Fixes dotnet#8768

elachlan · Feb 4, 2024 · 12bb875 · 12bb875
1 parent c8b7ff4
commit 12bb875
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Showing 2 changed files with 22 additions and 6 deletions.
diff --git a/...em.Windows.Forms.Primitives/src/System/LocalAppContextSwitches/LocalAppContextSwitches.cs b/...em.Windows.Forms.Primitives/src/System/LocalAppContextSwitches/LocalAppContextSwitches.cs
@@ -21,13 +21,15 @@ internal static partial class LocalAppContextSwitches
     internal const string TrackBarModernRenderingSwitchName = "System.Windows.Forms.TrackBarModernRendering";
     private const string DoNotCatchUnhandledExceptionsSwitchName = "System.Windows.Forms.DoNotCatchUnhandledExceptions";
     internal const string DataGridViewUIAStartRowCountAtZeroSwitchName = "System.Windows.Forms.DataGridViewUIAStartRowCountAtZero";
+    internal const string TreeNodePrevNodeDoNotUseFixedIndexWithSortedTreeViewSwitchName = "System.Windows.Forms.TreeNodeDoNotUseFixedIndexWithSortedTreeView";
 
     private static int s_scaleTopLevelFormMinMaxSizeForDpi;
     private static int s_anchorLayoutV2;
     private static int s_servicePointManagerCheckCrl;
     private static int s_trackBarModernRendering;
     private static int s_doNotCatchUnhandledExceptions;
     private static int s_dataGridViewUIAStartRowCountAtZero;
+    private static int s_treeNodePrevNodeDoNotUseFixedIndexWithSortedTreeView;
 
     private static FrameworkName? s_targetFrameworkName;
 
@@ -160,5 +162,15 @@ public static bool DataGridViewUIAStartRowCountAtZero
         get => GetCachedSwitchValue(DataGridViewUIAStartRowCountAtZeroSwitchName, ref s_dataGridViewUIAStartRowCountAtZero);
     }
 
+    /// <summary>
+    ///  Indicates whether TreeNode.PrevNode uses a fixed index to return its value.
+    ///  Thus mirroring the behaviour of the TreeNodeCollection.
+    /// </summary>
+    public static bool TreeNodePrevNodeDoNotUseFixedIndexWithSortedTreeView
+    {
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        get => GetCachedSwitchValue(TreeNodePrevNodeDoNotUseFixedIndexWithSortedTreeViewSwitchName, ref s_treeNodePrevNodeDoNotUseFixedIndexWithSortedTreeView);
+    }
+
     internal static void SetDataGridViewUIAStartRowCountAtZero(bool value) => s_dataGridViewUIAStartRowCountAtZero = value ? 1 : 0;
 }
diff --git a/src/System.Windows.Forms/src/System/Windows/Forms/Controls/TreeView/TreeNode.cs b/src/System.Windows.Forms/src/System/Windows/Forms/Controls/TreeView/TreeNode.cs
@@ -9,6 +9,7 @@
 using System.Runtime.InteropServices;
 using System.Runtime.Serialization;
 using System.Text;
+using System.Windows.Forms.Primitives;
 
 namespace System.Windows.Forms;
 
@@ -812,17 +813,20 @@ public TreeNodeCollection Nodes
             }
 
             // fixedIndex is used for perf. optimization in case of adding big ranges of nodes
-            int currentInd = _index;
-            int fixedInd = _parent.Nodes.FixedIndex;
+            int currentIndex = _index;
+            int fixedIndex = _parent.Nodes.FixedIndex;
 
-            if (fixedInd > 0)
+            if (fixedIndex > 0)
             {
-                currentInd = fixedInd;
+                if (!LocalAppContextSwitches.TreeNodePrevNodeDoNotUseFixedIndexWithSortedTreeView || TreeView is null || !TreeView.Sorted)
+                {
+                    currentIndex = fixedIndex;
+                }
             }
 
-            if (currentInd > 0 && currentInd <= _parent.Nodes.Count)
+            if (currentIndex > 0 && currentIndex <= _parent.Nodes.Count)
             {
-                return _parent.Nodes[currentInd - 1];
+                return _parent.Nodes[currentIndex - 1];
             }
             else
             {