[Proposal] Extension GetEnumerator (VS 16.8, .NET 5)

[333fred]

Extension GetEnumerator Recognition in foreach

• Proposed
• Prototype: N/A
• Implementation: Implemented
• Specification: Proposal Checked In

Discussion in #600

[jshall]

I see 2 pitfalls here:

• This would mean that a using directive could change the meaning of foreach in that C# file
• If there are multiple GetEnumerator extension methods with the same signature coming from different namespaces the compiler has an ambiguity to resolve that may not have existed previously

I would suggest that something like using foreach MyNamespace.MyExtensions; be required to trigger this change in foreach behavior. In this case MyNamespace.MyExtensions is the class that defines the relevant GetEnumerator or GetEnumerator<> methods.

[HaloFour]

@jshall

• This would mean that a using directive could change the meaning of foreach in that C# file

An extension GetEnumerator would only be considered last, so a using directive couldn't change the meaning of existing C# code.

• If there are multiple GetEnumerator extension methods with the same signature coming from different namespaces the compiler has an ambiguity to resolve that may not have existed previously

It's possible that you could import two namespaces that each have their own extension GetEnumerator, but as with extension methods that would result in a compiler error.

A number of existing language features, like await and deconstruction, already support being implemented by extension methods, so having to handle ambiguities in these cases is not new.

[YairHalberstadt]

Link to design meeting notes: https://github.com/dotnet/csharplang/blob/6e748b19f1076cc7109293f1038e6042e4ac7f06/meetings/2020/LDM-2020-03-23.md#allow-getenumerator-from-extension-methods

[YairHalberstadt]

The collection type is X, the enumerator type is E, and the element type is the type of the Current property.

Should the collection type be the type of the extension method parameter?

[leandromoh]

why simple dont use a GetEnumerable extension method to transform the currenty object into an enumerable (which has itself a GetEnumeator) ?

foreach(var item in source.GetEnumerable())

[quinmars]

why simple dont use a GetEnumerable extension method to transform the currenty object into an enumerable (which has itself a GetEnumeator) ?

Because

foreach (var i in 1..10)

is more appealing than:

foreach (var i in 1..10.ToEnumerable())

[333fred]

The actual spec text is now checked in, so I'll remove it from this issue.

[333fred]

This was shipped in C# 9. It still needs to be incorporated into the ECMA specification, which is what the Implemented Needs EMCA Spec tag is indicated.