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Recurring namespace errors, but project compiles

I've been having a recurring namespace error inside one of my projects. All the references that are having issues are inside the same project and appear to be properly namespaced. Clearing some VS cache files appears to fix the problem temporarily (AppData\Local\Microsoft\VisualStudio\12.0 and AppData\Roaming\Microsoft\VisualStudio\12.0). This isn't a show-stopper by any means; it just means I can't use intellisense or code navigation.
This is especially annoying because I have two files in the same folder referencing the same namespace, with the code content in those files namespaced identically, but VS cannot find the namespace I want in one file but it can in the other. E.g....
using System;
using ExampleNamespace.Model;
namespace ExampleNamespace.DAL{
..... code content .....
}
Is the same from one file to the other. The only difference is in one file, it cannot find ExampleNamespace.Model and all the types in the namespace are not valid.
Other things I've tried:
Resetting the target framework
Build Action on files is set to compile
Deleting cache files (as above)
Opening/closing VS
I've searched high and low for an answer, but haven't found one. My main question is why the project compiles at all if the IDE can't find a reference? Doesn't the IDE essentially pseudo-compile the code to do intellisense/syntax and bug high-lighting?

The type is defined in an assembly that is not referenced, how to find the cause?

I know the error message is common and there are plenty of questions on SO about this error, but no solutions have helped me so far, so I decided to ask the question. Difference to most of similar questions is me using App_Code directory.
Error message:
CS0012: The type 'Project.Rights.OperationsProvider' is defined in an
assembly that is not referenced. You must add a reference to assembly
'Project.Rights, version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.
Source File:
c:\inetpub\wwwroot\Test\Website\App_Code\Company\Project\BusinessLogic\Manager.cs
Following suggestions here and here, I have deleted all instances of Project.Rights.dll inside C:\Windows\Microsoft.NET/*.*
According to this, I checked if .cs files in question have build action set to "Compile". They do.
I have also double checked that the .cs file containing the "Project.Rights.OperationsProvider" type is deployed to App_Code directory.
For some reason, application is not looking for the type in the App_Code directory. Since I've deleted all instances of Project.Rights.dll (that I know of), I don't know which assembly the error message is mentioning.

When you get this error it isn't always obvious what is going on, but as the error says - you are missing a reference. Take the following line of code as an example:
MyObjectType a = new MyObjectType("parameter");
It looks simple enough and you probably have referenced "MyObjectType" correctly. But lets say one of the overloads for the "MyObjectType" constructor takes a type that you don't have referenced. For example there is an overload defined as:
public MyObjectType(TypeFromOtherAssembly parameter) {
// ... normal constructor code ...
}
That is at least one case where you will get this error. So, look for this type of pattern where you have referenced the type but not all the types of the properties or method parameters that are possible for functions being called on that type.
Hopefully this at least gets you going in the right direction!

Check target framework in the projects.
In my case "You must add a reference to assembly" actually meant, that caller and reference projects didn't have the same target framework. The caller project had .Net 4.5 , but referenced library had target 4.6.1.
I am sure, that MS compiler can be smarter and log more meaningful error message. I've added a suggestion to https://github.com/dotnet/roslyn/issues/14756

In my case this was because doing a NuGet package update had only updated references to a dll dependency in some but not all projects in my solution - resulting in conflicting versions. Using a grep-style tool to search text within *.csproj files in my solution it was then easy to see the projects that still needed to be updated.

When you get this error, it means that code you are using makes a reference to a type that is in an assembly, but the assembly is not part of your project so it can't use it.
Deleting Project.Rights.dll is the opposite of what you want. You need to make sure your project can reference the assembly. So it must either be placed in the Global Assembly Cache or your web application's ~/Bin directory.
Edit-If you don't want to use the assembly, then deleting it is not the proper solution either. Instead, you must remove all references to it in your code. Since the assembly isn't directly needed by code you've written, but instead by something else you're referencing, you'll have to replace that referenced assembly with something that doesn't have Project.Rights.dll as a dependency.

In my case, I was referencing a library that was being built to the wrong Platform/Configuration (I had just created the referenced library).
Furthermore, I was unable to fix the problem in Visual Studio Configuration Manager -- unable to switch and create new Platforms and Configurations for this library. I fixed it by correcting the entries in the ProjectConfigurationPlatforms section of the .sln file for that project. All its permutations were set to Debug|Any CPU (I'm not sure how I did that). I overwrote the entries for the broken project with the ones for a working project and changed the GUID for each entry.
Entries for functioning project
{9E93345C-7A51-4E9A-ACB0-DAAB8F1A1267}.Release|x64.ActiveCfg = Release|x64
{9E93345C-7A51-4E9A-ACB0-DAAB8F1A1267}.Release|x64.Build.0 = Release|x64
Entries for corrupted project
{94562215-903C-47F3-BF64-8B90EF43FD27}.Release|x64.ActiveCfg = Debug|Any CPU
{94562215-903C-47F3-BF64-8B90EF43FD27}.Release|x64.Build.0 = Debug|Any CPU
Corrupted entries now fixed
{94562215-903C-47F3-BF64-8B90EF43FD27}.Release|x64.ActiveCfg = Release|x64
{94562215-903C-47F3-BF64-8B90EF43FD27}.Release|x64.Build.0 = Release|x64
I hope this helps someone.

It just happened to me that different projects were referencing different copies of the same dll.
I made sure all referenced the same file on disk, and the error disappeared as I expected.

Unloading and reloading the class library in Visual Studio solved this for me.

For me, this was caused by the project both directly and indirectly (through another dependency) referencing two different builds of Bouncy Castle that had different assembly names. One of the Bouncy Castle builds was the NuGet package, the other one was a debug build of the source downloaded from GitHub. Both were nominally version 1.8.1, but the project settings of the GitHub code set the assembly name to BouncyCastle whereas the NuGet package had the assembly name BouncyCastle.Crypto. Changing the project settings, thus aligning the assembly names, fixed the problem.

It didn't work for me when I've tried to add the reference from the .NET Assemblies tab.
It worked, though, when I've added the reference with BROWSE to C:\Windows\Microsoft.NET\Framework\v4.0.30319

I had this issue on a newly created solution that used existing projects. For some reason, one project could not "see" one other project, even though it had the same reference as every other project, and the referenced project was also building. I suspect that it was failing to detect something having to do with multiple target frameworks, because it was building in one framework but not the other.
Cleaning and rebuilding didn't work, and restarting VS didn't work.
What ended up working was opening a "Developer Command Prompt for VS 2019" and then issuing a msbuild MySolution.sln command. This completed successfully, and afterwards VS started building successfully also.

one of main reason can be the property of DLL
you must before do any thing to check the specific version property if it true make it false
Reason:
maybe the source code joined with other (old)version when you build it , but this Library upgraded with new update the version now different in the Assembly Cash and your application forbidden to get new DLL ,and after disable specific version property your applacaten will be free to get the new version of DLL references

Maybe a library (DLL file) you are using requires another library. In my case, I referenced a library that contained a database entity model - but I forgot to reference the entity framework library.

This can also mean you use a library, which exposes (public) types that are defined in a library. Even when you do not use these specifically in your library (the one that doesn't build).
What this probably prevents is you writing code that uses a class (which in its signature has the types from a library not referenced) that you cannot use.

For me the reason why the error appeared was that the WebForm where the error was reported has been moved from another folder, but the name of its codefile class remained unchanged and didn't correspond to the actual path.
Initial state:
Original file path: /Folder1/Subfolder1/MyWebForm.aspx.cs
Original codefile class name: Folder1_Subfolder1_MyWebForm
After the file was moved:
File path: /Folder1/MyWebForm.aspx.cs
Codefile class name (unchanged, with the error shown): Folder1_Subfolder1_MyWebForm
The solution:
Rename your codefile class Folder1_Subfolder1_MyWebForm
to one corresponding with the new path: Folder1_MyWebForm
All at once - problem solved, no errors reporting..

The type 'Domain.tblUser' is defined in an assembly that is not
referenced. You must add a reference to assembly 'Domain,
Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.
**Solved:**
Add reference of my domain library layer to my web app libary layer
Note: Make sure your references are correct according to you DI container

In my case this was because I used
Implicit Operator
between BLL and DAL classes.when I want to use BLL Layer In Application Layer I got this error.
I changed
implicit operator
to
explicit operator
it be OK.
Thanks

In my case the version of the dll referenced was actually newer than the one that I had before.
I just needed to roll back to the previous release and that fixed it.

I have a similar problem, and I remove the RuntimeFrameworkVersion, and the problem was fixed.
Try to remove 1.1.1 or

My problem was that the Output Type for one of my projects was set to Console Application. To fix this, I right-clicked the project, chose Properties, clicked the Application tab, and change Output Type (from Console Application) to Class Library. After I re-compiled, this error went away.

Clean your solution and rebuild worked for me (in Visual Studio, these are options you get when you right click in your solution explorer), the error is gone in my project.

Type or namespace name does not exist in the namespace - yet the namespaces do exist

A project that I've been working on for a long time without any problems suddenly started throwing errors such as
The type or namespace name 'xxx' does not exist in the namespace 'yyy' (are you missing an assembly reference)?
In this case, the namespaces were all core .Net libraries such as System.Data, Linq, and IO that I've been using without any problems.
I've worked through all of the issues in this question such as making sure all the projects in the workspace are using the same version of .Net for the Target Framework setting.
I removed all the libraries in question and re-added them but the problem persisted. I doubt they libraries themselves are corrupted as I reference them in other projects within the solution.

After rolling back through my work with TFS I managed to work out what I'd done that had created this error: I'd added a folder called "System" to my project and put a class file in it.
This is an easy issue to reproduce: create a project, add a folder called system to it (it will still compile at this point) and then create a .cs file in it, that's when all the fun errors will occur.
The problem stems from the name of the "System" folder which leads to any files created in it being under the namespace ".System".
I can understand why having a folder / namespace called "System" would cause problems now but I think it would help if Visual Studio warned when creating a folder / namespace of such a name to stop this problem happening in the first place. I've logged a bug with MS, at least having this logged might help any other people who have made the same mistake as me!

I had the same issue after my system shut down unexpectedly and even though VS tried to restore, the problem popped up.
I had two projects in my Solution Explorer. To solve this, I right-clicked the project that was associated with the error message and selected 'Build'.
After that, the issue was resolved.
Just to add that the problem was permanently solved after I added a reference to the 'project' from the other 'project'.

I renamed the namespace of the file I was referencing from myproject.shared.constants to myproject.SOMETHING.shared.constants
After that It suggested "use myproject.SOMETHING.shared.constants" as suggestion. I renamed it back to myproject.shared.constants and then it worked.

Try to Clean and Then Rebuild the dll file .I was experiencing the same problem tried different suggestions from internet but none of them work.But it will. In case you don't know how follow these steps:
open your dll project file.
2.click on solution Explorer Right click on Your Dll Project Name you will find Clean and Rebuild Option.

Visual Studio saying name doesn't exist in current context

I am calling a static method on a class like
Foo.bar()
Visual studio's intellisense recognizes Foo and autocompletes bar for me (it highlights Foo and everything like it is working fine). Everything looks fine until I go to build the project, and it throws an error saying the name Foo doesn't exist in current context.
I am using this static method call in other files, so I know the class is ok. The situation is too big to post code, so I am mostly looking for reasons to start looking into that would cause intellisense to function normally but get errors on compile like this.

I've seen this error caused by differing versions of the .NET framework in the different projects. The Class Library I built was 4.5 and the application was 4.0, but the only error it gave was namespace errors. Changing the framework version on the class library and rebuilding it, then the application, resolved the error.

This can occur when namespaces, classes and variables become tangled when they have the same name. I have suffered with this before. Intellisense told me I was right, the compiler told me I was wrong! I trusted the compiler!
You have 2 options that I can think of
Search your code for Foo, and see it it is being used for something other than the static class.
Fully qualify the Foo.bar() call. MyApplication.This.That.Foo.bar();
Do it in that order...it's better to elegantly resolve the issue so you can just call Foo.bar() as this is more readable and maintainable than having MyApplication.This.That.Foo.bar(); all over the place!

In my case I was missing a } at the end of one of the methods in the middle of the code which was causing the program not see the rest of the code and complain about the Methods I have defined after that point.

Old thread I know, but I've encountered this issue when referencing a static method from within a unit test project - intellisense said the method was there, but when I tried to build/run the test (in Debug mode) I got the error 'name doesn't exist in current context'. In order to fix it I had to rebuild the project containing the referenced static method in Debug configuration (it had only previously been built in Release configuration) - after this the test built and ran OK.

I know this is a bit old topic, but I just experienced the same and for me it was because the file was not actually included in the solution.
I properly happened because I had renamed the class and then the file, which caused Visual Studio to still know the class and the namespace, but the compiler did not get the file as the renamed file was not included.

Consider doing a Clean and then a Build on the project with the problem. It is possible for the editor and Intellisense to correctly discover the class, while the compiler works with files that are out-of-date. (I had this same problem, and that's how I resolved it.)

this is an old article I know, but I just encountered this issue and has been puzzling me for couple of days, and eventually got to it: click on the class file, in Solution Explorer, then look at the Properties tab; make sure Build Action is set to "Compile".

Adjust the related file. If the error code in Default.aspx.cs, you need to change the top line in the file Default.aspx as below:
Replace "CodeFile=" with "CodeBehind"
Hope this can help.
-Thanks, Thai_FUV

I have run into this probelm a few times and so when I do, the first thing I check is if the assembly not recognized has any Nuget packages. In my cases they always have and I simply forgot to install the same packages in the assembly of which the reference to the un-recognized assembly is in. A re-build command and problem fixed. I hope this helps someone. This same error message can be given for multiple things so this particular case, may not apply. If you have not used Nuget than I would suggest trying the other answers

I also was running into this issue creating a data access layer and had static methods being called with the same symptoms: Intellisense finding it but not the compiler. I tried many of the above, including fixing the .Net version.
When adding the source files to the project I also changed the namespace.
With the file with the issue, I forgot to change the namespace to match when it was imported at another time.

Closing all tabs of MonoDevelop. Then Closing MonoDevelop. Finally opening MonoDevelop again solved the problem for me.

Mine was a little more convoluted solution. Project A referenced projects B and C: both references had Copy Local to true and both produced assemblies with identical names. When building the referencing project, the output assemblies from projects B and C were copied and one overwrote the other because they had the same name. VS was then looking for the references within the build directory and only found the assembly that had "won."

In my case I had to reload the project that was marked "missing".
Project > Unload Project
Project > Load Project
Clean, Build Solution

My solution to this problem that occurs every now and then:
Find the class that is giving you problems in the Solution Explorer and "Exclude From Project"
Rebuild that assembly (let's call it "A")
The project that used the file ("B") will ask you to "Reload" project, wait
Add the file back into assembly A, that you just removed it from, and rebuild
Now, reload project B
Then the file was found in VS and all was well.

Changing the id of the control resolved the issue for me. Apparently the id of the control existed in another part of the solution.

In my case, I was missing the following lines in my csproj file
<PropertyGroup Condition="'$(Configuration)|$(Platform)'=='Debug|AnyCPU'">
<DefineConstants>TRACE</DefineConstants>
<DebugType>full</DebugType>
<DebugSymbols>true</DebugSymbols>
</PropertyGroup>
Once I added this, I could see the variables while debugging

Unable to find the type or namespace name after moving files

I get this error message in Visual Studio when I'm trying to run the C# code:
The type or namespace name 'Shape' could not be found (are you missing a using directive or an assembly reference?)
Is this because I changed the location of some files in the Solution Explorer? From the beginning I had the files in a folder and then I moved them outside and erased the folder. I have tried to use rebuild solution and clean, but I still get this error.
Now I have all files in the same "level" in the Solution Explorer. What can I do to get it to work again?

A few things to try:
Make sure the class file containing the Shape class is marked as "Include in project" in the Solution Explorer.
In the file with the error, make sure you have an appropriate using statement referencing the namespace where the Shape class is located.
If the Shape class is in an external DLL, make sure it is referenced in your project.
Double-check what the namespace is for the class file that contains the Shape class. Depending on how you moved those files around, it may have changed and you will need to update the using statement where you use it.

This can be caused by many things. e.g. public, assembly referencing, and in a project or a solution folder. JohnFx has already handled some of these in his reply. But to get you "going" do the following:
Move all of the files into the same project within the solution. So you solution has a project and all of the files are in that one project.
This should at least get you going. Then experiment.
Good luck.