C# Compiler Warning 1685

Question

So, (seemingly) out of the blue, my project starts getting compiler warning 1685:

The predefined type 'System.Runtime.CompilerServices.ExtensionAttribute' is defined in multiple assemblies in the global alias; using definition from 'c:\Program Files\Reference Assemblies\Microsoft\Framework\v3.5\System.Core.dll'

Perplexed, I researched the MSDN article to figure out its cause. Here's the information I found:

Visual C# Reference: Errors and Warnings Compiler Warning (level 1) CS1685

Error Message The predefined type 'System.type name' is defined in multiple assemblies in the global alias; using definition from 'File Name'

This error occurs when a predefined system type such as System.int32 is found in two assemblies. One way this can happen is if you are referencing mscorlib from two different places, such as trying to run the.Net Framework versions 1.0 and 1.1 side-by-side.

The compiler will use the definition from only one of the assemblies. The compiler searches only global aliases, does not search libraries defined /reference. If you have specified /nostdlib, the compiler will search for Object, and in the future start all searches for predefined types in the file where it found Object.

Now I'm really scratching my head.

I'm not running two different versions of the .NET Framework (unless you count 2.0 and 3.5).
I'm not referencing any bizarre assemblies that might make me suspicious.
I don't recall making any changes to my application that would spur this change.
I've verified that all components target .NET Framework version v2.0.50727.

I'm open to suggestions, or ideas on how to correct this. I treat warnings as errors, and it's driving me crazy.

What really bugs me about it is that I don't know why it's occurring. Things that happen should have a discernable cause, and I should know why they happened. If I can't explain it, I can't accurately remedy it. Guesswork is never satisfactory.

The application is straightforward, consisting of a class library, and a windows forms application.

A C# class library DLL providing basic functionality encapsulating database access. This DLL references the following components:
- System
- System.Core
- System.Core.Data
- System.Data
- System.Data.DataSetExtensions
- System.Data.OracleClient
- System.Drawing
- System.Windows.Forms
- System.Xml
- System.Xml.Linq
A C# Windows Forms application providing the UI. This application references the following components:
- CleanCode
- CleanCodeControls (both of these provide syntax editor support, and are locally built against .NET 3.5).
- LinqBridge
- Roswell.Framework (the class library above)
- System
- System.Core
- System.Data
- System.Data.DataSetExtensions
- System.Data.OracleClient
- System.Deployment
- System.Design
- System.Drawing
- System.Windows.Forms
- System.Xml
- System.Xml.Linq

Let me know if you need further information and I'll gladly provide it.

see the following: http://stackoverflow.com/questions/546819/strange-warning-about-extensionattribute suggest you close the question it's an exact duplicate — ShuggyCoUk, Mar 02 '09 at 16:22
incidentally it is indeed LinqBridge which is causing this, you no longer need it at all — ShuggyCoUk, Mar 02 '09 at 16:23
I did a search on the warning number and couldn't find any questions that contained it; my apologies for the duplicate. Incidentally, LinqBridge WAS, in fact, the problem. Thank you! — Mike Hofer, Mar 02 '09 at 16:29
Good point. I have edited the linked question to contain it. — ShuggyCoUk, Mar 03 '09 at 01:02
I triggered this error by installing IIS with .NET 3.5 instead of 4.5 by accident. Fix was to add 4.5 back in in "Add Features ..." in control panel. — Ruskin, Jun 02 '15 at 14:14

score 102 · Answer 1 · edited Aug 29 '16 at 03:52

102

Another easy way to verify: In your code, temporarily use the class somewhere. Example:

System.Runtime.CompilerServices.ExtensionAttribute x = null;

When building, this will generate error:

The type 'System.Runtime.CompilerServices.ExtensionAttribute' exists in both 'c:\Program Files\Reference Assemblies\Microsoft\Framework\v3.5\System.Core.dll' and .....

And show you immediately the 2 sources causing the conflict.

edited Aug 29 '16 at 03:52

Noctis

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answered Jun 29 '11 at 09:22

Remco te Wierik

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Cool. In fact I didn't even need to build, as soon as I pasted that line into a code file, the intellisense warning told me all I needed to know! – demoncodemonkey Oct 21 '11 at 09:48
3

This is a better general answer. – O'Rooney Jul 15 '13 at 21:54
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Finally! A way to figure out what file is actually causing this warning. – nathansizemore Aug 22 '14 at 14:37

score 22 · Accepted Answer · answered Mar 02 '09 at 16:20

LINQBridge makes me immediately suspicious. The entire intent of this is to provide extension attribute/methods etc for 2.0 users. If you have 3.5 (System.Core.dll), don't use LINQBridge. If you do need LINQBridge in 3.5 for some obscure reason (and I can't think of one), then you might have to use an extern alias. But I really doubt you need it!

score 22 · Answer 3 · answered Mar 02 '09 at 16:32

22

Marc is almost certainly correct. Here's a way to verify

Open Reflector.exe
Add all of Non-System assemblies
F3 and search for ExtensionAttribute

If it pops up anywhere besides System.Core then you know where it's coming from.

answered Mar 02 '09 at 16:32

JaredPar

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I just used `grep -r ExtensionAttribute *` in my project directory and found the culprit that way. – palswim Sep 05 '13 at 18:19
@palswim how about windows ? – sam Mar 18 '23 at 13:06

score 9 · Answer 4 · answered Nov 09 '10 at 13:48

9

Another solution for this issue is to use a global alias for the whole assembly:

Reference -> Properties -> Aliases -> Replace 'global' with something else

answered Nov 09 '10 at 13:48

VitalyB

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score 5 · Answer 5 · answered May 03 '12 at 09:19

5

FYI: I had the same problem and was able to resolve it by using Resharper's "Optimize References" command, and then removing all unused references. Not completely sure why that worked, but it did.

answered May 03 '12 at 09:19

Efrain

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score 0 · Answer 6 · answered Feb 16 '23 at 15:26

My teams resolution for this CS1685 warning was removing the binding redirect for System.Text.Json.

<dependentAssembly>
   <assemblyIdentity name="System.Text.Json" publicKeyToken="cc7b13ffcd2ddd51" culture="neutral" />
   <bindingRedirect oldVersion="0.0.0.0-7.0.0.0" newVersion="7.0.0.0" />
</dependentAssembly>

score -4 · Answer 7 · answered Jan 08 '15 at 14:13

-4

Another solution for this issue => Right click project -> Properties -> Build -> Treat warnings as errors -> None

answered Jan 08 '15 at 14:13

Cipher

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I would never allow this. Proper code has no warnings. This feels like sweeping it under the carpet. This warning can and therefore should be solved. – Remco te Wierik Feb 18 '15 at 22:21

C# Compiler Warning 1685

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