Why do I get a warning icon when I add a reference to an MEF plugin project?

I wish to test the core class of a plugin by directly referencing the plugin project and instantiating the plugin class. When I create a test Console App project and add a project reference to the plugin project, I get a warning icon (yellow triangle with exclamation mark) next to the reference in the References list.

When I instead add a reference to the dll, the assembly build output of the plugin, I get no such warning. What could this warning be trying to tell me?

As mentioned in the question's comments, differing .NET Framework versions between the projects can cause this. Check your new project's properties to ensure that a different default version isn't being used.

Encountered the same issue with a ASP.Net Web App and two library class projects which needed to be referenced within the Web App. I had no information provided on why the build failed and the references were invalid.

Solution was to ensure all projects had the same Target Framework:

In Visual Studio 2015- Right Click project > Properties > Application > Target Framework

Save, Clean and Rebuild solution. The project references should no longer appear as yellow warnings and the solution will compile.

My Web App was targeting .Net 4.5 whereas the other two dependent library class projects targeted .Net v4.5.2

1. Make sure all versions are same for each projects click each projects and see the version here Project > Properties > Application > Target .NET framework

2. a. Go to Tools > Nuget Package Manager > Package Manager Console Type Update-Package -Reinstall (if not working proceed to 2.b)

   b. THIS IS CRITICAL BUT THE BIGGEST POSSIBILITY THAT WILL WORK. Remove < Target > Maybe with multiple lines < /Target > usually found at the bottom part of .csproj.

3. Save, load and build the solution.

For both of (or all of) the projects that you want to use together:

Right click on the project > Properties > Application > Target .NET framework

Make sure that both of (or all of) your projects are using the same .NET framework version.

Reinstall all packages in all projects of the current solution:

Update-Package -Reinstall

Try closing and opening VS.

Seems silly but after 1 hour of following the above and finding everything lined up OK. I restarted VS 2017 and the problems were gone.

Make sure you have the projects targeting the same framework version. Most of the times the reason would be that current project ( where you are adding reference of another project ) points to a different .net framework version than the rest ones.

For me, I ran into this issue when referencing a .NET Standard 2.0 class library in a .NET Framework 4.7.1 console application. Yes, the frameworks are different, but they are compatible (.NET Standard is supposed to jive with both .NET Core and .NET Framework.) I tried cleaning, rebuilding, removing and readding the project reference, etc... with no success. Finally, quitting Visual Studio and reopening resolved the issue.

Check NETFramework of the referred dll & the Project where you are adding the DLL. Ex: DLL ==> supportedRuntime version="v4.0" Project ==> supportedRuntime version="v3.0"

You will get warning icon. Solution : Make dll version consistence across.

It's been a long time since this question was asked but if someone is still interested - I recently ran into similar icons. I was compiling a C#.net project using VS 2008. I found VS could not locate the assemblies for those references. When I double clicked VS refreshed the references and removed the icons on some of those[EDIT: which it could NOW locate]. For remaining references, I had to compile the respective assemblies.

Adding my 2 cents to the @kad81 answer,

Go to Visual Studio -> BUILD -> Configuration Manager

In the "Active Solution Platform" drop down in top right hand corner (mine is VS 2012), if it is "Mixed Platforms", change it to the appropriate platform based upon your reference third party assemblies.

Then in each of the project in the list, make sure you select same platform for all the project. (if x86 not exist, then select "", then you can select "x86".)

Rebuild the library projects first and then referencing projects. Hope this helps.

In Asp.net core sometime it shows alert if you changes the project name space or name. To remove this kind of alerts you just Unload Project and load it again. If issue is still there means you it can not find your Assembly reference.

Using Visual Studio 2019 with all projects targeting .Net Core 3.1 the solution was to:

1. Clean / Build / Rebuild.
2. Restart Visual Studio 2019

Also happens if you explicitly reference a project that was already implicitly referenced.

i.e

• project a references project b
• project c references project a (which adds implicit ref. Expand and see)
• project c references project b

you will see an exclamation mark next to b under project references.

I had these icons for a different reason. We have one big solution for all our projects (nearly 100). I made a subselection of the projects I was interested in and made a new solution. However the references where project references instead of references to the compiled dll's....

After some research I found [this link on GitHub][1] which explains this is new behaviour in VS2015.

On the GitHub page they explain a workaround for converting project references to binary references.

[1]: https://github.com/dotnet/roslyn/issues/4358 "Visual Studio 2015 no longer uses a binary reference to a library when its project is loaded in the same solution"

To fix some not working stuff it has sense to remove some libraries sometimes, how would not that sound weird.

Anyways, I believe the problem is too wide and might be caused by different factors, so want to share my situation/solution.

I had a project (brought by customer) with Xamarin Forms and Telerik libraries. The thing was in general related to the components, which libraries are not included into packages folder, nor available via Nuget (paid ones).

The whole project References were "yellow", it looked horribly and scary.

The solution was just to remove those Telerik references (including a few controls in code which were using that). Right after that all the references magically got their common normal grey color and the errors (mostly) disappeared.

"Mostly" - because "all red around" error messages about "the element is not defined anywhere" sometimes happen still. That's weird, and brings inconvenience, but I still able to compile and run the project(s): just need to clean solution, restart Visual Studio, pray a little bit, clean again, remove obj/bin folders, restart again, and it works well.

The key thing is remove not available libraries references, as the error messages say absolutely another stuff. (For instance, something like "Xamarin.Build.Download.XamarinDownloadArchives not found or cannot find something" etc., but that just might mean you don't have some references available.

Then remove packages folder, reload/reopen the project/solution, go to "Manage Nuget Packages" and click "Restore" button.

In a multi-project solution, If every other thing failed... On the startUp project, check. Dependencies->Assemblies and see if the erring referenced project is there. Remove it and re-build.

I had the same issue in a solution with projects targting .NET Core 3.1, .NET Standard 2.0 and .NET Framework 4.8. The issue was on this last one.

The trick that solved the issue for me, was to change the target framework to .NET Framework 4.5, then back to .NET Framework 4.8.

I have absolutely no idea why this fixed the issue, but it did.

The IDE was Visual Studio 2019.

Open the YOURPROJECT.csproj file with a text editor then at the end of the file remove these lines inside the target tag, and then build the project again! be sure the Package folder is in the correct path which mentioned in < Reference > < HintPath >

<Error Condition="!Exists('.......

Enjoy it ;)

I also faced the same problem but my case was a bit different the ones above. I tried to open a project created in a different computer. I found that the path to package folder is not updated when you add a reference so restarting VS, changing .NET version, or any mentioned recommendation does not solve the problem. I opened the csproj file in notepad++ and corrected all the relative paths to packages folder. Then; all the warnings are gone. Hope it helps.

in VS 2017 Do a Clean then Build

Thank you all for the help. Here is a breakdown of how I fixed my problem:

Right click on your project > Properties

Under Application change the target Framework. In my case ImageSharp was using .Net 4.6.1. You can find this in your packages.config.

Go to your project references. You'll notice SixLabors has a yellow triangle. You have to update the NuGet package.

Right click on References > Manage NuGet Packages.

Update SixLabors.

You might have slight code updates (see below) but this fixed my problem.

https://stackoverflow.com/questions/57789399/convert-imagesharp-image-to-imagesharp-pixelformats-rgba32

In Visual Studio 2019, one of my projects target framework was .net core but it was referencing another project whose target framework was .net standard. I changed all of the projects to reference .net standard and the icons went away. To see what your project is right click it and click properties and look at Target framework. You can also normal click the project itself and look at the < TargetFramework > tag under < PropertyGroup >

I had created a new .sln which was put in a subfolder. The .nuget folder was missing from where that .sln file was added. Moving the .nuget folder from the root into the subfolder where my new .sln file was solved the issue for me.

I came back later and added the .sln file to the root and deleted the subfolder. Doing this originally would have solved the issue as well.

Based on the answer from @AljohnYamaro (sorry, couldn't comment on your answer, new account without enough reputation yet, but upvotaded you), I've checked the .csproj file.

On my file, besides the standard project reference:

<ProjectReference Include="..\ProjectA\ProjectA.csproj">
    <Private>true</Private>
    <CopyLocalSatelliteAssemblies>true</CopyLocalSatelliteAssemblies>
</ProjectReference>

There were also a directy link to the compiled dll from the referenced project:

<ItemGroup>
	<Reference Include="ProjectA">
	<HintPath>..\ProjectA\bin\Debug\netcoreapp3.1\ProjectA.dll</HintPath>
	</Reference>
</ItemGroup>

Removing this second reference solved the issue.

One of the reasons to get this annoying yellow triangle is that you are adding a reference to a project twice, meaning:

• Reference one: MyProjectOne (which contains already a reference to MyProjectTwo)
• Reference two: MyProjectTwo

By deleting the Reference two, the yellow triangle will disappear.

If you're using the newer style Sdk projects add OutputType to the ProjectGroup element with a value of Library in the project you're referencing. It'll also give you grief if it's in the project you're referencing and it references a project without the setting.

find your .csprojc file and open it.

find your packages path. I fixed my project by this issue.

..\packages\EntityFramework.6.1.3\lib\net45\EntityFramework.dll

this .. means vs will find this dll in parent directory.

you should confirm your package path, and your will fixed this issue.