From 23313b4fe1073621d2566196ec55d0b8db9cb1f0 Mon Sep 17 00:00:00 2001
From: Kloizdena <a>
Date: Sat, 20 Jun 2026 00:55:07 +0200
Subject: [PATCH] CSHARP-5611: Avoid buffer allocations in ObjectId.Parse

---
 src/MongoDB.Bson/BsonUtils.cs                 | 58 +++++++++++++++++--
 src/MongoDB.Bson/ObjectModel/ObjectId.cs      | 28 +++++++--
 .../ObjectModel/ObjectIdTests.cs              |  2 +-
 3 files changed, 75 insertions(+), 13 deletions(-)

diff --git a/src/MongoDB.Bson/BsonUtils.cs b/src/MongoDB.Bson/BsonUtils.cs
index 4b324286c10..1cbbe07a156 100644
--- a/src/MongoDB.Bson/BsonUtils.cs
+++ b/src/MongoDB.Bson/BsonUtils.cs
@@ -72,6 +72,24 @@ public static byte[] ParseHexString(string s)
             return bytes;
         }
 
+        /// <summary>
+        /// Parses a hex char span into its equivalent byte array.
+        /// </summary>
+        /// <param name="s">The hex char span to parse.</param>
+        /// <param name="bytes">The result span to fill with the byte equivalent of the hex string.</param>
+        public static void ParseHexString(ReadOnlySpan<char> s, Span<byte> bytes)
+        {
+            int expectedLength = GetByteLength(s.Length);
+            if (bytes.Length != expectedLength)
+            {
+                throw new FormatException($"Target should be {expectedLength} bytes long");
+            }
+            if (!TryParseHexString(s, bytes))
+            {
+                throw new FormatException("String should contain only hexadecimal digits.");
+            }
+        }
+
         /// <summary>
         /// Converts from number of milliseconds since Unix epoch to DateTime.
         /// </summary>
@@ -212,14 +230,43 @@ public static DateTime ToUniversalTime(DateTime dateTime)
         /// <returns>True if the hex string was successfully parsed.</returns>
         public static bool TryParseHexString(string s, out byte[] bytes)
         {
-            bytes = null;
-
             if (s == null)
             {
+                bytes = null;
                 return false;
             }
 
-            var buffer = new byte[(s.Length + 1) / 2];
+            var buffer = new byte[GetByteLength(s.Length)];
+            if (TryParseHexString(s.AsSpan(), buffer))
+            {
+                bytes = buffer;
+                return true;
+            }
+            else
+            {
+                bytes = null;
+                return false;
+            }
+        }
+
+        /// <summary>
+        /// Calculate the result byte length for the hex string length
+        /// </summary>
+        /// <param name="hexStringLength">The length of the hex string</param>
+        /// <returns>The required length to convert the hex string to bytes</returns>
+        internal static int GetByteLength(int hexStringLength)
+            => (hexStringLength + 1) / 2;
+
+        /// <summary>
+        /// Tries to parse a hex char span to a byte span.
+        /// </summary>
+        /// <param name="s">The hex chars.</param>
+        /// <param name="bytes">A byte span.</param>
+        /// <returns>True if the hex char span was successfully parsed.</returns>
+        public static bool TryParseHexString(ReadOnlySpan<char> s, Span<byte> bytes)
+        {
+            if (bytes.Length != GetByteLength(s.Length))
+                return false;
 
             var i = 0;
             var j = 0;
@@ -232,7 +279,7 @@ public static bool TryParseHexString(string s, out byte[] bytes)
                 {
                     return false;
                 }
-                buffer[j++] = (byte)y;
+                bytes[j++] = (byte)y;
             }
 
             while (i < s.Length)
@@ -246,10 +293,9 @@ public static bool TryParseHexString(string s, out byte[] bytes)
                 {
                     return false;
                 }
-                buffer[j++] = (byte)((x << 4) | y);
+                bytes[j++] = (byte)((x << 4) | y);
             }
 
-            bytes = buffer;
             return true;
         }
 
diff --git a/src/MongoDB.Bson/ObjectModel/ObjectId.cs b/src/MongoDB.Bson/ObjectModel/ObjectId.cs
index bb76b29b3a0..6347a748b60 100644
--- a/src/MongoDB.Bson/ObjectModel/ObjectId.cs
+++ b/src/MongoDB.Bson/ObjectModel/ObjectId.cs
@@ -87,8 +87,8 @@ public ObjectId(string value)
             {
                 throw new ArgumentNullException(nameof(value));
             }
-
-            var bytes = BsonUtils.ParseHexString(value);
+            Span<byte> bytes = stackalloc byte[12];
+            BsonUtils.ParseHexString(value.AsSpan(), bytes);
             FromBytesSpan(bytes, out _a, out _b, out _c);
         }
 
@@ -255,11 +255,27 @@ public static ObjectId Parse(string s)
         /// <returns>True if the string was parsed successfully.</returns>
         public static bool TryParse(string s, out ObjectId objectId)
         {
-            // don't throw ArgumentNullException if s is null
-            if (s != null && s.Length == 24)
+            if (s == null)
+            {
+                // don't throw ArgumentNullException if s is null
+                objectId = default(ObjectId);
+                return false;
+            }
+            return TryParse(s.AsSpan(), out objectId);
+        }
+
+        /// <summary>
+        /// Tries to parse a span of chars and create a new ObjectId.
+        /// </summary>
+        /// <param name="s">The span value.</param>
+        /// <param name="objectId">The new ObjectId.</param>
+        /// <returns>True if the string was parsed successfully.</returns>
+        public static bool TryParse(ReadOnlySpan<char> s, out ObjectId objectId)
+        {
+            if (s.Length == 24)
             {
-                byte[] bytes;
-                if (BsonUtils.TryParseHexString(s, out bytes))
+                Span<byte> bytes = stackalloc byte[12];
+                if (BsonUtils.TryParseHexString(s, bytes))
                 {
                     objectId = new ObjectId(bytes);
                     return true;
diff --git a/tests/MongoDB.Bson.Tests/ObjectModel/ObjectIdTests.cs b/tests/MongoDB.Bson.Tests/ObjectModel/ObjectIdTests.cs
index 45c03e93f97..b779f950388 100644
--- a/tests/MongoDB.Bson.Tests/ObjectModel/ObjectIdTests.cs
+++ b/tests/MongoDB.Bson.Tests/ObjectModel/ObjectIdTests.cs
@@ -427,7 +427,7 @@ public void TestTryParse()
         Assert.False(ObjectId.TryParse("102030405060708090a0b0c", out objectId1)); // too short
         Assert.False(ObjectId.TryParse("x102030405060708090a0b0c", out objectId1)); // invalid character
         Assert.False(ObjectId.TryParse("00102030405060708090a0b0c", out objectId1)); // too long
-        Assert.False(ObjectId.TryParse(null, out objectId1)); // should return false not throw ArgumentNullException
+        Assert.False(ObjectId.TryParse(default(string), out objectId1)); // should return false not throw ArgumentNullException
     }
 
     [Fact]