Extension GetEnumerator support for foreach loops.

Summary

Allow foreach loops to recognize an extension method GetEnumerator method that otherwise satisfies the foreach pattern, and loop over the expression when it would otherwise be an error.

Motivation

This will bring foreach inline with how other features in C# are implemented, including async and pattern-based deconstruction.

Detailed design

The spec change is relatively straightforward. We modify The foreach statement section to this text:

The compile-time processing of a foreach statement first determines the collection type, enumerator type and element type of the expression. This determination proceeds as follows:

  • If the type X of expression is an array type then there is an implicit reference conversion from X to the IEnumerable interface (since System.Array implements this interface). The collection type is the IEnumerable interface, the enumerator type is the IEnumerator interface and the element type is the element type of the array type X.

  • If the type X of expression is dynamic then there is an implicit conversion from expression to the IEnumerable interface (Implicit dynamic conversions). The collection type is the IEnumerable interface and the enumerator type is the IEnumerator interface. If the var identifier is given as the local_variable_type then the element type is dynamic, otherwise it is object.

  • Otherwise, determine whether the type X has an appropriate GetEnumerator method:

    • Perform member lookup on the type X with identifier GetEnumerator and no type arguments. If the member lookup does not produce a match, or it produces an ambiguity, or produces a match that is not a method group, check for an enumerable interface as described below. It is recommended that a warning be issued if member lookup produces anything except a method group or no match.
    • Perform overload resolution using the resulting method group and an empty argument list. If overload resolution results in no applicable methods, results in an ambiguity, or results in a single best method but that method is either static or not public, check for an enumerable interface as described below. It is recommended that a warning be issued if overload resolution produces anything except an unambiguous public instance method or no applicable methods.
    • If the return type E of the GetEnumerator method is not a class, struct or interface type, an error is produced and no further steps are taken.
    • Member lookup is performed on E with the identifier Current and no type arguments. If the member lookup produces no match, the result is an error, or the result is anything except a public instance property that permits reading, an error is produced and no further steps are taken.
    • Member lookup is performed on E with the identifier MoveNext and no type arguments. If the member lookup produces no match, the result is an error, or the result is anything except a method group, an error is produced and no further steps are taken.
    • Overload resolution is performed on the method group with an empty argument list. If overload resolution results in no applicable methods, results in an ambiguity, or results in a single best method but that method is either static or not public, or its return type is not bool, an error is produced and no further steps are taken.
    • The collection type is X, the enumerator type is E, and the element type is the type of the Current property.
  • Otherwise, check for an enumerable interface:

    • If among all the types Ti for which there is an implicit conversion from X to IEnumerable<Ti>, there is a unique type T such that T is not dynamic and for all the other Ti there is an implicit conversion from IEnumerable<T> to IEnumerable<Ti>, then the collection type is the interface IEnumerable<T>, the enumerator type is the interface IEnumerator<T>, and the element type is T.
    • Otherwise, if there is more than one such type T, then an error is produced and no further steps are taken.
    • Otherwise, if there is an implicit conversion from X to the System.Collections.IEnumerable interface, then the collection type is this interface, the enumerator type is the interface System.Collections.IEnumerator, and the element type is object.
  • Otherwise, determine whether the type 'X' has an appropriate GetEnumerator extension method:

    • Perform extension method lookup on the type X with identifier GetEnumerator. If the member lookup does not produce a match, or it produces an ambiguity, or produces a match which is not a method group, an error is produced and no further steps are taken. It is recommended that a warning be issues if member lookup produces anything except a method group or no match.
    • Perform overload resolution using the resulting method group and a single argument of type X. If overload resolution produces no applicable methods, results in an ambiguity, or results in a single best method but that method is not accessible, an error is produced an no further steps are taken.
      • This resolution permits the first argument to be passed by ref if X is a struct type, and the ref kind is in.
    • If the return type E of the GetEnumerator method is not a class, struct or interface type, an error is produced and no further steps are taken.
    • Member lookup is performed on E with the identifier Current and no type arguments. If the member lookup produces no match, the result is an error, or the result is anything except a public instance property that permits reading, an error is produced and no further steps are taken.
    • Member lookup is performed on E with the identifier MoveNext and no type arguments. If the member lookup produces no match, the result is an error, or the result is anything except a method group, an error is produced and no further steps are taken.
    • Overload resolution is performed on the method group with an empty argument list. If overload resolution results in no applicable methods, results in an ambiguity, or results in a single best method but that method is either static or not public, or its return type is not bool, an error is produced and no further steps are taken.
    • The collection type is X, the enumerator type is E, and the element type is the type of the Current property.
  • Otherwise, an error is produced and no further steps are taken.

For await foreach, the rules are similarly modified. The only change that is required to that spec is removing the Extension methods do not contribute. line from the description, as the rest of that spec is based on the above rules with different names substituted for the pattern methods.

Drawbacks

Every change adds additional complexity to the language, and this potentially allows things that weren't designed to be foreached to be foreached, like Range.

Alternatives

Doing nothing.

Unresolved questions

None at this point.