Error when using extension methods in C#

Question

I came across an issue that makes me think there is bug in the 3.0 framework. When I try to use extension methods I get the following error:

Missing compiler required member
'System.Runtime.CompilerServices.ExtensionAttribute..ctor'

When using this simple code:

public static class StringUtils {
    static void TestExtension(this String targetString) {

    }
}

The only way to make this compile error go away is to add the following code:

namespace System.Runtime.CompilerServices {
    public class ExtensionAttribute : Attribute { }
}

It's been a few months since I have used extensions methods, but I'm pretty sure I didn't have to do this. Has anyone else come across this issue?

Baziotes

I have the target framework set to 3.5 in the project properties. – Baziotes

In which case, I wonder if one of the assemblies you reference also declare this attribute (courtesy of being upgraded from .NET 2.0 with C# 3.0), and the compiler is having a hard time picking which one to use? Do you get this problem in a vanilla (clean) project with just the StringUtils etc? – Scott

(meaning: not System.Core.dll, and perhaps with an internal constructor) – Scott

Related: Catch-22 with self-defined ExtensionAttribute – Covarrubias

This error occurs if you need to target an old Framework version. Not sure how old, but I got it when I had to target .Net Framework 2.0. The above solution worked. – Pejsach

Answer 1

I just ran into this problem myself. In my case, it was because I converted a VS 2005/.Net 2.0 project to a VS 2008/.Net 3.5 project. The conversion tool kept references to System.Core 2.0, and I couldn't find an easy way to change the references to System.Core 3.5.

I ended up re-creating the project in VS 2008 from scratch, and it was created with proper references to System.Core 3.5

Viscoid

Answer 2

I have the exact same problem. The error System.Runtime.CompilerServices.ExtensionAttribute..ctor is rather cryptic, and could mean a number of different things.

However, for me It boiled down to the fact that I'm using Newtonsoft.Json.Net. I removed the reference to the file Newtonsoft.Json.Net20.dll, and the re-added it. After this my solution builds again.

The strangest thing is that when I tried to find out what was different after this procedure by using Subversion Diff, nothing appears to have changed.

So I really don't know what removing and re-adding this reference really does, but it does fix my build issue with this particular error message mentioned by the asker.

UPDATE 1:

For those that come across this again, as the comenters pointed out, the proper way to fix this is to Download Json.Net's ZIP, and there should be a 3.5 version, re-reference 3.5 every where you are using Json.Net and delete the old reference, as it is likely referencing an assembly that was built for older versions of .net.

UPDATE 2:

Charlie Flowers points out that the DLL NewtonSoft labels as being for 3.5 is actually not going to work with 3.5. You have to use the DLL they label as being for .net 2.0

Dymphia

If you are using Json.NET in a .NET 3.5 environment you should use the build for .NET 3.5 rather than 2.0 like you are currently. The 3.5 build doesn't include the ExtensionAttribute class. – Acarid

James You are right. I downloaded Json.Net's ZIP, and there was a 3.5 version. Thanks! – Dymphia

+1 I had exactly the same issue. As a side note to James Newton-King, I also use Json.NET 2.0 with .NET 3.5, because on fairly modern versions of mono (e.g. stock build from Ubuntu 9.10), json.net depends on some odd DLLs that aren't supported yet. So I compile it with the older version of json.net and it's been working fine for many months. – Annotate

+1 this was my problem too. I figured the newtonsoft.json.dll version of Json.NET was compiled against .net 1 – Selden

Man I just wasted an hour on this. Thanks for the answer. The dll newtonsoft labels as being for 3.5 is actually not going to work with 3.5. You have to use the dll they label as being for .net 2.0. – Gummous

@Charlie Flowers, Thanks for pointing that out too. I have updated the answer to give a heads-up to others in the same situation. – Dymphia

I got same error when I convert VS2008 project to VS2010 (still .NET 3,5). My project is using Momo.Cecil.dll. I end up with remove and readd that Mono.Cecil dll to my reference list. – Kant

Exactly the same problem for me too in a .NET 4 project. By Default the NUGET package references the 2.0 version. Just remove that and add the .net 4 dll from NuGet's packages folder – Positivism

NewtonSoft v2.0 doesn't work for me. Running .net 4.0 profile – Deflate

@Deflate As far as I know .Net 4.0 is not compatible with DLL files built for .Net 2.0 or 3.5 – Dymphia

Thats for that answer. I realised I was using the 2.0 version for JSON.net in my 3.5 project, deleting and updating the reference sovled my problem. – Greengage

Is there a solution that doesn't involve multiple assemblies? The ExtensionAttribute hack put me into a serious catch-22 situation... – Covarrubias

The solution is to manually compile project with this issue, because Visual Studio is stupid enough to add custom assemblies as references to compiler's command line (first Newtonsoft.Json.Net20.dll and then System.Core.dll) instead of put BCL assemblies first. – Superfuse

Hey, Thanks for your post...i'm also facing the same problem, when i removed Newtonsoft.Json.Net20.dll reference and reinserted it into project...my project built successfully. – Mylesmylitta

Chets: Yes, I've tried that several times (even restarted Visual Studio inbetween) - none of that works. Interesting is that this happened for me only when compiling Release version. – Superfuse

This should be the accepted answer. Newtonsoft.Json.Net20 was the culprit! – Plater

This issue was intermittent for me (closing the solution sometimes fixed it). I also got the warning The predefined type 'System.Runtime.CompilerServices.ExtensionAttribute' is defined in multiple assemblies in the global alias; using definition from 'd:\...\Newtonsoft.Json.dll'. Updating Newtonsoft to .NET 4.0 (which matched the rest of my solution) fixed the issue. – Muller

Answer 3

in VS, click Project (next to File,Edit,View), select Properties

then in Application tab (you'll notice you're already in 3.5), select the Target Framework to 2.0, then compile (it will error). then put it back again to 3.5, then compile again, the error will disappear

i think it is just a small glitch in Visual Studio, just fool the IDE :-)

Yokel

I had this problem and this was the only solution that worked for me (inside VS). – Lazybones

Definitely strange, but it worked for me as well! Can't argue with that, thank you! – Overbear

Answer 4

I had the same issue in a class library project that I had upgraded from VS 2008 to VS 2010 Beta 2. I hadn't added any extension methods to the project until after the upgrade, then I started seeing the same error.

Adding a class with the following code to the project solved the problem:

namespace System.Runtime.CompilerServices
{
    public class ExtensionAttribute : Attribute { }
}

Found the tip on this blog: http://blog.flexforcefive.com/?p=105

Lawton

The link is broken... Also, this hack puts you into a serious catch-22 situation – Covarrubias

Answer 5

Your framework isn't high enough for Extension Methods.
That's a hack for making extension methods work without being in 3.5

Gheber

Answer 6

What version of .NET are you targetting? The ExtensionAttribute class is shipped in System.Core.dll (.NET 3.5), but you can re-declare it yourself if you want to use extension methods in .NET 2.0/3.0 (with C# 3.0, obviously). In fact, LINQBridge does this.

[update] However, I'm slightly confused, because the error you should see is:

Error 1 Cannot define a new extension method because the compiler required type 'System.Runtime.CompilerServices.ExtensionAttribute' cannot be found. Are you missing a reference to System.Core.dll? [snipped some path stuff]

Scott

Answer 7

A missing System.Core reference will give these symptoms.

Kahaleel

Answer 8

For ongoing reference:

I have battled this exact same error message for the last two hours in the .Net 4.0 framework and finally tracked it down to a corrupted reference to the Microsoft.CSharp dll. It was such a frustratingly simple issue. I deleted the offending reference and added a "new" reference to that dll which was located at:

C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\.NETFramework\v4.0\Microsoft.CSharp.dll

Hopefully this will save someone else the time and aggravation of tracking down a solution to this error message.

Grith

This was the case for me too. However, I didn't have to find any files, I just had to "add reference" and search "csharp" under "assemblies". There, I was able to select the mentioned reference and everything worked fine from then on. – Zing

Answer 9

This problem is indeed caused by an incorrect reference to version 2 of System.Core . This is normally caused when upgrading from an earlier version of .NET to .NET 3.5 . If it is a website that you are experiencing this problem in then it can be remedied by following the steps below:

1) In web.config add a reference to System.Core v3.5:

<assemblies>
   <add assembly="System.Core, Version=3.5.0.0, Culture=neutral, PublicKeyToken=B77A5C561934E089"/>
</assemblies>

2) In web.config add the following as a child of configuration:

<configuration>
   <!--Some other config-->
   <assemblyBinding xmlns="urn:schemas-microsoft-com:asm.v1">
      <dependentAssembly>
         <assemblyIdentity name="System.Core" publicKeyToken="B77A5C561934E089"/>
         <bindingRedirect oldVersion="2.0.0.0-2.1.0.0" newVersion="3.5.0.0"/>
      </dependentAssembly>
   </assemblyBinding>
</configuration>

Abdullah

Answer 10

Extensions are introduced in C# 3.0, which on the other hand was introduced in .NET 3.5 so you can't really use them in .NET 3.0 directly.

Kentigerma

Answer 11

Is this a web site project, by chance? Try changing the target framework from .NET 3.5 to an earlier version, and then back to .NET 3.5.

Comintern

Answer 12

Try: Project, Add Reference, find System Core 3.5.0.0 in the list, and OK to add it.

Metaxylem

Answer 13

Just target your VS project to .NET framework 3.5 and above. Most probably you converted your project from a previous version.

Munitions

Answer 14

I had a version of Elmah.Sandbox that was targetting .net 2.0 in an existing .Net 4.0 project. I added an extension method to my project and the build failed with the "Missing compiler required member" error message.

The Elmah.Sandbox dll was included as part of an early version of ElmahR. Once this was removed, it all built again.

Tamer

Answer 15

In my case the reason was the wrong NET20 version of Microsoft's AntiXSSLibrary.dll. Replaced with NET35 - the error is gone.

Protrude

Answer 16

Edit your csproj and make sure those references are included (it wont work by simple 'add reference'...):

<Reference Include="System" />
<Reference Include="System.Core" />
<Reference Include="System.Xml.Linq" />
<Reference Include="System.Data.DataSetExtensions" />
<Reference Include="Microsoft.CSharp" />
<Reference Include="System.Data" />
<Reference Include="System.Net.Http" />
<Reference Include="System.Xml" />

Roughish