Use Spans and ReadOnlyCollection.Empty in TextBlock/Line, reduce allocs

src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/Text/ComplexLine.cs

@@ -160,7 +160,6 @@ internal override void Arrange(VisualCollection vc, Vector lineOffset)
             Debug.Assert(runs != null, "Cannot retrieve runs collection.");
 
             // Calculate offset shift due to trailing spaces
-            double adjustedXOffset = lineOffset.X + CalculateXOffsetShift();
             foreach (TextSpan<TextRun> textSpan in runs)
             {
                 TextRun run = textSpan.Value;
@@ -169,26 +168,24 @@ internal override void Arrange(VisualCollection vc, Vector lineOffset)
                     InlineObject inlineObject = run as InlineObject;
 
                     // Disconnect visual from its old parent, if necessary.
-                    Visual currentParent = VisualTreeHelper.GetParent(inlineObject.Element) as Visual;
-                    if (currentParent != null)
+                    if (VisualTreeHelper.GetParent(inlineObject.Element) is Visual currentParent)
                     {
                         ContainerVisual parent = currentParent as ContainerVisual;
                         Invariant.Assert(parent != null, "parent should always derives from ContainerVisual");
                         parent.Children.Remove(inlineObject.Element);
                     }
 
                     // Get position of inline object withing the text line.
-                    FlowDirection flowDirection;
-                    Rect rect = GetBoundsFromPosition(runDcp, inlineObject.Length, out flowDirection);
+                    Rect rect = GetBoundsFromPosition(runDcp, inlineObject.Length, out FlowDirection flowDirection);
                     Debug.Assert(DoubleUtil.GreaterThanOrClose(rect.Width, 0), "Negative inline object's width.");
 
                     ContainerVisual proxyVisual = new ContainerVisual();
-                    if (inlineObject.Element is FrameworkElement)
+                    if (inlineObject.Element is FrameworkElement frameworkElement)
                     {
                         FlowDirection parentFlowDirection = _owner.FlowDirection;
                         // Check parent's FlowDirection to determine if mirroring is needed
 
-                        DependencyObject parent = ((FrameworkElement)inlineObject.Element).Parent; 
+                        DependencyObject parent = frameworkElement.Parent; 
                         if(parent != null)
                         {
                             parentFlowDirection = (FlowDirection)parent.GetValue(FrameworkElement.FlowDirectionProperty);

src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/Text/Line.cs

@@ -160,10 +160,8 @@ internal Rect GetBoundsFromTextPosition(int characterIndex, out FlowDirection fl
         /// it uses those as the bounding rectangles. If not, it returns the rectangle for the first (and only) element
         /// of the text bounds.
         /// </remarks>
-        internal List<Rect> GetRangeBounds(int cp, int cch, double xOffset, double yOffset)
-        {
-            List<Rect> rectangles = new List<Rect>();
-
+        internal ReadOnlySpan<Rect> GetRangeBounds(int cp, int cch, double xOffset, double yOffset)
+        {       
             // Adjust x offset for trailing spaces
             double delta = CalculateXOffsetShift();
             double adjustedXOffset = xOffset + delta;
@@ -173,24 +171,27 @@ internal List<Rect> GetRangeBounds(int cp, int cch, double xOffset, double yOffs
             {
                 // We should not shift offset in this case
                 Invariant.Assert(DoubleUtil.AreClose(delta, 0));
-                System.Windows.Media.TextFormatting.TextLine line = _line.Collapse(GetCollapsingProps(_wrappingWidth, _owner.ParagraphProperties));
+                TextLine line = _line.Collapse(GetCollapsingProps(_wrappingWidth, _owner.ParagraphProperties));
                 Invariant.Assert(line.HasCollapsed, "Line has not been collapsed");
                 textBounds = line.GetTextBounds(cp, cch);
             }
             else
             {
                 textBounds = _line.GetTextBounds(cp, cch);
             }
+
             Invariant.Assert(textBounds.Count > 0);
 
+            Rect[] rectangles = new Rect[textBounds.Count];
 
-            for (int boundIndex = 0; boundIndex < textBounds.Count; boundIndex++)
+            for (int boundIndex = 0; boundIndex < rectangles.Length; boundIndex++)
             {
                 Rect rect = textBounds[boundIndex].Rectangle;
                 rect.X += adjustedXOffset;
                 rect.Y += yOffset;
-                rectangles.Add(rect);
+                rectangles[boundIndex] = rect;
             }
+
             return rectangles;
         }
 

src/Microsoft.DotNet.Wpf/src/PresentationFramework/MS/Internal/documents/HostedElements.cs

@@ -6,22 +6,18 @@
 // Enumerator class for returning descendants of TextBlock and FlowDocumentPage
 //
 
+using System.Diagnostics;
 using System.Collections;
-using MS.Internal.Documents;
 using System.Collections.Generic;
 using System.Collections.ObjectModel;
-using System;
-using System.Diagnostics;
-
-#pragma warning disable 1634, 1691  // suppressing PreSharp warnings
 
 namespace System.Windows.Documents
 {
     /// <summary>
     ///  Enumerator class for implementation of IContentHost on TextBlock and FlowDocumentPage.
     ///  Used to iterate through descendants of the content host
     /// </summary>
-    internal class HostedElements : IEnumerator<IInputElement>
+    internal sealed class HostedElements : IEnumerator<IInputElement>
     {
         //-------------------------------------------------------------------
         //
@@ -164,19 +160,13 @@ public IInputElement Current
         {
             get
             {
-                // Disable PRESharp warning 6503: "Property get methods should not throw exceptions".
-                // HostedElements must throw exception if Current property is incorrectly accessed
                 if (_textSegments == null)
                 {
-                    // Collection was modified 
-#pragma warning suppress 6503 // IEnumerator.Current is documented to throw this exception
                     throw new InvalidOperationException(SR.EnumeratorCollectionDisposed);
                 }
 
                 if (_currentPosition == null)
                 {
-                    // Enumerator not started. Call MoveNext to see if we can move ahead
-#pragma warning suppress 6503 // IEnumerator.Current is documented to throw this exception
                     throw new InvalidOperationException(SR.EnumeratorNotStarted);
                 }