Add Enumerable.Concat & Enumerable.Flatten methods

[eiriktsarpalis]

• Implements the API as approved in [API Proposal]: Add Enumerable.Concat & Flatten overloads #54220 (comment).
• Removes the unneeded ExceptionMessage enum infrastructure.

Note to reviewers: the feature is implemented in dfd7d00 and should be reviewed separately from 231b537 which contains infrastructural cleanups.

Fixes #54220.

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Issue Details

---

• Implements the API as approved in [API Proposal]: Add Enumerable.Concat & Flatten overloads #54220 (comment).
• Removes the unneeded ExceptionMessage enum infrastructure.

Fixes #54220.

Author:	eiriktsarpalis
Assignees:	eiriktsarpalis
Labels:	area-System.Linq
Milestone:	7.0.0

[stephentoub]

Removes the unneeded ExceptionMessage enum infrastructure.

Why is it "unneeded"?

[stephentoub]

Did you consider just doing:

return sources.SelectMany(e => e);

rather than implementing another iterator?

[eiriktsarpalis]

I did, ultimately it's tradeoff between this and a closure allocation and a few virtual calls. I should point out that the simple SelectMany implementation does inherit from Iterator<T>, but ultimately I didn't find any of the speed optimizations offered in this particular case to be useful in practice.

[stephentoub]

I did, ultimately it's tradeoff between this and a closure allocation and a few virtual calls

There's no closure allocation with sources.SelectMany(e => e). There is one delegate per T allocated for the process. And an extra delegate invocation per enumerable. From my perspective, I'd rather have less code to maintain and a smaller binary (and implicitly pick up any optimizations SelectMany adds in the future, e.g. if it were to special case T[]). Up to you, though.

[eiriktsarpalis]

Removes the unneeded ExceptionMessage enum infrastructure.

Why is it "unneeded"?

I would guess it's a mechanism that predates nameof. It's simply mapping predefined argument names to strings.

[stephentoub]

I would guess it's a mechanism that predates nameof

It's not. It's reducing the size of the generated generic instantiations. It's possible it's no longer the right tradeoff, but that would benefit from numbers.

[eiriktsarpalis]

It's not. It's reducing the size of the generated generic instantiations. It's possible it's no longer the right tradeoff, but that would benefit from numbers.

If I understand this correctly, it's an optimization replacing an ldstr instruction with an ldc.i4 in generic methods?

https://sharplab.io/#v2:C4LghgzgtgPgAgBgARwIwBYDcBYAUIpAZQAswAnABwBkwAjAOgCUBXAO2AEsoBTHXPOAGYUAJiQBhJAG88SOSmHdWzKEgByYHhGlIwAGlp6AxnoAme7noBmegOZ7iejnoBWegNZ6ANnqh7WegD2ehR6AI56ZHoQesB6zEgAvnzySLLyANoAUhzAAOJK3GQcRgAUwACeFNyBVqUc7ACUjQC66XJCKKjIALIAPAAqAHylaIKDQ0jujUgAvJOsmjV10ymZOfmFxWWV1bX1Ta3tCupL2moTo6jjw1Mz86da9O58iUA==