Remove enumerator allocations and LINQ from children change notifications.

src/Avalonia.Styling/StyledElement.cs

@@ -1,8 +1,8 @@
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.ComponentModel;
-using System.Linq;
 using Avalonia.Animation;
 using Avalonia.Collections;
 using Avalonia.Controls;
@@ -479,16 +479,16 @@ namespace Avalonia
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
-                    SetLogicalParent(e.NewItems.Cast<ILogical>());
+                    SetLogicalParent(e.NewItems);
                     break;
 
                 case NotifyCollectionChangedAction.Remove:
-                    ClearLogicalParent(e.OldItems.Cast<ILogical>());
+                    ClearLogicalParent(e.OldItems);
                     break;
 
                 case NotifyCollectionChangedAction.Replace:
-                    ClearLogicalParent(e.OldItems.Cast<ILogical>());
-                    SetLogicalParent(e.NewItems.Cast<ILogical>());
+                    ClearLogicalParent(e.OldItems);
+                    SetLogicalParent(e.NewItems);
                     break;
 
                 case NotifyCollectionChangedAction.Reset:
@@ -702,13 +702,32 @@ namespace Avalonia
             OnDataContextChanged(EventArgs.Empty);
         }
 
-        private void SetLogicalParent(IEnumerable<ILogical> children)
+        private void SetLogicalParent(IList children)
         {
-            foreach (var i in children)
+            var count = children.Count;
+
+            for (var i = 0; i < count; i++)
+            {
+                var logical = (ILogical) children[i];
+                
+                if (logical.LogicalParent is null)
+                {
+                    ((ISetLogicalParent)logical).SetParent(this);
+                }
+            }
+        }
+
+        private void ClearLogicalParent(IList children)
+        {
+            var count = children.Count;
+
+            for (var i = 0; i < count; i++)
             {
-                if (i.LogicalParent == null)
+                var logical = (ILogical) children[i];
+                
+                if (logical.LogicalParent == this)
                 {
-                    ((ISetLogicalParent)i).SetParent(this);
+                    ((ISetLogicalParent)logical).SetParent(null);
                 }
             }
         }
@@ -784,17 +803,6 @@ namespace Avalonia
             }
         }
 
-        private void ClearLogicalParent(IEnumerable<ILogical> children)
-        {
-            foreach (var i in children)
-            {
-                if (i.LogicalParent == this)
-                {
-                    ((ISetLogicalParent)i).SetParent(null);
-                }
-            }
-        }
-
         private void NotifyResourcesChanged(
             ResourcesChangedEventArgs? e = null,
             bool propagate = true)

src/Avalonia.Visuals/Visual.cs

@@ -1,4 +1,5 @@
 using System;
+using System.Collections;
 using System.Collections.Specialized;
 using Avalonia.Collections;
 using Avalonia.Data;
@@ -619,34 +620,30 @@ namespace Avalonia
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
-                    foreach (Visual v in e.NewItems)
-                    {
-                        v.SetVisualParent(this);
-                    }
-
+                    SetVisualParent(e.NewItems, this);
                     break;
 
                 case NotifyCollectionChangedAction.Remove:
-                    foreach (Visual v in e.OldItems)
-                    {
-                        v.SetVisualParent(null);
-                    }
-
+                    SetVisualParent(e.OldItems, null);
                     break;
 
                 case NotifyCollectionChangedAction.Replace:
-                    foreach (Visual v in e.OldItems)
-                    {
-                        v.SetVisualParent(null);
-                    }
-
-                    foreach (Visual v in e.NewItems)
-                    {
-                        v.SetVisualParent(this);
-                    }
-
+                    SetVisualParent(e.OldItems, null);
+                    SetVisualParent(e.NewItems, this);
                     break;
             }
         }
+        
+        private static void SetVisualParent(IList children, Visual parent)
+        {
+            var count = children.Count;
+
+            for (var i = 0; i < count; i++)
+            {
+                var visual = (Visual) children[i];
+                
+                visual.SetVisualParent(parent);
+            }
+        }
     }
 }