Metadata file '.dll' could not be found

I just had the same problem. Visual Studio isn't building the project that's being referenced.

Written Instructions:

Right click on the solution and click Properties. Click Configuration on the left. Make sure the check box under "Build" for the project it can't find is checked. If it is already checked, uncheck, hit apply and check the boxes again. (Optional) You had to do it for both Release and Debug modes on the solution properties.

Screen capture Instructions:

They say a picture is worth a thousand words. Click on the GIF to zoom in, and hopefully it will be easy to follow:

https://i.stack.imgur.com/F3Cs3.gif

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This can still happen in newer versions of Visual Studio (I just had it happen on Visual Studio 2013):

Another thing to try is to close Visual Studio and delete the .suo file that is next to the .sln file. (It will be re-generated the next time you Save all (or exit Visual Studio)).

I've had this problem when adding new projects to the solution on another machine and then pulling the revisions in, but the .suo file can be corrupted in other cases as well and lead to very strange Visual Studio behaviour, so deleting it is one of the things I always try.

Note that deleting the .suo file will reset the startup project(s) of the solution.

More on the .suo file is here.

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The suggested answer did not work for me. The error is a decoy for another problem.

I found out that I was targeting a slightly different version of .NET and this was flagged as a warning by the compiler, but it was causing building to fail. This should have been flagged as an error and not a warning.

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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 four errors of this kind (‘metadata file could not be found’) along with one 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 Visual Studio 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 two 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.

Section (2):

My particular case:

I tried all the steps above with various permutations and combinations with restarting Visual Studio a few times. But, it did not help me.

So, I decided to get rid of other error I was coming across ('Source File Could Not Be Opened (‘Unspecified error ‘)').

I came across a blog post: TFS Error–Source File Could Not Be Opened (‘Unspecified error ‘)

I tried the steps mentioned in that blog post, and I got rid of the error 'Source File Could Not Be Opened (‘Unspecified error ‘)' and surprisingly I got rid of other errors (‘metadata file could not be found’) as well.

Section (3):

Moral of the story:

Try all solutions as mentioned in section (1) above (and any other solutions) for getting rid of the error. If nothing works out, as per the blog mentioned in section (2) above, delete the entries of all source files which are no longer present in the source control and the file system from your .csproj file.

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In my case it was caused by a .NET Framework version mismatch.

One project was 3.5 and the other referencing project 4.6.1.

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Closing and reopening Visual Studio 2013 worked for me!

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Well, nothing in the previous answers worked for me, so it got me thinking about why am I clicking and hoping when as developers we should really try to understand what is going on here.

It seemed obvious to me that this incorrect meta data file reference must be held somewhere.

A quick search of the .csproj file showed the guilty lines. I had a section called that seemed to be hanging onto the old incorrect filepath.

<ItemGroup>
    <ProjectReference Include="..\..\..\MySiteOld\MySite.Entities\MySite.Entities.csproj">
        <Project>{5b0a347e-cd9a-4746-a3b6-99d6d010a6c2}</Project>
        <Name>Beeyp.Entities</Name>
    </ProjectReference>
...

So a simple fix really:

Backup your .csproj file. Find the incorrect paths in the .csproj file and rename appropriately.

Please make sure you backup your old .csproj before you fiddle.

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Visual Studio 2019 this worked for me:

Close Visual Studio Delete the hidden .vs folder Reopen Visual Studio and rebuild the solution.

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I also met this problem. Firstly you have to manually build you DLL project, by right-click, Build. Then it will work.

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In my case, I have my installed directory in mistaken ways.

If your solution path is something like "My Project%2c Very Popular%2c Unit Testing%2c Software and Hardware.zip", it cannot resolve the metadata file, perhaps we should prevent some invalid words like %2c.

Renaming the path into normal name resolved my issue.

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I got the same error "Metadata file '.dll' could not be found", and I tried several things described above, but the reason for the error was that I was referencing third-party DLL file which was targeting a .NET version higher that my project target .NET version. So the solution was to change the target framework of my project.

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I added a new project to my solution and started getting this.

The reason? The project I brought in was targeting a different .NET framework (4.6 and my other two were 4.5.2).

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For me, it was trying to find a DLL in a path that used to contain the Project, but we'd moved it to a new directory. The Solution had the correct path to the Project, but Visual Studio somehow kept looking in the old location.

Solution: Rename each problem Project - just add a character or whatever - then rename it back to its original name.

This must reset some global cache of some kind in Visual Studio, because this clears both this issue up and several like it, while things like Clean do not.

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In my case, the problem was caused by a simple build error,

error CS0067: The event 'XYZ' is never used

that, for any reason, did not show up in the error window.

Because of that, the Visual Studio build system seemed to miss the error and tried to build dependent projects, which in turn failed with the annoying metadata message.

The recommendation is -as stupid as it may sound-:

First look at your Output Window!

It took me half an hour before this idea hit me...

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It looks like such kind of errors related to the fact that Visual Studio doesn't provide correct information about an error. The developer doesn't even understand the reason for the failed build. It can be a syntax error or something else. In common, to solve such problems you should find the root of the problem (for example, look at the build log).

In my case the problem was in fact that the Error List window didn't show any errors. But really there were syntax errors; I found these errors in the Output window, and after fixing them, the problem was solved.

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For me it occurred when I included a new project to a solution.

Visual Studio automatically selects .NET framework 4.5.

I changed to version .NET 4.5.2 like the other libraries, and it worked.

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For me the following steps worked:

Find the project that is not building

Remove/add references to projects within the solution.

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I was pulling my hair out with this problem also, but after trying the previous answers the only thing that worked for me was to open each project in my solution 1 by 1 and build them individually.

Then I closed Visual Studio 2013, reopened my solution and it compiled fine.

It's strange, because if I clicked each project in my Solution Explorer and tried to build them that way, they all failed. I had to open them alone in their own solutions.

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In my case the issue was that I'd manually deleted a non-compilation file which was marked as "missing". Once I deleted the reference to the now-missing file and recompiled - all was well.

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If you have a space in your solution name, this will also cause the issue. Removing the space from your solution name, so path doesn't contain %20 will solve this.

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Coming back to this a few years later, this problem is more than likely related to the Windows maximum path limit:

Naming Files, Paths, and Namespaces, Maximum Path Length Limitation

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My instance of the problem was caused by a common project that had a duplicate class name in it (under a different filename). It is strange that Visual Studio could not detect that and instead just blew up the build process.

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I got this problem in Visual Studio 2012 in a solution that had many projects. Rebuilding each project in the solution manually in the same order as the Project Build Order (right-click and rebuild in Solution Explorer) fixed it for me.

Eventually I got to one that gave me a compile error. I fixed the error, and the solution would build correctly after that.

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I too had the same error. It hides as in the below path. The path which I referred to for the DLL file is like "D:\Assemblies Folder\Assembly1.dll".

But the original path in which the assembly referred was "D:\Assemblies%20Folder\Assembly1.dll".

Due to this path name variation, the assembly could not be retrieved from its original path and hence throws the "Metadata not found" error.

The solution is in Stack Overflow question How do I replace all the spaces with %20 in C#?.

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Most of the methods explained here did not solved the problem for me.

Finally, I fixed the problem by applying the following steps:

1. Close Visual Studio.

2. Delete all the contents in the bin folders of each project.

3. Open solution and rebuild.

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I'd faced the same problem. In my case I'd referenced to a class library project with higher .Net version than my project and VS failed to build the project and raised the same error you posted.

I simply set .Net version of my class library project(the one that had broken the build) identical to the .Net version of referenced project and problem solved.

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Just pointing out the blatantly obvious: if you don't have "Show output window when build starts" enabled, make sure you're noticing if your build is failing (small "build failed" error in lower left)!!!!

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I had this error when I was trying to publish a web application. Turned out that one of a class properties was wrapped into

#if DEBUG
    public int SomeProperty { get; set; }
#endif

but the property usage was not. The publishing was done in Release configuration without the DEBUG symbol, obviously.

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Based on the error message I don't believe the file path is being truncated. It looks to just be incorrect. If I'm reading the message correctly it appears to be looking for the DLL file at ...

WORK=-\Tools\VersionManagementSystem\BusinessLogicLayer\bin\Debug\BusinessLogicLayer.dll

This is not a valid path. Is it possible that you have a macro definition in the build process set to an invalid value?

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I had this issue because .nuget\NuGet.exe was not included in my repository. Although I enabled DownloadNuGetExe in NuGet.targets, it reported a proxy error when trying to download it. This caused the rest of the project builds to fail.

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