String support in EnumerableTryGetNonEnumeratedCount and Enumerable.Count

[NN---]

Description

System.String doesn't implement any collection interface: ICollection, IReadOnlyCollection, IReadOnlyList, IIListProvider.
This makes EnumerableTryGetNonEnumeratedCount and Enumerable.Count to work inefficienlty.

Regression?

No

Data

Enumerable.TryGetNonEnumeratedCount("abc", out var c);
Console.WriteLine(c); // 0

var count = Enumerable.Count("abc"); // Enumerates by character

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Issue Details

---

Description

System.String doesn't implement any collection interface: ICollection, IReadOnlyCollection, IReadOnlyList, IIListProvider.
This makes EnumerableTryGetNonEnumeratedCount and Enumerable.Count to work inefficienlty.

Regression?

No

Data

Enumerable.TryGetNonEnumeratedCount("abc", out var c);
Console.WriteLine(c); // 0

var count = Enumerable.Count("abc"); // Enumerates by character

Author:	NN---
Assignees:	-
Labels:	area-System.Linq, tenet-performance, untriaged
Milestone:	-

[krwq]

I'd suggest to create a PR. You will need to add something similar to this but for string https://github.com/dotnet/runtime/blob/main/src/libraries/System.Linq/src/System/Linq/Count.cs#L23-L26

We can talk about this under the PR. There might be some perf investigation we'd ask you to do (we will point to any instructions if needed)

[NN---]

Hello @krwq,
Do you tend to add check for string type in LINQ methods or adding interface to string ?
Adding interface covers much more than optimizing a single method.

[eiriktsarpalis]

How common is it for strings to be used as Linq inputs? Adding an additional type check for string would incur a performance penalty for every input calling into that method. Have you considered just making that check yourself?

int count = -1;
if (source is string s) 
     count = s.Length;
else 
      source.TryGetNonEnumeratedCount(out count);

Adding interface covers much more than optimizing a single method.

Adding a new interface implementation for a type as common as string would have negative impact on the runtime's performance. As a general rule we never do this.