在同一解决方案中找不到引用项目的类型或名称空间

Question

I am getting this unbelievably annoying issue in VS 2017. Somehow these kinds of issues are always plaguing me..

I opened up my solution today which contains several projects to find that one was just missing for no reason (it is always there when I open the solution).

So I simply re-added the project by right-clicking Solution explorer and adding an existing project - easy.

Now, when I try to run that project ( OrderGetter ), it fails to build with

The type or namespace name 'Utilities' could not be found (are you missing a using directive or an assembly reference?)

Utilties is another project in my solution:

It definitely exists and has always worked in the past. Why must this issue come and happen now?

Eveb weirder, The error keeps disappearing from the errors window, but when I try to build I get the popup telling me I have errors, and I have to check the output window to see what is going on.

I have tried the following:

• Clean solution
• Clean project
• Build/Rebuild solution
• Build/Rebuild projecct
• Close all tabs, restart VS
• Remove/re-add reference
• Unload project, delete reference from the csproj file and then re add it in the VS UI before reloading the project
  • Checked that the csproj GUIDs match

I can see the following in Solution.sln :

Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "MVCQCPage", "MVCQCPage\MVCQCPage.csproj", "{FF699AAD-EA97-4984-9DB2-70F2CA41CA16}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "SharedLibrary", "SharedLibrary\SharedLibrary.csproj", "{0E6DE04C-97A2-4C18-AB83-098C383DF9C7}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "AspNet.Identity.SQLite", "..\..\..\..\Downloads\Firefox\Asp.Net.Identity.SQLite-master\Asp.Net.Identity.SQLite-master\AspNet.Identity.SQLite\AspNet.Identity.SQLite.csproj", "{35DAC40A-38D0-43BD-8DCF-34E4061F0A75}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "AspNet.Identity.SQLite.EF", "..\..\..\..\Downloads\Firefox\Asp.Net.Identity.SQLite-master\Asp.Net.Identity.SQLite-master\AspNet.Identity.SQLite.EF\AspNet.Identity.SQLite.EF.csproj", "{D34D1279-F264-4A90-B4A5-2AAE4DE5D0D6}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "PublicSite", "..\..\..\..\Downloads\Firefox\Asp.Net.Identity.SQLite-master\Asp.Net.Identity.SQLite-master\PublicSite\PublicSite.csproj", "{18D9B36C-50A0-4753-9522-A1C282C70B3E}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "Utilities", "Utilities\Utilities.csproj", "{4473F366-D894-425D-AFE2-75EC883AC0E7}"
EndProject
Project("{F5034706-568F-408A-B7B3-4D38C6DB8A32}") = "Scripts", "Scripts\Scripts.pssproj", "{6CAFC0C6-A428-4D30-A9F9-700E829FEA51}"
EndProject
Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "TurboTime", "TurboTime\TurboTime.csproj", "{05FFA6B7-8ACE-4FDF-83DB-75FBF7C46E64}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "TurboTimeConsole", "TurboTimeConsole\TurboTimeConsole.csproj", "{F5B79240-155C-4E8B-B8F7-BFFBC9EFE9EB}"
EndProject
Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "OrderGetter", "OrderGetter\OrderGetter.csproj", "{BBD80E29-721A-4423-A21E-4B6C51D153FF}"
EndProject

And in Utilites.csproj I can see that the GUId is

<ProjectGuid>{4473F366-D894-425D-AFE2-75EC883AC0E7}</ProjectGuid>

And that OrderGetter.csproj is referencing

    <ProjectReference Include="..\Utilities\Utilities.csproj">
  <Project>{4473f366-d894-425d-afe2-75ec883ac0e7}</Project>
  <Name>Utilities</Name>
</ProjectReference>

So the GUIDs are the same except for one being upper case and the other lower case. What does that mean?

You can clearly see that the project is definitely being referenced:

Why is this happening?

Answer 1

If the above does not help, you can open the solution as well as the project files in a text editor and check if the project file have the correct references set.

Each project has a unique GUID

<ProjectGuid>{12D33B77-30A1-4D07-9DA4-8789AF6746F5}</ProjectGuid>

the referencing project uses this GUID along with the path to reference this project:

<ProjectReference Include="..\Data Binding Demo\C#\databindinglab.csproj">
  <Project>{12D33B77-30A1-4D07-9DA4-8789AF6746F5}</Project>
  <Name>databindinglab</Name>
</ProjectReference>

Check the affected projects and make sure file names and GUIDs match.

If this does not help, remove all references, save and close the project and then add each reference again.