CODEWITHSUNDEEP
c#.netcompiler-warnings
Mike Hofer
Mike Hofer

Reputation: 17032

C# Compiler Warning 1685

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.

  1. I'm not running two different versions of the .NET Framework (unless you count 2.0 and 3.5).

  2. I'm not referencing any bizarre assemblies that might make me suspicious.

  3. I don't recall making any changes to my application that would spur this change.

  4. 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.

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

Upvotes: 50

Views: 38308

Answers (8)

Ping Li
Ping Li

Reputation: 1

We got over 1000 such warnings. I believe it really slow down build performance. It would be nice if Microsoft could fix it or provide a workaround. Unfortunately the issue is logged but closed with a low priority :(

ASPNETCOMPILER : warning CS1685: The predefined type 'System.ObsoleteAttribute' is defined in multiple assemblies in the global alias; using definition from 'c:\Program Files (x86)\Reference Assemblies\Microsoft\Framework.NETFramework\v4.8\mscorlib.dll'

Upvotes: 0

ADH
ADH

Reputation: 3071

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>

Upvotes: 0

Remco te Wierik
Remco te Wierik

Reputation: 1466

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.

Upvotes: 105

Cipher
Cipher

Reputation: 352

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

Upvotes: -4

Efrain
Efrain

Reputation: 3364

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.

Upvotes: 5

VitalyB
VitalyB

Reputation: 12855

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

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

Upvotes: 9

JaredPar
JaredPar

Reputation: 755587

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

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

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

Upvotes: 22

Marc Gravell
Marc Gravell

Reputation: 1064254

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!

Upvotes: 22

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