Why would all of my references have yellow exclamation points on them in my .NET class library?

Question

I've been developing a class library for quite sometime and all of the sudden, sometime last week, I opened my project and all of my references have yellow exclamation points on them now (System.dll, System.Drawing.dll, etc..). I've tried removing the references and re-adding them to fix any broken reference paths, but they continue to show yellow exclamation points on them.

Nothing had changed since the last time I'd opened the project. The only thing that was different from the last time I'd opened the project was that this time I opened it from directly inside of .NET from another project. For example, I was working on a .NET 3.5 web project, then clicked File -> Open Recent Files -> My Other Solution. This closed my current current 3.5 web solution and opened the 2.0 class library solution and the problem appeared for the first time. I'm not sure how (or why) this would cause a problem, but I'm leaning toward the fact that Visual Studio got confused or something and now my assemblies are all invalid on this 2.0 class library project. (?)

What would cause this to happen and how can I fix it? I've looked around the web, but I only see that people have suggested to remove the references and re-add them; which I've done, to no avail.

I'm considering starting a new project and copying over all of my source files one by one, but would really like to avoid all this if possible.

Thanks in advance!

Solution 1

Using the Solution Explorer, right mouse click and select Unload Project and then select Edit (name of your csproj file) to be able to edit the .csproj file directly in VS.

Under one of the <ItemGroup> nodes you will find subnodes tagged Reference. Ensure that the HintPath node value points to a valid path. Also double check the nodes SpecificVersion and Private for valid values.

Hopefully evaluating these values will help you resolve your problem.

Solution 2

Just to add to this. I recently had an issue where I was adding references that had the TargetFramework set to .NET 4.0 to a Visual Studio 2008 project. All you get is a yellow exclamation mark and explanation.

Once I realised I picked the wrong release binaries it seemed obvious!

Solution 3

Check weather the project .NET framework is equal or greater than reference .Net framework

Solution 4

my problem was difference in target .NET framework versions set on the projects. the first was set to .NET 4.0 and the second was .NET 4.5 changing it solved my problem

Solution 5

I got the same issue with a project opened in Visual Studio 2015 recently.

The error came out when I've relocated a few times the physical location of the solutions default package repositories and deliberately changed the value of the repositoryPath in the NuGet.Config

In my case, I needed to remove an element whose path pointed to by the condition attribute is invalid that is inside the Target section of the csproj file.

<Error Condition="!Exists('..\..\..\..\..\Packages\Microsoft.Bcl.Build.1.0.21\build\Microsoft.Bcl.Build.targets')" 
       Text="$([System.String]::Format('$(ErrorText)', '..\..\..\..\..\Packages\Microsoft.Bcl.Build.1.0.21\build\Microsoft.Bcl.Build.targets'))" />

Also, this error is reproduced when the usual Package folder is removed or relocated elsewhere. One should then update this value along with the ItemGroups.

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Comments

[Admin]

Is one project using .Net 2.0 while another project in the same solution uses .Net 3.5? And one project references the other?

[Luc]

Thanks for your tip. I did what you said and everything looks correct in the <ItemGroup>. I simply changed all of my relative references (ie. '..\..\..\') to absolute references (ie. 'c:\') and the exclamation points magically went away. :) Thanks again!

[pfeds]

Ha, that was exactly the issue I was having - Thanks.

[Contango]

This fixed the issue for me - project was configured for .NET 4.5, and I should have configured the project for .NET 4.5.1.

[pashute]

as @reza_baiat said (I missed that one)

[Matt Borja]

Had the same problem here; .NET 4.5 project referencing a .NET 4.5.1 project wouldn't compile until I did this. Thanks!

[shivesh suman]

A symptom to this is that types are recognized by IntelliSence and you can write code in Visual Studio using the types in referenced library however the code would not compile and you see a yellow exclamation mark for that reference assembly. Unfortunately there is no tool tip to tell us what is wrong.

[FloverOwe]

Luc, wouldn't such hard-wiring of paths make cross-deployments difficult? Maybe the default isn't set up right. Where does it get the default "current" app path and how can we verify and adjust it? Thanks