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

Question

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

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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

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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

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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

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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

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I have a similar problem  and I remove the RuntimeFrameworkVersion  and the problem was fixed   Try to remove 1 1 1 or

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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

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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

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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

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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

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I had this issue on a newly created solution that used existing projects  For some reason  one project could not  quot see quot  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  quot Developer Command Prompt for VS 2019 quot  and then issuing a msbuild MySolution sln command  This completed successfully  and afterwards VS started building successfully also

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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

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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

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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

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When you get this error  it means that code you are using makes a reference to an 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

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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

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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

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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

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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

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