dotnet · eiriktsarpalis · Jul 16, 2024 · Jul 2, 2024 · Jul 2, 2024 · Jul 2, 2024
diff --git a/src/libraries/System.Linq/src/System/Linq/OfType.SpeedOpt.cs b/src/libraries/System.Linq/src/System/Linq/OfType.SpeedOpt.cs
@@ -52,17 +52,21 @@ public override TResult[] ToArray()
 
             public override List<TResult> ToList()
             {
-                var list = new List<TResult>();
+                SegmentedArrayBuilder<TResult>.ScratchBuffer scratch = default;
+                SegmentedArrayBuilder<TResult> builder = new(scratch);
 
                 foreach (object? item in _source)
                 {
                     if (item is TResult castItem)
                     {
-                        list.Add(castItem);
+                        builder.Add(castItem);
                     }
                 }
 
-                return list;
+                List<TResult> result = builder.ToList();
+                builder.Dispose();
+
+                return result;
             }
 
             public override TResult? TryGetFirst(out bool found)

diff --git a/src/libraries/System.Linq/src/System/Linq/SegmentedArrayBuilder.cs b/src/libraries/System.Linq/src/System/Linq/SegmentedArrayBuilder.cs
@@ -5,6 +5,7 @@
 using System.Diagnostics;
 using System.Linq;
 using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
 
 namespace System.Collections.Generic
 {
@@ -251,6 +252,27 @@ public readonly T[] ToArray()
             return result;
         }
 
+        /// <summary>Creates a list containing all of the elements in the builder.</summary>
+        public readonly List<T> ToList()
+        {
+            List<T> result;
+            int count = Count;
+
+            if (count != 0)
+            {
+                result = new List<T>(count);
+
+                CollectionsMarshal.SetCount(result, count);
+                ToSpanInlined(CollectionsMarshal.AsSpan(result));
+            }
+            else
+            {
+                result = [];
+            }
+
+            return result;
+        }
+
         /// <summary>Creates an array containing all of the elements in the builder.</summary>
         /// <param name="additionalLength">The number of extra elements of room to allocate in the resulting array.</param>
         public readonly T[] ToArray(int additionalLength)

diff --git a/src/libraries/System.Linq/src/System/Linq/Select.SpeedOpt.cs b/src/libraries/System.Linq/src/System/Linq/Select.SpeedOpt.cs
@@ -3,6 +3,7 @@
 
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Runtime.CompilerServices;
 using System.Runtime.InteropServices;
 using static System.Linq.Utilities;
 
@@ -31,15 +32,19 @@ public override TResult[] ToArray()
 
             public override List<TResult> ToList()
             {
-                var list = new List<TResult>();
+                SegmentedArrayBuilder<TResult>.ScratchBuffer scratch = default;
+                SegmentedArrayBuilder<TResult> builder = new(scratch);
 
                 Func<TSource, TResult> selector = _selector;
                 foreach (TSource item in _source)
                 {
-                    list.Add(selector(item));
+                    builder.Add(selector(item));
                 }
 
-                return list;
+                List<TResult> result = builder.ToList();
+                builder.Dispose();
+
+                return result;
             }
 
             public override int GetCount(bool onlyIfCheap)
@@ -657,7 +662,7 @@ public override IEnumerable<TResult2> Select<TResult2>(Func<TResult, TResult2> s
                 return sourceFound ? _selector(input!) : default!;
             }
 
-            private TResult[] LazyToArray()
+            private TResult[] ToArrayNoPresizing()
             {
                 Debug.Assert(_source.GetCount(onlyIfCheap: true) == -1);
 
@@ -691,24 +696,39 @@ public override TResult[] ToArray()
                 int count = _source.GetCount(onlyIfCheap: true);
                 return count switch
                 {
-                    -1 => LazyToArray(),
+                    -1 => ToArrayNoPresizing(),
                     0 => [],
                     _ => PreallocatingToArray(count),
                 };
             }
 
+            private List<TResult> ToListNoPresizing()
+            {
+                Debug.Assert(_source.GetCount(onlyIfCheap: true) == -1);
+
+                SegmentedArrayBuilder<TResult>.ScratchBuffer scratch = default;
+                SegmentedArrayBuilder<TResult> builder = new(scratch);
+
+                Func<TSource, TResult> selector = _selector;
+                foreach (TSource input in _source)
+                {
+                    builder.Add(selector(input));
+                }
+
+                List<TResult> result = builder.ToList();
+                builder.Dispose();
+
+                return result;
+            }
+
             public override List<TResult> ToList()
             {
                 int count = _source.GetCount(onlyIfCheap: true);
                 List<TResult> list;
                 switch (count)
                 {
                     case -1:
-                        list = new List<TResult>();
-                        foreach (TSource input in _source)
-                        {
-                            list.Add(_selector(input));
-                        }
+                        list = ToListNoPresizing();
                         break;
                     case 0:
                         list = new List<TResult>();

diff --git a/src/libraries/System.Linq/src/System/Linq/SelectMany.SpeedOpt.cs b/src/libraries/System.Linq/src/System/Linq/SelectMany.SpeedOpt.cs
@@ -48,15 +48,19 @@ public override TResult[] ToArray()
 
             public override List<TResult> ToList()
             {
-                var list = new List<TResult>();
+                SegmentedArrayBuilder<TResult>.ScratchBuffer scratch = default;
+                SegmentedArrayBuilder<TResult> builder = new(scratch);
 
                 Func<TSource, IEnumerable<TResult>> selector = _selector;
-                foreach (TSource element in _source)
+                foreach (TSource item in _source)
                 {
-                    list.AddRange(selector(element));
+                    builder.AddRange(selector(item));
                 }
 
-                return list;
+                List<TResult> result = builder.ToList();
+                builder.Dispose();
+
+                return result;
             }
         }
     }

diff --git a/src/libraries/System.Linq/src/System/Linq/SkipTake.SpeedOpt.cs b/src/libraries/System.Linq/src/System/Linq/SkipTake.SpeedOpt.cs
@@ -491,25 +491,30 @@ public override TSource[] ToArray()
 
             public override List<TSource> ToList()
             {
-                var list = new List<TSource>();
-
                 using (IEnumerator<TSource> en = _source.GetEnumerator())
                 {
                     if (SkipBeforeFirst(en) && en.MoveNext())
                     {
                         int remaining = Limit - 1; // Max number of items left, not counting the current element.
                         int comparand = HasLimit ? 0 : int.MinValue; // If we don't have an upper bound, have the comparison always return true.
 
+                        SegmentedArrayBuilder<TSource>.ScratchBuffer scratch = default;
+                        SegmentedArrayBuilder<TSource> builder = new(scratch);
                         do
                         {
                             remaining--;
-                            list.Add(en.Current);
+                            builder.Add(en.Current);
                         }
                         while (remaining >= comparand && en.MoveNext());
+
+                        List<TSource> result = builder.ToList();
+                        builder.Dispose();
+
+                        return result;
                     }
                 }
 
-                return list;
+                return [];
             }
 
             private bool SkipBeforeFirst(IEnumerator<TSource> en) => SkipBefore(_minIndexInclusive, en);

diff --git a/src/libraries/System.Linq/src/System/Linq/Where.SpeedOpt.cs b/src/libraries/System.Linq/src/System/Linq/Where.SpeedOpt.cs
@@ -55,18 +55,22 @@ public override TSource[] ToArray()
 
             public override List<TSource> ToList()
             {
-                var list = new List<TSource>();
+                SegmentedArrayBuilder<TSource>.ScratchBuffer scratch = default;
+                SegmentedArrayBuilder<TSource> builder = new(scratch);
 
                 Func<TSource, bool> predicate = _predicate;
                 foreach (TSource item in _source)
                 {
                     if (predicate(item))
                     {
-                        list.Add(item);
+                        builder.Add(item);
                     }
                 }
 
-                return list;
+                List<TSource> result = builder.ToList();
+                builder.Dispose();
+
+                return result;
             }
 
             public override TSource? TryGetFirst(out bool found)
@@ -199,17 +203,21 @@ public static TSource[] ToArray(ReadOnlySpan<TSource> source, Func<TSource, bool
 
             public static List<TSource> ToList(ReadOnlySpan<TSource> source, Func<TSource, bool> predicate)
             {
-                var list = new List<TSource>();
+                SegmentedArrayBuilder<TSource>.ScratchBuffer scratch = default;
+                SegmentedArrayBuilder<TSource> builder = new(scratch);
 
                 foreach (TSource item in source)
                 {
                     if (predicate(item))
                     {
-                        list.Add(item);
+                        builder.Add(item);
                     }
                 }
 
-                return list;
+                List<TSource> result = builder.ToList();
+                builder.Dispose();
+
+                return result;
             }
 
             public override TSource? TryGetFirst(out bool found)
@@ -398,17 +406,21 @@ public static TResult[] ToArray(ReadOnlySpan<TSource> source, Func<TSource, bool
 
             public static List<TResult> ToList(ReadOnlySpan<TSource> source, Func<TSource, bool> predicate, Func<TSource, TResult> selector)
             {
-                var list = new List<TResult>();
+                SegmentedArrayBuilder<TResult>.ScratchBuffer scratch = default;
+                SegmentedArrayBuilder<TResult> builder = new(scratch);
 
                 foreach (TSource item in source)
                 {
                     if (predicate(item))
                     {
-                        list.Add(selector(item));
+                        builder.Add(selector(item));
                     }
                 }
 
-                return list;
+                List<TResult> result = builder.ToList();
+                builder.Dispose();
+
+                return result;
             }
 
             public override TResult? TryGetFirst(out bool found) => TryGetFirst(_source, _predicate, _selector, out found);
@@ -538,19 +550,23 @@ public override TResult[] ToArray()
 
             public override List<TResult> ToList()
             {
-                var list = new List<TResult>();
+                SegmentedArrayBuilder<TResult>.ScratchBuffer scratch = default;
+                SegmentedArrayBuilder<TResult> builder = new(scratch);
 
                 Func<TSource, bool> predicate = _predicate;
                 Func<TSource, TResult> selector = _selector;
                 foreach (TSource item in _source)
                 {
                     if (predicate(item))
                     {
-                        list.Add(selector(item));
+                        builder.Add(selector(item));
                     }
                 }
 
-                return list;
+                List<TResult> result = builder.ToList();
+                builder.Dispose();
+
+                return result;
             }
 
             public override TResult? TryGetFirst(out bool found)