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Read the backing array through a compiled accessor in CollectionsMarshal.AsSpan #707

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Merged
SimonCropp merged 1 commit into from
Oct 12, 2026
Merged

Read the backing array through a compiled accessor in CollectionsMarshal.AsSpan #707

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25 changes: 22 additions & 3 deletions src/Polyfill/CollectionsMarshal.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -8,6 +8,7 @@ namespace System.Runtime.InteropServices;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Linq.Expressions;
using System.Reflection;

/// <summary>
Expand Down Expand Up @@ -37,14 +38,32 @@ public static Span<T> AsSpan<T>(List<T>? list)
return default;
}

var items = (T[]) ItemsAccessor<T>.Field.GetValue(list)!;
var items = ItemsAccessor<T>.Get(list);
return new(items, 0, list.Count);
}

static class ItemsAccessor<T>
{
public static readonly FieldInfo Field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
// FieldInfo.GetValue is slow, so the field read is compiled once per T
public static readonly Func<List<T>, T[]> Get = Create();

static Func<List<T>, T[]> Create()
{
var field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
try
{
var list = Expression.Parameter(typeof(List<T>), "list");
return Expression
.Lambda<Func<List<T>, T[]>>(Expression.Field(list, field), list)
.Compile();
}
catch (Exception)
{
// Expressions can not be compiled here, so read the field through reflection
return list => (T[]) field.GetValue(list)!;
}
}
}

#endif
Expand Down
22 changes: 19 additions & 3 deletions src/Split/net461/CollectionsMarshal.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ namespace System.Runtime.InteropServices;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Linq.Expressions;
using System.Reflection;
/// <summary>
/// An unsafe class that provides a set of methods to access the underlying data representations of collections.
Expand All @@ -30,13 +31,28 @@ public static Span<T> AsSpan<T>(List<T>? list)
{
return default;
}
var items = (T[]) ItemsAccessor<T>.Field.GetValue(list)!;
var items = ItemsAccessor<T>.Get(list);
return new(items, 0, list.Count);
}
static class ItemsAccessor<T>
{
public static readonly FieldInfo Field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
public static readonly Func<List<T>, T[]> Get = Create();
static Func<List<T>, T[]> Create()
{
var field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
try
{
var list = Expression.Parameter(typeof(List<T>), "list");
return Expression
.Lambda<Func<List<T>, T[]>>(Expression.Field(list, field), list)
.Compile();
}
catch (Exception)
{
return list => (T[]) field.GetValue(list)!;
}
}
}
#endif
/// <summary>
Expand Down
22 changes: 19 additions & 3 deletions src/Split/net462/CollectionsMarshal.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ namespace System.Runtime.InteropServices;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Linq.Expressions;
using System.Reflection;
/// <summary>
/// An unsafe class that provides a set of methods to access the underlying data representations of collections.
Expand All @@ -30,13 +31,28 @@ public static Span<T> AsSpan<T>(List<T>? list)
{
return default;
}
var items = (T[]) ItemsAccessor<T>.Field.GetValue(list)!;
var items = ItemsAccessor<T>.Get(list);
return new(items, 0, list.Count);
}
static class ItemsAccessor<T>
{
public static readonly FieldInfo Field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
public static readonly Func<List<T>, T[]> Get = Create();
static Func<List<T>, T[]> Create()
{
var field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
try
{
var list = Expression.Parameter(typeof(List<T>), "list");
return Expression
.Lambda<Func<List<T>, T[]>>(Expression.Field(list, field), list)
.Compile();
}
catch (Exception)
{
return list => (T[]) field.GetValue(list)!;
}
}
}
#endif
/// <summary>
Expand Down
22 changes: 19 additions & 3 deletions src/Split/net47/CollectionsMarshal.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ namespace System.Runtime.InteropServices;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Linq.Expressions;
using System.Reflection;
/// <summary>
/// An unsafe class that provides a set of methods to access the underlying data representations of collections.
Expand All @@ -30,13 +31,28 @@ public static Span<T> AsSpan<T>(List<T>? list)
{
return default;
}
var items = (T[]) ItemsAccessor<T>.Field.GetValue(list)!;
var items = ItemsAccessor<T>.Get(list);
return new(items, 0, list.Count);
}
static class ItemsAccessor<T>
{
public static readonly FieldInfo Field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
public static readonly Func<List<T>, T[]> Get = Create();
static Func<List<T>, T[]> Create()
{
var field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
try
{
var list = Expression.Parameter(typeof(List<T>), "list");
return Expression
.Lambda<Func<List<T>, T[]>>(Expression.Field(list, field), list)
.Compile();
}
catch (Exception)
{
return list => (T[]) field.GetValue(list)!;
}
}
}
#endif
/// <summary>
Expand Down
22 changes: 19 additions & 3 deletions src/Split/net471/CollectionsMarshal.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ namespace System.Runtime.InteropServices;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Linq.Expressions;
using System.Reflection;
/// <summary>
/// An unsafe class that provides a set of methods to access the underlying data representations of collections.
Expand All @@ -30,13 +31,28 @@ public static Span<T> AsSpan<T>(List<T>? list)
{
return default;
}
var items = (T[]) ItemsAccessor<T>.Field.GetValue(list)!;
var items = ItemsAccessor<T>.Get(list);
return new(items, 0, list.Count);
}
static class ItemsAccessor<T>
{
public static readonly FieldInfo Field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
public static readonly Func<List<T>, T[]> Get = Create();
static Func<List<T>, T[]> Create()
{
var field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
try
{
var list = Expression.Parameter(typeof(List<T>), "list");
return Expression
.Lambda<Func<List<T>, T[]>>(Expression.Field(list, field), list)
.Compile();
}
catch (Exception)
{
return list => (T[]) field.GetValue(list)!;
}
}
}
#endif
/// <summary>
Expand Down
22 changes: 19 additions & 3 deletions src/Split/net472/CollectionsMarshal.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ namespace System.Runtime.InteropServices;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Linq.Expressions;
using System.Reflection;
/// <summary>
/// An unsafe class that provides a set of methods to access the underlying data representations of collections.
Expand All @@ -30,13 +31,28 @@ public static Span<T> AsSpan<T>(List<T>? list)
{
return default;
}
var items = (T[]) ItemsAccessor<T>.Field.GetValue(list)!;
var items = ItemsAccessor<T>.Get(list);
return new(items, 0, list.Count);
}
static class ItemsAccessor<T>
{
public static readonly FieldInfo Field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
public static readonly Func<List<T>, T[]> Get = Create();
static Func<List<T>, T[]> Create()
{
var field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
try
{
var list = Expression.Parameter(typeof(List<T>), "list");
return Expression
.Lambda<Func<List<T>, T[]>>(Expression.Field(list, field), list)
.Compile();
}
catch (Exception)
{
return list => (T[]) field.GetValue(list)!;
}
}
}
#endif
/// <summary>
Expand Down
22 changes: 19 additions & 3 deletions src/Split/net48/CollectionsMarshal.cs
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Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ namespace System.Runtime.InteropServices;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Linq.Expressions;
using System.Reflection;
/// <summary>
/// An unsafe class that provides a set of methods to access the underlying data representations of collections.
Expand All @@ -30,13 +31,28 @@ public static Span<T> AsSpan<T>(List<T>? list)
{
return default;
}
var items = (T[]) ItemsAccessor<T>.Field.GetValue(list)!;
var items = ItemsAccessor<T>.Get(list);
return new(items, 0, list.Count);
}
static class ItemsAccessor<T>
{
public static readonly FieldInfo Field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
public static readonly Func<List<T>, T[]> Get = Create();
static Func<List<T>, T[]> Create()
{
var field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
try
{
var list = Expression.Parameter(typeof(List<T>), "list");
return Expression
.Lambda<Func<List<T>, T[]>>(Expression.Field(list, field), list)
.Compile();
}
catch (Exception)
{
return list => (T[]) field.GetValue(list)!;
}
}
}
#endif
/// <summary>
Expand Down
22 changes: 19 additions & 3 deletions src/Split/net481/CollectionsMarshal.cs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ namespace System.Runtime.InteropServices;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Linq.Expressions;
using System.Reflection;
/// <summary>
/// An unsafe class that provides a set of methods to access the underlying data representations of collections.
Expand All @@ -30,13 +31,28 @@ public static Span<T> AsSpan<T>(List<T>? list)
{
return default;
}
var items = (T[]) ItemsAccessor<T>.Field.GetValue(list)!;
var items = ItemsAccessor<T>.Get(list);
return new(items, 0, list.Count);
}
static class ItemsAccessor<T>
{
public static readonly FieldInfo Field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
public static readonly Func<List<T>, T[]> Get = Create();
static Func<List<T>, T[]> Create()
{
var field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
try
{
var list = Expression.Parameter(typeof(List<T>), "list");
return Expression
.Lambda<Func<List<T>, T[]>>(Expression.Field(list, field), list)
.Compile();
}
catch (Exception)
{
return list => (T[]) field.GetValue(list)!;
}
}
}
#endif
/// <summary>
Expand Down
22 changes: 19 additions & 3 deletions src/Split/netcoreapp2.0/CollectionsMarshal.cs
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Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -6,6 +6,7 @@ namespace System.Runtime.InteropServices;
using System.Collections.Generic;
using System.Diagnostics;
using System.Diagnostics.CodeAnalysis;
using System.Linq.Expressions;
using System.Reflection;
/// <summary>
/// An unsafe class that provides a set of methods to access the underlying data representations of collections.
Expand All @@ -30,13 +31,28 @@ public static Span<T> AsSpan<T>(List<T>? list)
{
return default;
}
var items = (T[]) ItemsAccessor<T>.Field.GetValue(list)!;
var items = ItemsAccessor<T>.Get(list);
return new(items, 0, list.Count);
}
static class ItemsAccessor<T>
{
public static readonly FieldInfo Field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
public static readonly Func<List<T>, T[]> Get = Create();
static Func<List<T>, T[]> Create()
{
var field = typeof(List<T>)
.GetField("_items", BindingFlags.Instance | BindingFlags.NonPublic)!;
try
{
var list = Expression.Parameter(typeof(List<T>), "list");
return Expression
.Lambda<Func<List<T>, T[]>>(Expression.Field(list, field), list)
.Compile();
}
catch (Exception)
{
return list => (T[]) field.GetValue(list)!;
}
}
}
#endif
/// <summary>
Expand Down
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