Why can t I reference System ComponentModel DataAnnotations

Question

I m trying to use DataAnnotations in my WPF project to specify a maximum length of strings  with the following   using System ComponentModel DataAnnotations    However  I get the error     The type or namespace name  DataAnnotations  does not exist in the   namespace  System ComponentModel   are you missing an assembly   reference     I ve seen other examples where DataAnnotations does exist in this namespace  I m using C 4  Is there any reason why I can t use this  What can I do to fix it

User · Answer

I also have this problem  That is very stupid when i add a namespace the same with System  I try to remove all references  but it is not resolved  I use  global  System ComponentModel   it is working as well  When i remove my namespace  this problem has been resolved

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I found that I cannot reference System ComponentModel DataAnnotations from Silverlight 5 with the below version at  1    I found that Silverlight 5 assemblies cannot use  NET assemblies  it gives the error  You can t add a reference to System ComponentModel DataAnnotations as it was not built against the Silverlight runtime        I plan to workaround this by hopefully installing the Silverlight 5 package found at  2  below   If this fails I will update this post    UPDATE   it failed   I installed everything relating to Silverlight 5 and I don t have the Silverlight version of the  dll assembly System ComponentModel DataAnnotations    Too bad  UPDATE II   I found an old  dll having this name from a previous installation of Silverlight developer s kit for Visual Studio 2008 or 2010  I added this file and it seems to  work   in that IntelliSense is now recognizing attributes on class members  such as   Display Name    My Property Name      Whether or not this works for everything else in this  dll I don t know     1   Microsoft Visual Studio Professional 2013 Version 12 0 21005 1 REL Microsoft  NET Framework Version 4 5 51641   Installed Version  Professional   2  http   go microsoft com fwlink  LinkId 229318

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System ComponentModel DataAnnotations is contained in its own assembly so you need to make sure you have it refernced  Just simply   1   Right click on Soloution and choose add   2   Choose reference from the list   3   Search   System ComponentModel DataAnnotation   and tick the check box on its left hand side and press ok    Job done  shouldnt have any refernce errors

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If you don t have it in references  like I did not  you can also add the NuGet System ComponentModel Annotations to get the assemblies and resolve the errors   Adding it here as this answer still top of Google for the error

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You have to reference the assembly in which this namespace is defined  it is not referenced by default in the visual studio templates   Open your reference manager and add a reference to the System ComponentModel DataAnnotations assembly  Solution explorer -  Add reference -  Select  Net tab -  select System ComponentModel DataAnnotations from the list

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This error occurs when the reference to the  System dll  got removed Solution to the problem is very simple add the reference to  System dll  The dll is normally available in the following location  C  WINDOWS Microsoft NET Framework v2 0 50727   Add the reference your problem will get solved

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To Reference System ComponentModel DataAnnotations  In a code file to have Using System ComponentModel DataAnnotations  at the top of the file such as   using System ComponentModel DataAnnotations    Add a  NET reference to your project by right clicking the project in solution explorer      Hope this helps  This question helped me

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I searched for help on this topic as I came across the same issue   Although the following may not be the Answer to the question asked originally in 2012 it may be a solution for those who come across this thread   A way to solve this is to check where your project is within the solution  It turns out for my instance  I was trying to install a NuGet package but it wouldn t and the listed error came up  that my project file was not included within the solution directory although showing in the solution explorer  I deleted the project from the directory out of scope and re-added the project but this time within the correct location

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For  Net Core in Visual Studio 2019 try this  see VS suggestion It worked for me  hope it ll work for you as well

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Use the FrameWork version 4 5 and above for your project then problem solved Because this namespace is under 4 5 and above

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I upgraded from Silverlight 4 to Silverlight 5 and then I was having this issue   Although I had a reference to  System ComponentModel DataAnnotations  under  References  in my project  it had a yellow yield sign by it that indicated the previously referenced assembly could not be found   It turned out that the properties of the  System ComponentModel DataAnnotations  reference indicated  Specific Version   True   when I changed this to  Specific Version   False  it fixed the issue   Right click on the  System ComponentModel DataAnnotations  assembly under  References  and select  Properties  from the context menu   Check that the property value for  Specific Version   False    It must have been referencing the old Silverlight 4 assembly which was no longer available after the upgrade to Silverlight 5

User · Answer

I also had the same problem and I resolved by adding the reference in one of my projects which didn t had the mentioned reference  If you have 2-3 projects in your solution  then check by adding this reference to the other projects

User · Answer

If using  NET Core or  NET Standard  use      Manage NuGet Packages       instead of      Add Reference

User · Answer

I had same problem  I solved this problem by following way    Right click on page  select Property  in build action select Content   Hope that this solution may help you

User · Answer

If you tried to update visual studio from vs2008 to vs2010  And your app uses framework 3 5  and you don t want to upgrade it   and also used WCF RIA Services BETA    I have bad news    you MUST upgrade to WCF RIA Services v1  BETA does not work on vs2010     and due to this    you also have to install Silverlight 4   upgrade to framework 4 0  See this  http   blog nappisite com 2010 05 updating-visual-studio-2008net-35-ria html

[c#] Why can't I reference System.ComponentModel.DataAnnotations?

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