, but only this worked:
delete the .pdb file for the solution.
delete the offending .obj files (for the file being reported out of sync)
build the solution.
This fixed the issue for all builds moving forward for me.

In Visual Studio 2017 deleting the hidden .vs folder in the resolved this issue for me.

This is how I fixed the problem in Visual Studio 2010:
1) Change the 'Solutions Configurations' option from "Debug" to "Release"
2) Start debugging
3) Stop debugging and switch the 'Solutions Configurations' option back to "Debug"
This worked for me. Step 3 is optional - it was working fine when I changed it to "Release" but I wanted to change it back.

My solution:
I had included an existing project from a different solution in a new solution file.
I did not notice that when the existing project was rebuilt, it was putting the final output into the NEW solution's output directory. I had a linker path defined to look into the OLD solution's output directory.
Switching my project to search in the new solution's output directory fixed this issue for me.

I had this problem, and it turns out I was running my console application as a windows application. Switching the output type back to console fixed the issue.

I had the same problem. To fix it I used the "Release Mode" to debug in VS2013. Which is sufficient for me, because I'm working in a node js\c++ addon.

My problem was that I had two projects in my solution. The second one was a test project used to call the first one. I had picked the path to the references from the bin folder's release folder.
So whenever I made a change to the first project's code and rebuilt it, it would update the dlls in the debug folder but the calling project was pointing to the release folder, giving me the error, "the source file is different from when the module was built."
Once I deleted the reference to the main project's dll in the release folder and set it to the dll in the debug folder, the issue went away.

In my case, the #Eliott's answer doesn't work.
To solve this problem I had Exclude/Include From Project my deficient file, andalso Clean and Rebuild the solution.
After these actions, my file with my last modifications and the debugger are restored.
I hope this help.

solution:-
the problem is:-
if your some projects in a solution , refer to some other projects,
then sometimes the dll of some projects, will not update automatically, whenever you build the solution,
some projects will have previous build dlls, not latest dlls
you have to go manually and copy the dll of latest build project into referenced project

I was using Visual Studio 2013 and I had an existing project under source control.
I had downloaded a fresh copy from source control to a new directory.
After making changes to the fresh copy, when building I received the error in question.
My solution:
1) Open Documents\IISExpress\config\applicationhost.config
2) Update virtualDirectory node with directory to the fresh copy and save.

My problem was that I had a webservice in the project and I changed the build path.
Restoring the default build path solved my issue.

I had this same problem and I followed the majority of the guidance in the other answers posted here, nothing seemed to work for me.
I eventually opened IIS and recycled the application pool for my web application. I have IIS version 8.5.9600, I right-clicked my web application, then: Deploy > Recycle > Recycle application pool > OK.
That seems to have fixed it, breakpoints now being hit as expected. I think that doing this along with deleting the bin and obj folders helped my situation.
Good luck!

I know this is an old question but I just had the same problem and wanted to post here in case it helps someone else. I got a new computer and the IT dept merged my old computer with the new one. When I set up TFS, I mapped a different local path than what I was previously using, to an additional internal drive. The old path still existed from the merged data on my hard drive so I could still build and run. My IIS paths were also pointing to the old directory. Once I updated IIS to the correct path, I was able to debug just fine. I also deleted the old directory for good measure.

I also experienced that. I just open the obj folder on the project and then open the debug folder delete the .pdb file and that's all.

This error also happens if you try to make changes to a source file that is not part of the project.
I was debugging a method from a .dll of another one of my projects, where Visual Studio had quite helpfully loaded the source because the .dll had been built on the same machine and it knew the path to the source. Obviously, changing such a file isn't going to do anything unless you rebuild the referenced project.

Delete all breakpoints.
Rebuild.
Done

At Visual Studio 2015, using C++, what fixed for me the the source file is different from when the module was built problem was
restart Visual Studio.

Check if the location you pointed to using mex() in Matlab is correct (contains lib and obj files which are modified to the last date you compiled the library in Visual studio).
If this is not the case:
Make sure you are compiling Visual studio in a mode that saves .lib files :
properties -> Config properties -> General -> Config type -> static library
properties -> Config properties -> General -> Target extension=.lib (instead of exe)
Make sure the output and intermediate directories match the Matlab directory in
properties -> Config properties -> General -> Output directory
properties -> Config properties -> General -> Intermediate directory

I get this issue when debugging sometimes w/ Visual Studio but when the application is served by IIS. (we have to develop in this form for some complicated reasons that have to do with how the original developer setup this project.)
When I change the file and rebuild, that fixes it a lot of the time. I know that sounds silly, but I was just trying to debug some code to see why it's doing something weird when I haven't changed it in a while, and I tried a dozen things from this page, but it was fixed just by changing the file..

In my case, the problem was that the debugger exe path was pointing to a net5.0 bin folder. I am using net6.0, so I should've updated the exe path back when I updated the target framework. Works fine now.

Debug-> start without debugging.
This option worked for me. Hope this helps!