Better conversion from collection expression element
Note
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Summary
Updates to the better conversion rules to be more consistent with params
, and better handle current ambiguity scenarios. For example, ReadOnlySpan<string>
vs ReadOnlySpan<object>
can currently
cause ambiguities during overload resolution for [""]
.
Detailed Design
The following are the better conversion from expression rules. These replace the rules in https://github.com/dotnet/csharplang/blob/main/proposals/csharp-12.0/collection-expressions.md#overload-resolution.
These rules are:
Given an implicit conversion
C₁
that converts from an expressionE
to a typeT₁
, and an implicit conversionC₂
that converts from an expressionE
to a typeT₂
,C₁
is a better conversion thanC₂
if one of the following holds:E
is a collection expression, andC₁
is a better collection conversion from expression, or
E
is not a collection expression and one of the following holds:
E
exactly matchesT₁
andE
does not exactly matchT₂
E
exactly matches both or neither ofT₁
andT₂
, andT₁
is a better conversion target thanT₂
E
is a method group, ...
We add a new definition for better collection conversion from expression, as follows:
Given:
E
is a collection expression with element expressions[EL₁, EL₂, ..., ELₙ]
T₁
andT₂
are collection typesE₁
is the element type ofT₁
E₂
is the element type ofT₂
CE₁ᵢ
are the series of conversions fromELᵢ
toE₁
CE₂ᵢ
are the series of conversions fromELᵢ
toE₂
If there is an identity conversion from E₁
to E₂
, then the element conversions are as good as each other. Otherwise, the element conversions to E₁
are better than the element conversions to E₂
if:
- For every
ELᵢ
,CE₁ᵢ
is at least as good asCE₂ᵢ
, and - There is at least one i where
CE₁ᵢ
is better thanCE₂ᵢ
Otherwise, neither set of element conversions is better than the other, and they are also not as good as each other.
Conversion comparisons are made using better conversion from expression ifELᵢ
is not a spread element. IfELᵢ
is a spread element, we use better conversion from the element type of the spread collection toE₁
orE₂
, respectively.
C₁
is a better collection conversion from expression than C₂
if:
- Both
T₁
andT₂
are not span types, andT₁
is implicitly convertible toT₂
, andT₂
is not implicitly convertible toT₁
, or E₁
does not have an identity conversion toE₂
, and the element conversions toE₁
are better than the element conversions toE₂
, orE₁
has an identity conversion toE₂
, and one of the following holds:T₁
isSystem.ReadOnlySpan<E₁>
, andT₂
isSystem.Span<E₂>
, orT₁
isSystem.ReadOnlySpan<E₁>
orSystem.Span<E₁>
, andT₂
is an array_or_array_interface with element typeE₂
Otherwise, neither collection type is better, and the result is ambiguous.
Note
These rules mean that methods that expose overloads that take different element types and without a conversion between the collection types are ambiguous for empty collection expressions. As an example:
public void M(ReadOnlySpan<int> ros) { ... }
public void M(Span<int?> span) { ... }
M([]); // Ambiguous
Scenarios:
In plain English, the collection types themselves must be either the same, or unambiguously better (ie, List<T>
and List<T>
are the same, List<T>
is unambiguously better than IEnumerable<T>
, and List<T>
and HashSet<T>
cannot be compared), and
the element conversions for the better collection type must also be the same or better (ie, we can't decide between ReadOnlySpan<object>
and Span<string>
for [""]
, the user needs to make that decision). More examples of this are:
T₁ |
T₂ |
E |
C₁ Conversions |
C₂ Conversions |
CE₁ᵢ vs CE₂ᵢ |
Outcome |
---|---|---|---|---|---|---|
List<int> |
List<byte> |
[1, 2, 3] |
[Identity, Identity, Identity] |
[Implicit Constant, Implicit Constant, Implicit Constant] |
CE₁ᵢ is better |
List<int> is picked |
List<int> |
List<byte> |
[(int)1, (byte)2] |
[Identity, Implicit Numeric] |
Not applicable | T₂ is not applicable |
List<int> is picked |
List<int> |
List<byte> |
[1, (byte)2] |
[Identity, Implicit Numeric] |
[Implicit Constant, Identity] |
Neither is better | Ambiguous |
List<int> |
List<byte> |
[(byte)1, (byte)2] |
[Implicit Numeric, Implicit Numeric] |
[Identity, Identity] |
CE₂ᵢ is better |
List<byte> is picked |
List<int?> |
List<long> |
[1, 2, 3] |
[Implicit Nullable, Implicit Nullable, Implicit Nullable] |
[Implicit Numeric, Implicit Numeric, Implicit Numeric] |
Neither is better | Ambiguous |
List<int?> |
List<ulong> |
[1, 2, 3] |
[Implicit Nullable, Implicit Nullable, Implicit Nullable] |
[Implicit Numeric, Implicit Numeric, Implicit Numeric] |
CE₁ᵢ is better |
List<int?> is picked |
List<short> |
List<long> |
[1, 2, 3] |
[Implicit Numeric, Implicit Numeric, Implicit Numeric] |
[Implicit Numeric, Implicit Numeric, Implicit Numeric] |
CE₁ᵢ is better |
List<short> is picked |
IEnumerable<int> |
List<byte> |
[1, 2, 3] |
[Identity, Identity, Identity] |
[Implicit Constant, Implicit Constant, Implicit Constant] |
CE₁ᵢ is better |
IEnumerable<int> is picked |
IEnumerable<int> |
List<byte> |
[(byte)1, (byte)2] |
[Implicit Numeric, Implicit Numeric] |
[Identity, Identity] |
CE₂ᵢ is better |
List<byte> is picked |
int[] |
List<byte> |
[1, 2, 3] |
[Identity, Identity, Identity] |
[Implicit Constant, Implicit Constant, Implicit Constant] |
CE₁ᵢ is better |
int[] is picked |
ReadOnlySpan<string> |
ReadOnlySpan<object> |
["", "", ""] |
[Identity, Identity, Identity] |
[Implicit Reference, Implicit Reference, Implicit Reference] |
CE₁ᵢ is better |
ReadOnlySpan<string> is picked |
ReadOnlySpan<string> |
ReadOnlySpan<object> |
["", new object()] |
Not applicable | [Implicit Reference, Identity] |
T₁ is not applicable |
ReadOnlySpan<object> is picked |
ReadOnlySpan<object> |
Span<string> |
["", ""] |
[Implicit Reference] |
[Identity] |
CE₂ᵢ is better |
Span<string> is picked |
ReadOnlySpan<object> |
Span<string> |
[new object()] |
[Identity] |
Not applicable | T₁ is not applicable |
ReadOnlySpan<object> is picked |
ReadOnlySpan<InterpolatedStringHandler> |
ReadOnlySpan<string> |
[$"{1}"] |
[Interpolated String Handler] |
[Identity] |
CE₁ᵢ is better |
ReadOnlySpan<InterpolatedStringHandler> is picked |
ReadOnlySpan<InterpolatedStringHandler> |
ReadOnlySpan<string> |
[$"{"blah"}"] |
[Interpolated String Handler] |
[Identity] - But constant |
CE₂ᵢ is better |
ReadOnlySpan<string> is picked |
ReadOnlySpan<string> |
ReadOnlySpan<FormattableString> |
[$"{1}"] |
[Identity] |
[Interpolated String] |
CE₂ᵢ is better |
ReadOnlySpan<string> is picked |
ReadOnlySpan<string> |
ReadOnlySpan<FormattableString> |
[$"{1}", (FormattableString)null] |
Not applicable | [Interpolated String, Identity] |
T₁ isn't applicable |
ReadOnlySpan<FormattableString> is picked |
HashSet<short> |
Span<long> |
[1, 2] |
[Implicit Constant, Implicit Constant] |
[Implicit Numeric, Implicit Numeric] |
CE₁ᵢ is better |
HashSet<short> is picked |
HashSet<long> |
Span<short> |
[1, 2] |
[Implicit Numeric, Implicit Numeric] |
[Implicit Constant, Implicit Constant] |
CE₂ᵢ is better |
Span<short> is picked |
Open questions
How far should we prioritize ReadOnlySpan
/Span
over other types?
As specified today, the following overloads would be ambiguous:
C.M1(["Hello world"]); // Ambiguous, no tiebreak between ROS and List
C.M2(["Hello world"]); // Ambiguous, no tiebreak between Span and List
C.M3(["Hello world"]); // Ambiguous, no tiebreak between ROS and MyList.
C.M4(["Hello", "Hello"]); // Ambiguous, no tiebreak between ROS and HashSet. Created collections have different contents
class C
{
public static void M1(ReadOnlySpan<string> ros) {}
public static void M1(List<string> list) {}
public static void M2(Span<string> ros) {}
public static void M2(List<string> list) {}
public static void M3(ReadOnlySpan<string> ros) {}
public static void M3(MyList<string> list) {}
public static void M4(ReadOnlySpan<string> ros) {}
public static void M4(HashSet<string> hashset) {}
}
class MyList<T> : List<T> {}
How far do we want to go here? The List<T>
variant seems reasonable, and subtypes of List<T>
exist aplenty. But the HashSet
version has very different semantics, how sure are we that it's actually "worse"
than ReadOnlySpan
in this API?
C# feature specifications