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[API Proposal]: Add Enumerable.Concat & Flatten overloads #54220
Description
Background and Motivation
When working with different APIs, often times it increases code readability to aggregate results and then linearize them. For example, I could write the code:
Command.NameInfo.OnlineAccounts = new List<OnlineAccount>();
Command.NameInfo.OnlineAccounts.AddRange(Input.WebAddresses.Select(x => CreateWebAddress(x)));
Command.NameInfo.OnlineAccounts.AddRange(Input.EmailAddresses.Select(x => CreateEmailAddress(x)));or I could write:
Command.NameInfo.OnlineAccounts = new[] {
Input.WebAddresses.Select(x => CreateWebAddress(x)),
Input.EmailAddresses.Select(x => CreateEmailAddress(x))
}.Concat().ToList();The second is much more clear and concise.
Proposed API
namespace System.Linq
{
public static class Enumerable
{
public static IEnumerable<T> Concat<T>(this IEnumerable<TSource> first, IEnumerable<TSource> second);
+ public static IEnumerable<T> Concat<T>(params IEnumerable<T>[] sources);
+ public static IEnumerable<T> Concat<T>(this IEnumerable<IEnumerable<T>> sources);
}
}Usage Examples
var source = Enumerable.Range(0, 2);
Enumerable.Concat(source, source, source, source); // 0, 1, 0, 1, 0, 1, 0, 1
Enumerable.Repeat(source, 3).Concat(); // 0, 1, 0, 1, 0, 1Alternative Designs
Just use SelectMany(x => x)
Risks
None other than System.Linq size concerns.
Activity
eiriktsarpalis commentedon Jun 15, 2021
I'm assuming
SelectMany()andConcat()are just alternative names for the same proposed method? Why not just stick with one? I personally would prefer exposingConcat()as an overload, since F# uses the same name for its equivalent method.svick commentedon Jun 16, 2021
Note that another way to write this code is to use the existing overload of
Concat:Or:
But you do have to choose between the fluent assymetric form and the non-fluent symmetric form. So I think the proposed overload of
Concatwould still be a good addition.eiriktsarpalis commentedon Jun 16, 2021
Agreed, @TonyValenti if you could update the OP following the API proposal template we can consider this proposal for .NET 7.
danielchalmers commentedon Aug 12, 2021
I often make a
Flatten<T>(this IEnumerable<T> l)=>l.SelectMany(x=>x)extension so I would love to see this in the BCL!eiriktsarpalis commentedon Aug 13, 2021
@danielchalmers please consider upvoting the original post, it provides a good signal on whether a particular proposal is popular!
20 remaining items
[-][API Proposal]: Add Enumerable.Concat overloads for variadic arguments and nested enumerables[/-][+][API Proposal]: Add Enumerable.Concat & Flatten overloads[/+]eiriktsarpalis commentedon Nov 15, 2021
Changed to
api-needs-workdue to feedback in #61230. cc @stephentoubTahirAhmadov commentedon Nov 27, 2021
@bartonjs did you consider the solution I proposed here ?
LeaFrock commentedon Mar 23, 2023
No progress for .NET 8? Does
IQueryableblock it?JBrown526 commentedon Sep 17, 2024
It would be nice to have a
Flatten()or parameterlessSelectMany()implementation at least, as the current usage ofnestedCollections.SelectMany(x => x);` never seems particularly intuitive or meaningful for mejulealgon commentedon Sep 17, 2024
With collection literals and the spread operator, I assume this example would be completely nullified since you can just do:
Or am I missing something?
Thaina commentedon Sep 18, 2025
I would like to add my voice to
Flatten()or parameterlessSelectMany()too. I think the old BCL was not support this because it lack contravariant feature? If so we now have contravariant it should be allowed?