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.

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.

• 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.

Answer 1

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!

Answer 2

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.

Answer 3

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

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

Answer 4

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.

Answer 5

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

Answer 6

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.