Add ref accessors for List<T>

[kzrnm]

Background and Motivation

Add ref accessors for List<T> same as Dictionary<TKey, TValue>. #27062.

Motivation

Updating struct causes struct copies. It may be expensive.

struct LargeStruct
{
    // Other members...
    
    public int MyInt { get; set; }

    // Other members...
}

LargeStruct value= list[index]; // struct copy
value.MyInt++;
list[index] = value; // another struct copy

Proposed API

namespace System.Runtime.InteropServices
{
    public static class CollectionsMarshal
    {
        /// <summary>
        ///   Gets a reference to the value at the given <paramref name="index"/>.
        /// </summary>
        public static ref T GetValueRef<T>(List<T> list, int index);
    }
}

Usage Examples

ref LargeStruct value = ref CollectionsMarshal.GetValueRef(list, index); // avoid struct copy
value.MyInt++;

Alternative Designs

It can also be resolved using CollectionsMarshal.AsSpan, but it doesn't correspond to API of Dictionary<TKey, TValue>.
I think this is non-intuitive.

// List<T> use AsSpan
ref var listRef = ref CollectionsMarshal.AsSpan(list)[index];

// Dictionary<TKey, TValue> use ref accessor
ref var dictRef = ref CollectionsMarshal.GetValueRef(dictionary, key);

Risks

[ghost]

Tagging subscribers to this area: @eiriktsarpalis
See info in area-owners.md if you want to be subscribed.

Issue Details

---

Background and Motivation

Add ref accessors for List<T> same as Dictionary<TKey, TValue>. #27062.

Motivation

Updating struct causes struct copies. It may be expensive.

struct LargeStruct
{
    // Other members...
    
    public int MyInt { get; set; }

    // Other members...
}

LargeStruct value= list[index]; // struct copy
value.MyInt++;
list[index] = value; // another struct copy

Proposed API

namespace System.Runtime.InteropServices
{
    public static class CollectionsMarshal
    {
        /// <summary>
        ///   Gets a reference to the value at the given <paramref name="index"/>.
        /// </summary>
        public static ref T GetValueRef<T>(List<T> list, int index);
    }
}

Usage Examples

ref LargeStruct value = ref CollectionsMarshal.GetValueRef(list, index); // avoid struct copy
value.MyInt++;

Alternative Designs

It can also be resolved using CollectionsMarshal.AsSpan, but it doesn't correspond to API of Dictionary<TKey, TValue>.
I think this is non-intuitive.

// List<T> use AsSpan
ref var listRef = ref CollectionsMarshal.AsSpan(list)[index];

// Dictionary<TKey, TValue> use ref accessor
ref var dictRef = ref CollectionsMarshal.GetValueRef(dictionary, key).MyInt++;

Risks

Author:	naminodarie
Assignees:	-
Labels:	api-suggestion, area-System.Collections, untriaged
Milestone:	-

[huoyaoyuan]

It can also be resolved using CollectionsMarshal.AsSpan, but it doesn't correspond to API of Dictionary<TKey, TValue>.
I think this is non-intuitive.

I think AsSpan is already a perfect solution. Considering boundary check, checking span boundary is somehow more institutive than Unsafe.IsNullRef.

The list and dictionary methods lives in the same class CollectionMarshal, so there should be no difficulty of discovery.

[eiriktsarpalis]

I think AsSpan is already a perfect solution. Considering boundary check, checking span boundary is somehow more institutive than Unsafe.IsNullRef.

The list and dictionary methods lives in the same class CollectionMarshal, so there should be no difficulty of discovery.

Both great points. Unlike Dictionary I don't think there is much benefit in exposing ref accessors for List. I will close this issue, but we can always revisit should we discover a compelling reason to include it.

[kzrnm]

Thanks for reviewing!