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C# with WPF -- Using project reference and adding to reference list does not work

In a specific project (meant for web-services) in my .net solution (Visual Studio), I have added a using directive:
using ourSystem.Common.WPF
and I also added the corresponding reference (pointing to the DLL) to the project file.
The web-services project compiles, builds, and works just fine until I try to add the WPF project.
When I add the WPF project, I get the error message:
Error CS0234 The type or namespace name 'WPF' does not exist in the namespace 'ourSystem.Common' (are you missing an assembly reference?)
I have looked at similar questions on stack overflow and web posts elsewhere, but nothing seems to work.
It appears that the problem must be some sort of incompatibility between files.
NOTE: The Visual Studio Solution (VS2019) I am working on has several projects, and this DLL/namespace works in many of the other projects, just not the project that I am trying to add it to.
Both the WPF project and the project I am trying to reference it in are v4.5.2
Thus, at least for that attribute, they should be compatible.
For what it's worth, I have also tried unloading projects, and reloading them. This often changes the error count from about 4 (all the same message) to well over 100, and it seems to lose all references to everything.
Edit added: It turns out the relative structure of the many projects within the solution (which I didn't describe in the original post of this question) was key in finding an answer. See my own answer below for details, if interested.
Any suggestions on how to fix this problem?

I was able to fix it essentially as follows:
I refactored the directory structure.
It had been
A-> Top Level GUI project that had called B & C
B-> Folder with the projects used by A including WPF, but not Web Service Project.
C-> Folder with Web Service Project
I then moved all of C to the same layer as B.
Having them in the same folder seemed to 'placate' the compiler.
I am not claiming that this was absolutely needed, nor that it should always be done, however it worked in my case.
I had to also redo some code within the C project to account for different layering within C's own folders (especially packages folder), as it still had legacy code as if it were the top-level project. The following link was useful in that: (especially the answer by Nikita R.)
How to fix re-layering problem

How to decompile ASP.NET / C# Web Application

I've just inherited a web application whose source has long been lost (originally written in 2010 and shelved). The application has a few .dll assemblies that related to the application itself e.g. "applicationCORE.dll", "applicationBI.dll", "applicationDATA.dll" and "application.dll"
I've seen this question and the suggested tool (Just Decompile) is brilliant and created a .sln and .csproj file for the first assembly that I decompiled. My question is how do I merge the various projects that would be created through decompiling with the compiled web application files (.aspx) also, how do I resolve the references in the .aspx files i.e. referencing the codebehind file that no longer exists e.g. "default.aspx" references "default.aspx.cs" while the decompiler creates a "default.cs" file. Is it safer to rename the .cs file or should I update the reference?
Finally, will each dll appear as a separate project within the solution?
I realise this may be perceived as a duplicate question however there doesn't appear to be a resource online that walks a developer through the process.

Following David's advice, I managed to get the application running from decompiled assemblies. Here's the process I followed to get it working
I had already decompiled the various assemblies into projects using a Reflector (on a trial).
I created a blank Web forms application in Visual Studio
I added the .aspx pages from the website to the project through visual studio
Then added the .cs files from the decompiled 'application.dll' project (since this is the website project within the solution. Some files had to be renamed to match the codebehind references in the `.aspx. files
Each additional project e.g. applicationCore.dll was then added to the solution
Each project's references needed to be updated and references to the newly added projects must be added to the startup project
Since the website was built so long ago, there were 1,000's of syntax errors. The easiest way to resolve them was to use Notepad++ and the Find and Replace. To be safe, I did this file-by-file by following the errors from Visual Studio rather than a batch find and replace
When trying to build I noticed errors where required assemblies were missing so I changed the build output directory of the sub-projects to the bin folder of the web project
I added the connection strings and settings from the original website's web.config. I did this line by line to make sure I didn't break anything and so that I could trace the result of each addition
Finally I had a successful build!
Additional Steps
There were also syntax errors which I assumed were due to the decompiling process. Some external references needed to be added and there were slight changes due to the age of the project e.g. asp:AjaxScriptControl changed to asp:ScriptControl (after adding the package using Nuget). I also had to install Crystal Reports for this application and will have to purchase a Telerik licence as there are UI components being used (although I'll see if I can use an open / native alternative as I work through the app).
I've logged in using credentials (I did have to set the correct start page) and tried a few basic CRUD operations. There are silly issues that have to be resolved e.g. the authentication doesn't work properly and there's no redirect if you access a protected page but these things are relatively minor compared to the issues I faced initially.
What I must say is each error was resolved using questions and answers from this site! This was all completed in just under 6 hours.

References not working on my computer but fine on colleagues with same solution

I'm currently developing an app on our companies portal and yesterday started noticing that I wasn't able to reference external classes or methods from references.
My colleague put the solution on his computer and committed his code with all the references working fine, and I opened up his code and the references have stopped working and still can't be found.
I have deleted the website cache and other user options but had no luck so far.
One reference file is an external VB class file and the other was simply calling "request" from Web.UI.UserControls. (I got around this by using HttpContext.Current.Request.QueryString , even though "Request" worked on my colleagues computer)
Is there any options on my Visual Studio that would make the same code break compared to his? I contemplated a reinstall but it's a lot of time wasted and the problem may surface again.
Edit. I'm on Visual Studio 2013 Ultimate

You can enable assembly binding log, and use fuslogvw.exe to see what is going on behind the scenes. It will allow you to know more details on the error and think in possible solutions.
http://msdn.microsoft.com/en-us/library/e74a18c4(v=VS.100).aspx

Getting mysterious build error after upgrading solution to Windows 8.1

I have used windows store apps 8.0 in my project and upgraded it to be 8.1. And also upgraded Visual Studio from 2012 to 2013.
First, I got an error that cannot use the Microsoft.VCLibs Version 11.0 and 12.0. So I removed the 11.0 version.
And then I got this errors:
I tried to find some information on the internet, but nothing.
Also when I double clicked on the error it doesn't bring me to the error's spot.
I have tried to do many things, and nothing helped, so I decided to write this post.
Can someone help me resolve this errors?
EDIT:
I am adding some more information, maybe this can lead to the problem:
Solved:
With the help of Hans Passant the errors have fixed. The problem was that I had this duplicate on my resource file:
I did it twice because one is for buttons (that has content), and one is for message dialogs on code behind.
In Visual Studio 2012 this code was working fine, but somehow, not in Visual Studio 2013.
I deleted the row of Cancel.Content and the errors disappeared and it works fine :)

Here is somebody else with the same problem.
I'll just copy Caplan's guidance. Look through the .resw file(s) in your project and look for a duplicated "Cancel" resource. I should note that intentionally forcing a duplicate resource produced another message when I built the program on VS2013:
1>MakePri : error 0x80073b0f: Processing Resources with error: Duplicate Entry.
1>MakePRI : error 0xdef00532: Conflicting values for resource 'Resources/String1'
Same message from either duplicating the resource in the same .resw file and by making a copy of the .resw file and adding it to the project. So Caplan's explanation isn't exactly a slam-dunk. I'd pay attention to other projects in your solution that might also have a resource named "Cancel". A likely scenario when your solution has a dependency on VCLibs, that sounds like you are mixing C# and C++/CX.

I came across this MSDN URL which seemed to be pretty helpful.
2 snippets in particular you could try:
You could actually have a duplicate resource entry in your project file.
Your obj files could have become corrupt, you need to clear out your obj files (although I haven't seen this problem for a while in my
team - it might be fixed in 2008).
If you are using a custom culture, perhaps with the .Net RegisterCulture method, ensure that the culture has been registered on
your computer. If you've reinstalled your OS and installed VS2008 and
you can't build due to this problem it could be because you need to
register your custom cultures before you build. This error seems to
be thrown when msbuild cannot find the culture.
Alternatively:
PS - There is also a Visual Studio 2012 Update 3 available that could possibly help?
Visual Studio 2012 Update 3
I hope this helps.

A similar error can also occur if you update a Nuget package in one project, but not in another dependent project also using the same Nuget package.
i.e. it can be a version mismatch error when referencing different versions of a dependency across two dependent projects.

I don't know if you have tried re-installing VS 2013, I was getting the same error but it was resolved when I re-installed.

In my case, after retargeting a working Win8 App for Win81 App, was UI string. Imho it is correct but soehow the VS2013 has some problems with it.
Locations SomeString
Locations.Text SomeStringForUI
Removing the one of them solved my problem.

Recently i got a similar build exception on my UWP Project
MakePri : error 0x80073b0f: Processing Resources failed with error: Doppelter Eintrag.
MakePRI : error 0xdef00532: Conflicting values for resource 'System.Design/ClassComments1'
I found a lot of information about the error but no solution. It looks like the error must be in the entry application project but it isn't.
In my case the VS integrated automaticly a reference to System.Activities because a made a mistake.
After i deleted the wrong reference, it works fine again.

Got this error in a UWP Xamarin.Forms project when a fellow developer referenced System.Web in a PCL that the Xamarin.Forms PCL references.
For more info:
Add System.Web Reference To A Windows 10 Universal App

When compiling solution, previously working assembly reference / using no longer works [duplicate]

My C# WinForms solution has two projects.
A DLL which is the main project I'm working on, and an executable WinForms I call "Sandbox" so that I can compile/run/debug the DLL easily in one go.
I'm working in .Net 4.0 for both projects.
Everything was working fine until I added some seemingly innocent code, and a reference to System.Web in the DLL.
Now my Sandbox project can't see the namespace of the DLL project. I didn't change anything which I believe should have affected this.
If I delete the project reference to the DLL from the Sandbox references and re-add it, then the red underlines all disappear and the colour coding comes back for all my classes etc; but as as soon as I try to build the solution, the whole thing falls apart again.
When I right-click the DLL project in the Sandbox's references and view in object browser, I can see the namespace and all the stuff in there.
I have a feeling this might be some sort of bug?
Is this some sort of VS2010 bug? I had this same issue a few months ago and I could only fix it at the time by making a whole new project and re-importing my files. This time, however, I have a bajillion files and will only do that as a last resort!
Edit:
After panickedly going through and undoing all my changes, trying to find what caused the problems, it seems to be this line:
string url = "http://maps.google.com?q=" + HttpUtility.UrlEncode(address);
If I comment out this line, then I get no namespace errors and the project builds fine. I can't see anything wrong with this line though.

I'm ready to declare this a bug in VS2010, this has bitten way too many programmers already. The fix is easy: Project + Properties, Application tab, change Target Framework to ".NET Framework 4" instead of the Client Profile that is selected by default.
System.Web is not included in the client profile. Having this option in the first place is quite silly, the client profile is only 15% smaller than the full version of .NET 4.0. Having it selected by default is even sillier. But I digress.
UPDATE: mercifully this all got fixed in VS2012. Which no longer makes the client profile the default for a new project. And the client profile got retired completely in .NET 4.5, good riddance.

Check to make sure that both projects are using the non-client profile for their target framework (go to each project's properties to do this).

One possibility is that the target .NET Framework version of the class library is higher than that of the project.
I faced this problem, and I solved it by closing visual studio, reopening visual studio, cleaning and rebuilding the solution. This worked for me. On some other posts, I have read the replies and most of users solved the problem by following this way.

Try building only the project with the Sandbox dll first independently.
Then point your executable project to the required dll and ensure copy local is set to true. in reference settings.
Tthen build the executable project.

Changing the target framework from the ".NET Framweork 4 Client Profile" to ".NET Framework 4" worked for me with a similar problem. I agree that the client profile doesn't seem to have much of an advantage to using it. I seem to get nailed with weird errors that I hunt for until I remember that Visual Studio defaults to the client profile. I guess the moral of the story when getting an error is: if "Rebuild Solution" doesn't work, check the Target framework...

If you tried already doing the Framework change, and still not worked, I hope this works for you (as it did for me): Simply add the necessary references from within your projects. Very obvious but I was doing it wrong until I found what was the issue.

I just had this issue and it turned out to be I had multiple namespaces being used that had the same object name (i.e. business objects had the same names as mvc models);
Fully qualifying the names fixed the issue for me.