Resolving extension methods/LINQ ambiguity

Question

I'm writing an add-in for ReSharper 4. For this, I needed to reference several of ReSharper's assemblies. One of the assemblies (JetBrains.Platform.ReSharper.Util.dll) contains a System.Linq namespace, with a subset of extension methods already provided by System.Core.

When I edit the code, it creates an ambiguity between those extensions, so that I cannot use OrderBy, for instance. How could I solve this? I would like to use the core LINQ extensions, and not the ones from ReSharper.

I get the following error when trying to compile:

The call is ambiguous between the following methods or properties: 'System.Linq.Enumerable.OrderBy<string,int>(System.Collections.Generic.IEnumerable<string>, System.Func<string,int>)' and 'System.Linq.Enumerable.OrderBy<string,int>(System.Collections.Generic.IEnumerable<string>, System.Func<string,int>)'

EDIT: I tried the suggestion below, unfortunately without luck. In the meanwhile, I "solved" the problem by removing references to System.Core. This way I could use the extensions provided by ReSharper DLL files.

I uploaded a sample program where I just imported the ReSharper DLL files I needed. I changed the alias of System.Core to SystemCore, added the extern alias directive, but it still didn't work. If I missed something, please let me know. P.S. The references are to ReSharper v4.1 DLL files installed in the default directroy in "C:\Program Files\JetBrains\ReSharper\v4.1\...".

Answer 1

It is really a compiler error.

I had the same problem, and I solved it just by cleaning and rebuilding the project. After that the problem disappeared.

Answer 2

I found this same sort of ambiguity when use PagedList in MVC (.Net 4.5, MVC 5). I found that if I took the object for the argument that was ambiguous and cast it explicitly first, the problem was resolved. To if the ambiguity was between a method that takes System.Linq.Enumerable and one that takes System.Collections.Generic.IEnumerable as the parameter in question, and the source is of type System.Collections.Generic.IEnumerable, I don't use the extension method on it. I cast it.In this example, my repository method returns a List:

searchRequest.CaseSearchResults = csr.SelectMatchingCases(searchRequest);
var results = searchRequest.CaseSearchResults.AsEnumerable<CaseSearchResult>();
int pageNum = (int)(ViewBag.PageNum ?? 1);
var pageResults =results.ToPagedList<CaseSearchResult>(pageNum, 5);

Calling the extension method on searchRequest.CaseSearchResults caused the ambiguity error; explicitly casting to results and then calling the extension on that worked.

Answer 3

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.

Answer 4

I had the same problem, even with extern alias, and I raised it as a compiler bug on Connect. The workaround for the time being is to forgo the extension method syntax.

The bug is fixed for Visual Studio 2010.