7

I am currently going through a tutorial using Visual Studio 11 beta. When trying to set the max length of a field value in one of my classes:

[MaxLength(50)]
public string LastName { get; set; }

It errors out and wont let me compile because the MaxLength() function exists in two places:

Error 4 The type 'System.ComponentModel.DataAnnotations.MaxLengthAttribute' exists in both 'c:\Users\me\Documents\Visual Studio 11\ContosoUniversity\packages\EntityFramework.4.1.10331.0\lib\net40\EntityFramework.dll' and 'c:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.5\System.ComponentModel.DataAnnotations.dll'

I have tried to remove both files but that just causes more issues because other code in my project is dependent upon them.

Is there a way I can tell it to use one or the other?

All of these approaches don't seem to be working for me.. Refer to the comments under the answers.. Any other ideas?

Thanks

tereško
  • 58,060
  • 25
  • 98
  • 150
Trevor
  • 16,080
  • 9
  • 52
  • 83
  • What about the full name space, Are they the same too? – humblelistener May 31 '12 at 20:37
  • Thank you for the help. It looks like some if not all of your answers would work out. I'll be able to test it on my other computer tomorrow and then choose an answer. Thank you. – Trevor Jun 01 '12 at 20:57
  • Unfortunately this approach is till not working, the system still detects two occurrences of System.ComponentModel.DataAnnotations.MaxLengthAttribute And I tried to create an extern Alias via the command prompt - by going to the dll and typing /r:GridV1=grid.dll but It didn't recognize the command or something. – Trevor Jun 04 '12 at 19:51
  • @L.B When the error message says that two types with the same fully namespace qualified name exist in different assemblies, it won't help to supply the full namespace. It is the same in both cases. His problem is that he references too many assemblies, and he gets duplicates from that. – Jeppe Stig Nielsen Oct 19 '13 at 09:52

7 Answers7

6

Use using at the top of your code:

using MaxLength = System.ComponentModel.DataAnnotations
Ufuk Hacıoğulları
  • 37,978
  • 12
  • 114
  • 156
Saeed Neamati
  • 35,341
  • 41
  • 136
  • 188
  • When I try this I still get this error "Error 1 'System.ComponentModel.DataAnnotations' is not an attribute class" – Trevor Jun 08 '12 at 12:39
  • Then simply reference it's fully qualified name ;). – Saeed Neamati Jun 09 '12 at 04:50
  • 3
    Giving the full namespace with the name is of no use when both types have the same fully namespace qualified name. He should check if he references too many assemblies. In the ***unlikely*** case that it is really a good idea to reference two distinct assemblies which both have a `System.ComponentModel.DataAnnotations.MaxLengthAttribute` class, he should use `extern alias ThisOneHere;` and then `[ThisOneHere::System.ComponentModel.DataAnnotations.MaxLength(50)]` as suggested by Martin Ernst's answer. – Jeppe Stig Nielsen Oct 19 '13 at 09:59
6

Just Uninstall EntityFramework from packages and reinstall it(EntityFramework). It works for me. Just follow the steps mentioned below:

1.Right click on reference 2.Click on manage nugetpackages

1.Right click on reference 2.Click on manage nugetpackages.

3.Click on Uninstall

3.Click on Uninstall

  1. go in online section and type entity frame work in search box 5. Click on install button

4. go in online section and type entity frame work in search box 5. Click on install button

Kurt Van den Branden
  • 11,995
  • 10
  • 76
  • 85
vebs
  • 325
  • 1
  • 4
  • 11
  • Yep, this is the only solution that worked for me, and it also avoids having to add extra code, aliases etc. – Caio Campos Dec 09 '16 at 05:44
  • This also worked for me. In my case, I had upgraded my project from .NET 4.0 to 4.5. And my Error List was showing a couple of nugget packages (including EntityFramework specifically) needed to be reinstalled as a result. It presented that as a Warning, but I ignored it until your post. – jozolo May 04 '17 at 20:20
3

Qualify the attribute with the desired namespace

[System.ComponentModel.DataAnnotations.MaxLength(50)]
public string LastName { get; set; }
Olivier Jacot-Descombes
  • 104,806
  • 13
  • 138
  • 188
  • I still get "Error 2 The type or namespace name 'MaxLength' does not exist in the namespace 'System.ComponentModel.DataAnnotations' (are you missing an assembly reference?)" And it still exists in both locations "Error 3 The type 'System.ComponentModel.DataAnnotations.MaxLengthAttribute' exists in both 'c:\Users\me\Documents\Visual Studio 11\ContosoUniversity2\packages\EntityFramework.4.1.10331.0\lib\net40\EntityFramework.dll' and 'c:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.5\System.ComponentModel.DataAnnotations.dll'" – Trevor Jun 08 '12 at 12:41
  • 1
    Obvoiusly it is in the same namespace in both libraries. It is not a good idea to reference two different versions of the same library. Remove one reference if you can. Otherwise use the procedure that I described in this SO post http://stackoverflow.com/a/10841652/880990 to resovle the namespace clash. – Olivier Jacot-Descombes Jun 08 '12 at 14:48
  • I was having trouble understanding until I read the article in your other post. Thank you. – Trevor Jun 11 '12 at 12:53
2

In case, if you are getting the same error in latest environment (VS 2017/.NET Framework 4.6.x) and with Entity Framework like 6.1 or 6.2, here is the solution;

Downgrade your Entity Framework to 6.0. It'll work.

marc_s
  • 732,580
  • 175
  • 1,330
  • 1,459
skillworks
  • 332
  • 3
  • 10
1

MaxLength is not a function, it's an Attribute.
You can use the using directive in each file to specify the current correct context.
Or just type the full namespace, e.g. System.ComponentModel.DataAnnotations.MaxLength

Erez Robinson
  • 794
  • 4
  • 9
1

Try using extern alias http://msdn.microsoft.com/en-us/library/ms173212.aspx to differentiate between the two assemblies

Also check out http://bartdesmet.net/blogs/bart/archive/2006/10/07/4502.aspx near the bottom of the page is an example

Martin Ernst
  • 5,629
  • 2
  • 17
  • 14
  • I tried going to the dll and assigning an alias but I got a the following error "the file name, directory name, or volume label is incorrect." – Trevor Jun 08 '12 at 12:48
1

This question is now the top SO answer for this question so I figured I would answer it generally here.

The The type 'BLAH' exists in both error often pops up in the following occassions:

1. DUPLICATE FILES - (often very simple) This is notoriously the case with .dll files. In most cases of duplication, deletion of one of the duplicate files is the easiest and best solution

2. NON-DUPLICATE FILES - (more complicated, such as the original poster's case) you unfortunately need to edit and often recompile/find alternate versions of the files so that conflicts like this do not arise. Luckily more often than not, qualifying the attribute with the same namespace will fix conflicts like this.

For example if your code is conflicted on something called MaxLength that is mentioned in several files, then qualify it on necessary files to make it very clear which MaxLength you want to use like so: System.ComponentModel.DataAnnotations.MaxLength. This should help to clear things up, so nothing gets conflicted when trying to run your code

Chris Klingler
  • 5,258
  • 2
  • 37
  • 43