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=, Culture=neutral, PublicKeyToken=null'.

Source File:


Following suggestions here and here, I have deleted all instances of Project.Rights.dll inside C:WindowsMicrosoft.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.


Thank you for visiting the Q&A section on Magenaut. Please note that all the answers may not help you solve the issue immediately. So please treat them as advisements. If you found the post helpful (or not), leave a comment & I’ll get back to you as soon as possible.

Method 1

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!

Method 2

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

Method 3

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.

Method 4

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.

Method 5

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.

Method 6

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.

Method 7

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.

Method 8

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:WindowsMicrosoft.NETFrameworkv4.0.30319

Method 9

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.

Method 10

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

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

Method 11

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.

Method 12

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.

Method 13

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..

Method 14

The type ‘Domain.tblUser’ is defined in an assembly that is not
referenced. You must add a reference to assembly ‘Domain,
Version=, Culture=neutral, PublicKeyToken=null’.

 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

Method 15

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


explicit operator

it be OK.

Method 16

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.

Method 17

I have a similar problem, and I remove the RuntimeFrameworkVersion, and the problem was fixed.

Try to remove 1.1.1 or

Method 18

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.

Method 19

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

Method 20

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.

All methods was sourced from or, is licensed under cc by-sa 2.5, cc by-sa 3.0 and cc by-sa 4.0

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