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Is there a way to track which assembly definition a script belongs to?

I upgraded my project to Unity 2021 and have tons of the same error:
error CS0246: The type or namespace name could not be found (are you missing a using directive or an assembly reference?)
This is for both packages and my own scripts. I added an assembly definition in my main scripts folder and tried to add a reference but I couldn't find the proper one to fix any issue.
For example, I'm using the RootMotion FinalIK and PuppetMaster assets. All scripts referencing these have using RootMotion.FinalIK and using RootMotion.Dynamics However, the script doesn't recognize RootMotion.Dynamics and every variable type using RootMotion.FinalIK cannot be found.
Any ideas?

The fix was to just restart Unity and VSCode.

C# - Recompiling old code, running into errors (are you missing an assembly reference?)

First and foremost, I've never worked with C#, or any compiled language for that matter. So I'm hoping I'm just doing something wrong here.
We have an old in house app, written in c# that has some smtp settings hard coded that need to be changed. I have the source files, and even the .sln visual studio project file. So I open the project in vs, make the required changes, and go to build- however I'm running into errors (Build -> Build 'Autodocs' (the name of our app)):
Build started...
1>------ Rebuild All started: Project: AutoDocs, Configuration: Debug Any CPU ------
1>C:\shared\New folder\AutoDocs\Microsoft\Office\Interop\Word\Documents.cs(10,38,10,54): error CS0234: The type or namespace name 'CustomMarshalers' does not exist in the namespace 'System.Runtime.InteropServices' (are you missing an assembly reference?)
1>C:\shared\New folder\AutoDocs\AutoDocs\modules\DWStorer.cs(7,7,7,15): error CS0246: The type or namespace name 'DocuWare' could not be found (are you missing a using directive or an assembly reference?)
========== Rebuild All: 0 succeeded, 1 failed, 0 skipped ==========
So does anybody have any suggestions for a c# newbie like me? Anything I should have to do when copying a project dir from the server to my pc to build it?
Thank you in advance.

Do you know what kind of program it is? Windows? ASP.NET? MVC?
The errors are shown, but let's go through them:
1>C:\shared\New folder\AutoDocs\Microsoft\Office\Interop\Word\Documents.cs(10,38,10,54):
error CS0234: The type or namespace name 'CustomMarshalers' does not exist in the
namespace 'System.Runtime.InteropServices' (are you missing an assembly reference?)
First, look in this document:
C:\shared\New folder\AutoDocs\Microsoft\Office\Interop\Word\Documents.cs
Somewhere in that file, when viewing in Visual Studio, you will see the word CustomMarshalers, and it needs to be referenced.
So, in your project, you will need to right-click the References section and add that missing reference (if it isn't there already):
Also, at the top of your Documents.cs file, you need to have a line like this:
using System.Runtime.InteropServices;
See Microsoft's documentation here: System.Runtime.InteropServices.CustomMarshalers
Here is your second error:
>C:\shared\New folder\AutoDocs\AutoDocs\modules\DWStorer.cs(7,7,7,15):
error CS0246: The type or namespace name 'DocuWare' could not be found
(are you missing a using directive or an assembly reference?)
Similar to the first error, this one is having an error in the DWStorer.cs file because it cannot resolve the keyword DocuWare.
DocuWare is a third party tool that must be added to your references and added to the top of your DWStorer.cs file.
DocuWare
You may have to go to the link above to download their tool if you do not have it in your project somewhere.

Can't get access to my folder Working in VS2015 .NET 4.5.2

as you can see in the Picture i can't get access to my App_Globalresources and it keeps throwing errors
The type or namespace name 'App_GlobalResources' could not be found (are you missing a using directive or an assembly reference?)
i tried everything (Restarting VS,Rebuilding the project) nothing works
and inside that folder is a .rsex file which i used to write some errors
is there anything that i can do to fix it?

Rather than using:
typeof(App_GlobalResources.Resource)
You should be using:
typeof(Resources.Resouce)
Working in VS2015 .NET 4.5.2.

Getting namespace does not exist error for all the references added in my project

Whenever I take the latest code from TFS and tries to build existing solution, It fails all the project building and says that : The type or namespace name 'XYZ' does not exist in the sampace 'a.b.c' (are you missing an assembly reference?)
This is happening with all the references and only in my system. On the other systems all these code works fine.
Can it be a Visual Studio issue or some setting which might have got changed?

Why can't the type or namespace be found on IIS 7 server when it works fine on the local machine?

I am using DynamicPDF creator, Visual Studio 2010, and IIS 7.
On my local machine, the line "using ceTe.DynamicPDF;" and subsequent lines work fine, but
when I try to load the page on my server, I get the following error:
"Compiler Error Message: CS0246: The type or namespace name 'ceTe' could not be found (are you missing a using directive or an assembly reference?)"
I have added the .dll file as a reference, and intellisense even works.
I am wondering if it is a problem with assemblies. Any ideas about what I should do to get this working on my server?

May be you lost some of dlls, you can check dll which contains ceTe. ... by dependency walker.