[Api Proposal] Add List<T> AsSpan to CollectionsMarshal

@jkotas

Its not a "safe" operation, though it is a useful one to gain access to the prebuilt array; without first copying using .ToArray()

namespace System.Runtime.InteropServices
{
    public partial static class CollectionsMarshal
    {
        public Span<T> AsSpan(List<T> list);
        public Memory<T> AsMemory(List<T> list);
    }
}

/cc @jkotas

I do not think this should be on MemoryMarshal. This should be on a new one, like CollectionsMarshal.

This should not be an extension method. No other methods on *Marshal are extension methods to make you do more typing.

The PowerCollections in corefxlab would be a good place to experiment with this: dotnet/corefxlab#2406

Updated

The PowerCollections in corefxlab would be a good place to experiment with this

Would then need to go via private reflection?

@jkotas

@jkotas is reflection what you have in mind?

@benaadams curious, do you happen to have a motivating scenario?

curious, do you happen to have a motivating scenario?

List<T> is a very convenient datastructure for creating an array of an unknown size. However, to use it as an array you need to copy the entire contents again with .ToArray().

AsSpan/AsMemory would give the "array" correctly bounded (size of List rather than size of backing array); without the copy.

@jkotas

@jkotas is reflection what you have in mind?

I had in mind all the different list and array builders in "Common". They are all variants of List with different performance characteristics.

All internal though 😢

Would it make sense to propose a "ListBuilder"/"SpanBuilder" type then? It could have roughly the same surface area as List, along with an AsSpan/AsMemory method. Once those are called, mutation is no longer allowed.

@ahsonkhan

@ahsonkhan

@safern

@safern this does not seem to me essential in order to ship 3.0. Please correct me if I'm wrong.

any updates, just wondering? Sounds like a great API 😃

@benaadams

@benaadams We understand that this a logical extension of the current implementation of the type. Could you list some compelling scenarios? We can always come up with potential scenarios.

Creating a Memory/Span (Array-like) of an unknown upfront size. List<T> is currently the main builder type; but getting an array from it .ToArray() causes a secondary allocation.

The alternative would be to expose an alternative builder type https://github.com/dotnet/corefx/issues/31597#issuecomment-413059736

FYI, it's currently being discussed here.

I missed this when I had a list and wanted to do something (been a while... can't remember what the exact situation was) which could be vectorised (since you need to take pointer of the underlying array). It didn't have to be exposed as a Span... but just my thought.

Video

Approved shape:

namespace System.Runtime.InteropServices
{
    public partial static class CollectionsMarshal
    {
        public static Span<T> AsSpan(List<T> list);
        public static ReadOnlySpan<T> AsReadOnlySpan(List<T> list);

        // We don't want this one:
        // public Memory<T> AsMemory(List<T> list);
    }
}

Re-opening until API is exposed from the ref assembly.

The “approved shape” wasn’t completely implemented with dotnet/coreclr#26867. Should we update the “approved shape” above so it is in sync with the code?

Re-flagging as api-ready-for-review to address the AsReadOnlySpan change and also do a quick review on the nullable annotations change (see dotnet/coreclr#26903).

namespace System.Runtime.InteropServices
{
    public partial static class CollectionsMarshal
    {
        // Updated with nullable annotation
        public static Span<T> AsSpan(List<T>? list);
   
        // Since the AsSpan(...) overload is no longer changing the internal version,
        // we don't need this one anymore
        // public static ReadOnlySpan<T> AsReadOnlySpan(List<T> list);

        // We don't want this one:
        // public Memory<T> AsMemory(List<T> list);
    }
}

Video

We decided to not increment the version, which is why we removed the AsReadOnlySpan() overload.

namespace System.Runtime.InteropServices
{
    public partial static class CollectionsMarshal
    {
        public static Span<T> AsSpan(List<T>? list);
    }
}

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[Api Proposal] Add List<T> AsSpan to CollectionsMarshal #27061

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[Api Proposal] Add List<T> AsSpan to CollectionsMarshal #27061

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jkotas commented on Aug 4, 2018

benaadams commented on Aug 4, 2018

danmoseley commented on Aug 15, 2018

benaadams commented on Aug 15, 2018

jkotas commented on Aug 15, 2018

benaadams commented on Aug 15, 2018

morganbr commented on Aug 15, 2018

danmoseley commented on Aug 15, 2018

danmoseley commented on May 30, 2019

Gnbrkm41 commented on Aug 2, 2019

terrajobst commented on Aug 27, 2019

benaadams commented on Aug 27, 2019

TylerBrinkley commented on Aug 27, 2019

Gnbrkm41 commented on Aug 27, 2019

terrajobst commented on Sep 24, 2019

ahsonkhan commented on Sep 25, 2019

eerhardt commented on Sep 25, 2019

ahsonkhan commented on Sep 26, 2019

terrajobst commented on Oct 1, 2019

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