From 6d9eb1c54dbcc7c6bde3017d666cec74141ba4ae Mon Sep 17 00:00:00 2001
From: Art Leonard <artl@Arts-MacBook-Pro-M5.local>
Date: Fri, 17 Jul 2026 12:29:28 -0700
Subject: [PATCH] Defer CBOR reader map key tracking

Co-authored-by: Copilot App <223556219+Copilot@users.noreply.github.com>

Copilot-Session: d9c75d62-dda8-4115-be1b-ef2f101a6d61
---
 .../System/Formats/Cbor/Reader/CborReader.Map.cs    | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Reader/CborReader.Map.cs b/src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Reader/CborReader.Map.cs
index 1ca66a0537015a..e5dd5aced4bf1f 100644
--- a/src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Reader/CborReader.Map.cs
+++ b/src/libraries/System.Formats.Cbor/src/System/Formats/Cbor/Reader/CborReader.Map.cs
@@ -160,7 +160,18 @@ private void ValidateKeyUniqueness((int Offset, int Length) currentKeyEncodingRa
         {
             Debug.Assert(_currentKeyOffset != null);
 
-            HashSet<(int Offset, int Length)> keyEncodingRanges = GetKeyEncodingRanges();
+            HashSet<(int Offset, int Length)>? keyEncodingRanges = _keyEncodingRanges;
+            if (keyEncodingRanges is null)
+            {
+                if (_previousKeyEncodingRange is null)
+                {
+                    _previousKeyEncodingRange = currentKeyEncodingRange;
+                    return;
+                }
+
+                keyEncodingRanges = GetKeyEncodingRanges();
+                keyEncodingRanges.Add(_previousKeyEncodingRange.Value);
+            }
 
             if (!keyEncodingRanges.Add(currentKeyEncodingRange))
             {