Resolving extension methods/LINQ ambiguity

This is probably one of those rare cases where it makes sense to use an extern alias.

In the properties page for the reference to System.Core (i.e. under References, select System.Core, right-click and select "Properties"), change the "Aliases" value to "global,SystemCore" (or just "SystemCore" if it's blank to start with).

Then in your code, write:

extern alias SystemCore;
using SystemCore::System.Linq;

That will make all the relevant types etc in System.Core.dll's System.Linq namespace available. The name "SystemCore" here is arbitrary - you could call it "DotNet" or something else if that would make it clearer for you.

This isn't really an answer, but may provide an easier way for others to reproduce the issue (from the command-line - you could do it with two projects in Visual Studio if you want).

1) Create BadLinq.cs and build it as BadLinq.dll:

using System.Collections.Generic;

namespace System.Linq
{
    public static class Enumerable
    {
        public static IEnumerable<T> Where<T>(this IEnumerable<T> source, 
                                              Func<T,bool> predicate)
        {
            return null;
        }
    }
}

2) Create Test.cs:

extern alias SystemCore;

using System;
using SystemCore::System.Linq;

static class Test
{
    static void Main()
    {
        var names = new[] { "Larry", "Curly", "Moe" };

        var result = names.Where(x => x.Length > 1);
    }
}

3) Compile Test.cs specifying the extern alias:

csc Test.cs /r:BadLinq.dll /r:SystemCore=System.Core.dll

This fails with:

Test.cs(11,28): error CS1061: 'System.Array' does not contain a definition for 'Where' and no extension method 'Where' accepting a first argument of type 'System.Array' could be found (are you missing a using directive or an assembly reference?)

If you change it to not try to use an extension method (i.e. Enumerable.Where) it works fine with the extern alias.

I think this may be a compiler bug. I've emailed a private mailing list which the C# team reads - I'll update this answer or add a new one when I hear back.

This is no longer an issue, since I am able to use the LINQ extensions, as provided by ReSharper DLL files, even while targeting .NET 3.0.

Mr. Skeet was right again! I am able to use full LINQ syntax, while targeting .NET 3.0 in the project's properties and not referencing System.Core!

In order for ReSharper to be as compatible as possible with the variety of solutions it is used with, it is built against .NET 2.0. LINQ, etc. came in in C# 3.0, so they are not available in that version of the Framework. So, JetBrains added in their own version.

The solution is to build your addin against .NET 2.0 as well.

I had the ambiguous reference problem using System.ComponentModel. Visual Studio was complaining that a DLL file exists in both v2 and v4. I was able to resolve it by removing the reference to the System DLL file and readding it.

Resolving extension methods/LINQ ambiguity

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