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Editorial Team
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Asked: 2026-06-03T21:37:16+00:00

The type ‘System.ComponentModel.DataAnnotations.MaxLengthAttribute’ exists in both [path…]\packages\EntityFramework.4.3.1\lib\net40\EntityFramework.dll and ‘c:\Program Files (x86)\Reference Assemblies\Microsoft\Framework \.NETFramework\v4.5\System.ComponentModel.DataAnnotations.dll’ Now,

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 The type 'System.ComponentModel.DataAnnotations.MaxLengthAttribute' 
 exists in both 

 [path...]\packages\EntityFramework.4.3.1\lib\net40\EntityFramework.dll 

 and

'c:\Program Files (x86)\Reference Assemblies\Microsoft\Framework
\.NETFramework\v4.5\System.ComponentModel.DataAnnotations.dll'

Now, I have read on msdn that its safe to exclude the EntityFramework reference (which was added through the nuget package). However, when I do that, I cant create a DBContext properly as the DbModelBuilder class lives in the EntityFramework dll. Also, some other critical classes are missing when I remove the EntityFramework refference so this is old and irrelevant solution now.

Update (disambiguation): Both System.ComponentModel.DataAnnotations.dll and EntityFramework.dll include System.ComponentModel.DataAnnotations.MaxLengthAttribute. The problem is that each dll also includes other classes that are critical to EF code-first design. For example:

EntityFramework.dll:
 - System.Data.Entity.DbModelBuilder

System.ComponentModel.DataAnnotations.dll:
 - System.ComponentModel.DataAnnotations.RegularExpressionAttribute
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1 Answer

  1. Editorial Team

    Add this statement to top of your class

     using System.ComponentModel.DataAnnotations;

    System.ComponentModel.DataAnnotations namespace is distibuted across the EntityFramework.dll and System.ComponontModel.DataAnnotations.dll. So you need to add a reference to both of that in your project to make use of DataAnnotations.

    The MaxLenth attribute is present in EntityFramework.dll. So make sure you have that reference to that dll present in your project references section.

    enter image description here

    EDIT : As of .NET framework 4.5, this namespace is moved to the System.ComponentModel.DataAnnotations.dll. So If you use .NET Framework 4.5 with Entity Framework 4.3.1 or less, You will run in to this conflict. The solution is to switch to Entity framework 1.50 beta 1/ 2 release if you want to stick with .NET 4.5 or downgrade to .NET 4 to use EntityFramework 4.3.1.

    From the msdn documentations.

    Starting with Entity Framework 5.0 Beta 1, the EntityFramework.dll
    does not contain definitions for data annotations. These definitions
    were moved to System.ComponentModel.DataAnnotations.dll and are
    defined in the System.ComponentModel.DataAnnotations.Schema namespace.

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