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[Proposal]: Dictionary expressions #7822
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As noted by open question 2, I think losing support for |
Would an extension method not suffice here? var dictionary = [ "foo": "bar", "fizz": "buzz" ].WithComparer(StringComparer.OrdinalIgnoreCase);
// or
var dictionary = StringComparer.OrdinalIgnoreCase.CreateDictionary([ "foo": "bar", "fizz": "buzz" ]); |
Would an extension method approach to specifying comparers be able work without creating a second dictionary? |
@GabeSchaffer Likely yes (though we would still need to get extensions working with collection exprs). Specifically, the keys/values in the expr woudl likely be passed to the extension method as a ReadOnlySpan of KeyValuePairs. These would normally just be on the stack, and would be given as a contiguous chunk of data for the dictionary to create its internal storage from once. |
Apologies for my lack of understanding, but it seems like a builder approach to augmentation could be made to work, like with That seems like it could make a syntax like this possible: var dictionary = Dictionary.Create(StringComparer.OrdinalIgnoreCase, [ "foo": "bar", "fizz": "buzz" ]); Another options that seems feasible is to allow constructor arguments to be specified in parens after the collection expression: [ "foo": "bar", "fizz": "buzz" ](StringComparer.OrdinalIgnoreCase) // function call syntax for passing ctor args An uglier possibility is to use a semicolon: [ "foo": "bar", "fizz": "buzz"; StringComparer.OrdinalIgnoreCase ] // ; delimits comparer I think if we want to have hope of being able to specify a comparer in a pattern, the latter two are better. |
Wow I had no idea the C# team were so young ;)
|
How about reusing the existing dictionary syntax? Given that we already have var dict = new Dictionary<K, V> { [key] = value } We can somehow use a similar syntax: var dict = { [key]: value } This is also matching how TypeScript defines an interface with indexed field:
And we can even support dictionary patterns along with existing patterns as well: var obj = new C();
if (obj is {
[string key]: 42,
["foo"]: int value,
SomeProp: float x
}) ... // matching an object which has a indexed value 42 while its key is string, and a key "foo" whose value is int
class C
{
public float SomeProp { get; }
public int this[string arg] => ...
} As well as the case where the indexer has multiple parameters: if (obj is {
["foo", "bar"]: 42
}) |
What about spreads, those will have to be iterated and placed on the stack before calling the extension method? Or is there a way to pass them in somehow? |
I don't see how that can be implemented in any way other than to copy every element into a contiguous array in order to create a |
I think dictionary-like objects that have only one indexer are so common that it's worth to consider special, more succinct, syntax that doesn't require unnecessary Actually, your proposed syntax is not far from what we can do now in C#: Dictionary<string, int> dict = new() {
["aaa"] = 1,
["bbb"] = 1
};
Actually I was kinda surprised that C# doesn't have pattern matching for indexers. This code doesn't work: Dictionary<string, int> dict = new() {
["aaa"] = 1,
["bbb"] = 2
};
if (dict is { ["aaa"] : 1 })
{
} But this feels looks like another "pattern matching improvements" feature, not strictly related to dictionary expressions we discuss in this thread. |
I suggest using object notation, like TypeScript. Like: {
Type = "Person",
Name = "John"
} |
Can we make the "add/overwrite" be switchable? Say, we default to the safe "add" approach, and if overwrite is desired: var d = [(k1: v1)!, (..d1)!]; Here, PS. In addition to the above, if it should be "overwrite" for the whole list: var d = ([k1: v1, ..d1])!; |
That syntax looks a bit weird. If an "or overwrite" operation was added, I'd prefer to augment the IDictionary<string, string> d = GetDictionary();
(string k, string v) = GetNewKVP();
IDictionary<string, string> d2 = d with { [k1]: v1 }; But maybe that would be confusing, as one would have to remember that spreading is adding and |
@colejohnson66 with your approach, you would always have to create a temp dictionary variable to then "overwrite" things onto it to produce the final desired result, which effectively changes it from a "collection expression" to a "procedural code block". |
You can do this today Dictionary<string, int> _nameToAge = new(OptionalComparer) {
["Cyrus"] = 21,
["Dustin"] = 22,
["Mads"] = 23,
}; or just I'd rather see dictionary/indexer patterns to complete the matching side of the existing var x = new Dictionary() { .. }; There's endless optimization possibilities for collection expressions, not so much with dictionaries, and spreads with maps are just confusing or so rare at best. Having said that, I think immutable dictionaries could use some less awkward initialization API, if possible. |
I would like to add my take on the IEqualtiyComparer argument: var dict =
(new SomeEqualityComparer())
[
// Whatever syntax we decide on here
["Cyrus"] = 21,
["Dustin"] = 22,
["Mads"] = 23,
] This could also work for constructor parameters on other types, if that is found necessary. |
What about using the JSON-like syntax for declaration: private readonly Dictionary<string, int> _nameToAge =
{
"Cyrus": 21,
"Dustin": 22,
"Mads": 23
}; But for pattern matching, use the indexer syntax suggested by // Match a KV pair in the dictionary.
if (_nameToAge is { ["Cyrus": 69] })
// Match a property of the dictionary.
if (_nameToAge is { Length: 0 }) |
@glen-84 One of the goals of collection expressions were parity with pattern matching, so this (hopefully) won't happen. |
This is probably as close to parity as you'll get when going with this JSON-like syntax. I think it's intuitive – you're matching elements of the dictionary, not the dictionary itself. Otherwise it's back to option 1. 🙂 |
At least when it comes to expressions for creating a dictionary, in order to avoid the confusion of having too many options (especially ones that are almost the same), I think it would help to stay close to this existing dictionary initializer syntax: new Dictionary<string, int>()
{
["Cyrus"] = 21,
["Dustin"] = 22,
["Mads"] = 23,
}; For example: [
["Cyrus"] = 21,
["Dustin"] = 22,
["Mads"] = 23,
] This avoids the confusion of yet another style, and it separates key and value more clearly than the other existing collection initializer syntax: new Dictionary<string, int>()
{
// Meh for complex keys/values, especially when their expressions have commas and braces of their own
{ 111, new StudentName() { FirstName="Sachin", LastName="Karnik", ID=211 } },
{ 112, new StudentName() { FirstName="Dina", LastName="Salimzianova", ID=317 } },
{ 113, new StudentName() { FirstName="Andy", LastName="Ruth", ID=198 } },
}; |
This adds a lot of annoying extra characters to type, and tries to address a non-existent goal - to maintain similarity to existing approaches. Collection expressions (including dictionary expressions) are meant to come up with a standardized new syntax for all (most) collections, not try to continue older approaches. With respect to the syntax above, you can already do: new()
{
["Cyrus"] = 21,
["Dustin"] = 22,
["Mads"] = 23,
}; |
Well, it's the same thing for lists and arrays. I can also currently do: new() {1, 2, 3} // generates whatever collection target type is
new[] {1, 2, 3} // generates an array But collection expressions were still added. |
Just clarifying, which of these would we allow in pattern matching: [
// Type validation and accessing by value
[string name] = 21,
// Discarding
[var _] = 22,
// Accessing unknown values
["Mads"] = var age
] |
It saves typing even in the simple |
This syntax is well known. We can create a standard without reinventing the wheel. It's not a rule that we cant reuse old syntax. |
Personally, I never use the
There is no rule that we can't reuse old syntax, but there is zero reason to do so, and the old syntax is more typing. So, if it's worse than the proposed new one, and we are under no obligation to reuse old syntax, why reuse it? |
Another thought - perhaps the |
@TahirAhmadov What makes it worse? If you have autocomplete, it's 1 extra character you have to type, 2 without. It's not the end of the world. It also conveys how it really works with the override semantics, which is far more useful than saving typing. And again, it doesn't have to replace the current to create a standard. We do not have to reinvent the wheel. ["Cyrus"] = 21 is just as intuitive. |
@Ai-N3rd hmm that's incorrect. |
That being said, you make a good point about explicit overwrite sementics. As for spreads, perhaps we could do something like this: [
// Overwrite
[..someDict],
// Overwrite
["Cyrus"] = 21,
// Add
..differentDict,
// Add
"Dustin": 22
] This would be more confusing and less safe, but I can see the idea. |
That's actually another reason to avoid the |
As for the entire dictionary having overwrite semantics, that could look like this: var dict =
[[
// Still has overwrite semantics
"Cyrus": 21,
"Dustin": 22,
"Mads": 23,
]] This would allow people to choose to use this syntax with overwrite semantics, although might again, be somewhat confusing. I just threw something up as an idea. |
We need other people to add their opinions, however. We can't decide between the two of us. |
This is already legal Syntax today. |
@CyrusNajmabadi It can't compile though, because those can't be lvalues, so on that basis wouldn't we be free to change the syntax model for that pattern? |
That's something we could consider :) |
I actually like that syntax quite a bit. It's a mix of the new collection expression syntax with the existing KVP syntax from dictionary initializers. |
I'm proposing that reducing dissimilar syntaxes is a valuable additional goal. Options for avoiding the added complexity of many different syntaxes are worth discussing. More details below.
This does add three characters (
Exactly - you can, and you may occasionally do so, such as when you want to use This is especially true because you won't always want to use a collection initializer, as the behaviors differ. A collection initializer uses the default constructor and then repeatedly calls
There are plenty of people still using collection initializers. I do, because I prefer to be super consistent in using var items = new[] { 1, 2, 3, };
Item[] items = [1, 2, 3, ]; // Inconsistent with all my other local variable declarations There are more factors at play that determine which option is preferred. Collection initializers are definitely still commonly used.
In and of itself, that's great, but there is also the bigger picture to consider. The learning curve to C# is already way steeper than it was 10 years ago. The greater the number of subtly different options available, the greater the cognitive load. Considering the fact that collection initializers still have a place, as discussed above, there is value in reuse. The Rather than disregarded, the value of reuse should be weighed against the cost (the three extra characters). |
@Timovzl I just read the OP again and the primary motivating scenario for "overwrite" semantics is the ability to do I hope they don't decide to go with "overwrite" by default, though. |
Interesting consideration, overwrite vs. add. Possibly this one proposed use case pushes the balance somewhat in favor of overwrite by default?
I like that feature. For that one, with dictionaries being what they are, I'd expect values from What are the arguments against overwrite by default? Only accidental duplicates in manually added entries, or more? |
Would this have some kind of |
I recently noticed that Dictionary<string,string> d1 = [new: (comparer), "a": "b"];
FrozenDictionary<string,string> d2 = [new: (comparer, true), "a": "b"];
// maybe use "init" so that it's clear we're not calling the collection's ctor?
FrozenDictionary<string,string> d2 = [init: (comparer, true), "a": "b"]; |
Can you link to what you're referring to? Ah. Found it: dotnet/runtime#81194 |
@CyrusNajmabadi PS. Posted these links then noticed you edited your comment. |
This looks quite pleasing and clear. |
This allows us to introduce the init syntax to other collections as well. Ex. A HashSet |
In the "Motivation" section , I can see Because collections expressions are probably often used to represent some kind of static data, it would be good, to also support |
@wertzui I wouldn't be worried about that; they're even considering supporting simply |
Dictionary Expressions
Summary
Collection Expressions were added in C# 12. They enabled a lightweight syntax
[e1, e2, e3, .. c1]
to create many types of linearly sequenced collections, with similar construction semantics to the existing[p1, p2, .. p3]
pattern matching construct.The original plan for collection expressions was for them to support "dictionary" types and values as well. However, that was pulled out of C# 12 for timing reasons. For C# 13 we would to bring back that support, with an initial suggested syntax of
[k1: v1, k2: v2, .. d1]
. As a simple example:The expectation here would be that this support would closely track the design of collection expressions, with just additions to that specification to support this
k:v
element syntax, and to support dictionary-like types as the targets of these expressions.Motivation
Collection-like values are hugely present in programming, algorithms, and especially in the C#/.NET ecosystem. Nearly all programs will utilize these values to store data and send or receive data from other components. Currently, almost all C# programs must use many different and unfortunately verbose approaches to create instances of such values.
An examination of the ecosystem (public repos, nuget packages, and private codebases we have access to), indicate that dictionaries are used as a collection type in APIs around 15% of the time. While not nearly as ever-present as the sequenced collections (like arrays, spans, lists, and so on), this is still substantial, and warrants the equivalently pleasant construction provided by collection-expressions.
Like with linear collections, there are numerous different types of dictionary-like types in the .net ecosystem. This includes, but is not limited to, simply constructed dictionaries like
Dictionary<TKey, TValue>
andConcurrentDictionary<TKey, TValue>
, but also interfaces likeIDictionary<TKey, TValue>
andIReadOnlyDictionary<TKey, TValue>
, as well as immutable versions like `ImmutableDictionary<TKey, TValue>. Supporting all these common forms is a goal for these expressions.Detailed design
Main specification here: https://github.com/dotnet/csharplang/blob/main/proposals/collection-expressions-next.md
The only grammar change to support these dictionary expressions is:
Spec clarifications
dictionary_element
instances will commonly be referred to ask1: v1
,k_n: v_n
, etc.Conversions
The following implicit collection literal conversions exist from a collection literal expression:
...
To a type that implements
System.Collections.IDictionary
where:System.Int32
and namecapacity
.Ei
:Ei
isdynamic
and there is an applicable indexer setter that can be invoked with twodynamic
arguments, orEi
is a typeSystem.Collections.Generic.KeyValuePair<Ki, Vi>
and there is an applicable indexer setter that can be invoked with two arguments of typesKi
andVi
.Ki:Vi
, there is an applicable indexer setter that can be invoked with two arguments of typesKi
andVi
.Si
:Si
isdynamic
and there is an applicable indexer setter that can be invoked with twodynamic
arguments, orSystem.Collections.Generic.KeyValuePair<Ki, Vi>
and there is an applicable indexer setter that can be invoked with two arguments of typesKi
andVi
.To an interface type
I<K, V>
whereSystem.Collections.Generic.Dictionary<TKey, TValue>
implementsI<TKey, TValue>
and where:Ei
, the type ofEi
isdynamic
, or the type ofEi
is a typeSystem.Collections.Generic.KeyValuePair<Ki, Vi>
and there is an implicit conversion fromKi
toK
and fromVi
toV
.Ki:Vi
there is an implicit conversion fromKi
toK
and fromVi
toV
.Si
, the iteration type ofSi
isdynamic
, or the iteration type isSystem.Collections.Generic.KeyValuePair<Ki, Vi>
and there is an implicit conversion fromKi
toK
and fromVi
toV
.Syntax ambiguities
dictionary_element
can be ambiguous with aconditional_expression
. For example:This could be interpreted as
expression_element
where theexpression
is aconditional_expression
(e.g.[ (a ? [b] : c) ]
). Or it could be interpreted as adictionary_element
"k: v"
wherea?[b]
isk
, andc
isv
.Alternative designs
Several discussions on this topic have indicated a lot of interest and enthusiasm around exploring how close this feature is syntactically (not semantically) to JSON. Specifically, while we are choosing
[k: v]
for dictionary literals, JSON chooses{ "key": value }
. As"key"
is already a legal C# expression, this means that[ "key": value ]
would be nearly identical to JSON (except for the use of brackets instead of braces). While it would make it so we would have two syntaxes for collection versus dictionary expressions, we should examine this space to determine if the benefits we could get here would make up for the drawbacks.Specifically, instead of reusing the
collection_expression
grammar, we would introduce:You could then write code like:
Or:
Or
The downside here is:
In order:
First, the above syntax already conflicts with
int[] a = { 1, 2, 3, 4 }
(an array initializer). However, we could trivially sidestep this by saying that if the initializer contained a spread_element or dictionary_element it was definitely a dictionary. If it did not (it only contains normal expressions, or is empty), then it will be interpreted depending on what the target type is.Second, this could definitely impact future work wanted in the language. For example, "block expressions" has long been something we've considered. Where one could have an expression-valued "block" that could allow statements and other complex constructs to be used where an expression is needed. That said, such future work is both speculative, and itself could take on some new syntax. For example, we could mandate that an expression block have syntax like
@{ ... }
.Third, the pattern form here presents probably the hardest challenges. We would very much like patterns and construction to have a syntactic symmetry (with patterns in the places of expressions). Such symmetry would motivate having a pattern syntax of
{ p1: p2 }
. However, this is already completely the completely legal pattern syntax for a property pattern. In other words, one can already writeif (d is { Length: 0 })
. Indeed, it was all the ambiguities with{ ... }
patterns in the first place that motivated us to use[ ... ]
for list patterns (and then collection-expressions). We will end up having to resolve these all again if we were to want this to work. It is potentially possible, but will likely be quite subtle with various pitfalls. Alternatively, we can come up with some new syntax for dictionary-patterns, but we would then break our symmetry goal.Regardless, even with the potential challenges above, there is a great attractiveness on aligning syntactically with JSON. This would make copying JSON into C# (particularly with usages in API like Json.net and System.Text.Json) potentially no work at all. However, we may decide the drawbacks are too high, and that it is better to use the original syntactic form provided in this specification.
That said, even the original syntactic form is not without its own drawbacks. Specifically, if we use
[ ... ]
(just with a new element type, then we will likely have to address ambiguities here when we get to the "natural type" question.For example:
and so on.
Open Questions
The space of allowing merging, with "last one wins" seems pretty reasonable to us. However, we want a read if people would prefer that throw, or if there should be different semantics for dictionaries without spreads for example.
IEqualityComparer
. Should that be something supported somehow in this syntax. Or would/should that require falling back out to a normal instantiation?Design Meetings
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