Avoid boxing iterators in hot paths

src/Avalonia.Base/Rendering/Composition/CompositingRenderer.cs

@@ -225,7 +225,7 @@ public class CompositingRenderer : IRendererWithCompositor
             sortedChildren.Dispose();
         }
         else
-            foreach (var ch in v.GetVisualChildren())
+            foreach (var ch in visualChildren)
             {
                 var compositionChild = ch.CompositionVisual;
                 if (compositionChild != null)

src/Avalonia.Base/StyledElement.cs

@@ -803,8 +803,11 @@ namespace Avalonia
 
             if (theme.HasChildren)
             {
-                foreach (var child in theme.Children)
-                    ApplyStyle(child, null, type);
+                var children = theme.Children;
+                for (var i = 0; i < children.Count; i++)
+                {
+                    ApplyStyle(children[i], null, type);
+                }
             }
         }
 
@@ -816,8 +819,11 @@ namespace Avalonia
             
             if (host.IsStylesInitialized)
             {
-                foreach (var style in host.Styles)
-                    ApplyStyle(style, host, FrameType.Style);
+                var styles = host.Styles;
+                for (var i = 0; i < styles.Count; ++i)
+                {
+                    ApplyStyle(styles[i], host, FrameType.Style);
+                }
             }
         }
 
@@ -826,8 +832,11 @@ namespace Avalonia
             if (style is Style s)
                 s.TryAttach(this, host, type);
 
-            foreach (var child in style.Children)
-                ApplyStyle(child, host, type);
+            var children = style.Children;
+            for (var i = 0; i < children.Count; i++)
+            {
+                ApplyStyle(children[i], host, type);
+            }
         }
 
         private void ReevaluateImplicitTheme()

src/Avalonia.Base/Styling/OrSelector.cs

@@ -71,9 +71,9 @@ namespace Avalonia.Styling
             var activators = new OrActivatorBuilder();
             var neverThisInstance = false;
 
-            foreach (var selector in _selectors)
+            for (var i = 0; i < _selectors.Count; i++)
             {
-                var match = selector.Match(control, parent, subscribe);
+                var match = _selectors[i].Match(control, parent, subscribe);
 
                 switch (match.Result)
                 {
@@ -108,16 +108,19 @@ namespace Avalonia.Styling
 
         internal override void ValidateNestingSelector(bool inControlTheme)
         {
-            foreach (var selector in _selectors)
-                selector.ValidateNestingSelector(inControlTheme);
+            for (var i = 0; i < _selectors.Count; i++)
+            {
+                _selectors[i].ValidateNestingSelector(inControlTheme);
+            }
         }
 
         private Type? EvaluateTargetType()
         {
             Type? result = null;
 
-            foreach (var selector in _selectors)
+            for (var i = 0; i < _selectors.Count; i++)
             {
+                var selector = _selectors[i];
                 if (selector.TargetType == null)
                 {
                     return null;