Enumerable.Single and Enumerable.SingleOrDefault overloaded implementations are inconsistent

[rverdier]

There is an inconsistency between the implementations of Enumerable.Single() and Enumerable.Single(predicate). Similarly there is another similar inconsistency between the implementations of Enumerable.SingleOrDefault() and Enumerable.SingleOrDefault(predicate)

The overloads which accept a predicate enumerate the whole sequence before eventually throwing, even if more than one matching elements were already found:

For consistency and performance reasons, it may be a good idea to make Enumerable.Single(predicate) and Enumerable.SingleOrDefault(predicate) methods able to fail fast and to throw an InvalidOperationException as soon as a second matching element is found.

If that seems like a reasonable change, I would gladly submit a pull request.

[MarcinJuraszek]

There is a really old Connect report about that by Jon Skeet that was _Won't Fix_ed: https://connect.microsoft.com/VisualStudio/feedback/details/639955

[rverdier]

Hello Marcin,

I found that Connect issue, as well as this one:

https://connect.microsoft.com/VisualStudio/feedback/details/810457

In the issue you mention, the reason why it was marked as "Won't fix" seems to be related to bug triage/schedule/priority. The answer even states (in 2011) that this issue will be re-opened for a future release. So I thought that in 2015, given the context – i.e. CoreFx being open-source, Microsoft accepting pull requests, etc. – it could be a good thing to fix this minor issue.

[ellismg]

@AlekseyTs fixing this to early out seems reasonable, what do you think?

[herecydev]

👍

[JonHanna]

This is a subset of dotnet/corefx#2349, and as such was fixed by dotnet/corefx#2350.