TryAddWithoutValidation for multiple values could be more efficient

[MihaZupan]

The method takes an IEnumerable<string>. Things we could improve:

• Special-case IList<string> or some other type to avoid boxing the enumerator
• Avoid double entry lookups in case of more than one value
• Avoid unnecessary List resizes if we already know the final size

For example

TryAddWithoutValidation("foo", "1");
TryAddWithoutValidation("foo", new[] { "2", "3" });

Will currently allocate the enumerator, a List<string> of size 2, and a List<string> of size 4.

We can reduce that down to allocating a single List<string> of size 3.

An example of how this can be achieved:
https://gist.github.com/MihaZupan/800df7a324a66a8e814c3ec91f3d39b0

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Issue Details

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The method takes an IEnumerable<string>. Things we could improve:

• Special-case IList<string> or some other type to avoid boxing the enumerator
• Avoid double entry lookups in case of more than one value
• Avoid unnecessary List resizes if we already know the final size

For example

TryAddWithoutValidation("foo", "1");
TryAddWithoutValidation("foo", new[] { "2", "3" });

Will currently allocate the enumerator, a List<string> of size 2, and a List<string> of size 4.

We can reduce that down to allocating a single List<string> of size 3.

An example of how this can be achieved:
https://gist.github.com/MihaZupan/800df7a324a66a8e814c3ec91f3d39b0

Author:	MihaZupan
Assignees:	-
Labels:	area-System.Net.Http, tenet-performance
Milestone:	-

[karelz]

Triage: Worth investment, but the method is not commonly used, so not priority.

[deeprobin]

I am confident that this will grant a performance advantage. As @karelz mentioned, this does not seem to be a method on the hot path.

@MihaZupan
Do you feel like submitting a pull request and also creating a benchmark in dotnet/performance?

[MihaZupan]

It's not a priority for us for now, but I'll happily review a PR if you are interested :)