[NRBF] Don't use Unsafe.As when decoding DateTime(s)

src/libraries/System.Formats.Nrbf/src/System/Formats/Nrbf/ArraySinglePrimitiveRecord.cs

@@ -240,7 +240,7 @@ private static List<T> DecodeFromNonSeekableStream(BinaryReader reader, int coun
             }
             else if (typeof(T) == typeof(DateTime))
             {
-                values.Add((T)(object)Utils.BinaryReaderExtensions.CreateDateTimeFromData(reader.ReadInt64()));
+                values.Add((T)(object)Utils.BinaryReaderExtensions.CreateDateTimeFromData(reader.ReadUInt64()));
             }
             else
             {

src/libraries/System.Formats.Nrbf/src/System/Formats/Nrbf/NrbfDecoder.cs

@@ -257,7 +257,7 @@ private static SerializationRecord DecodeMemberPrimitiveTypedRecord(BinaryReader
             PrimitiveType.Single => new MemberPrimitiveTypedRecord<float>(reader.ReadSingle()),
             PrimitiveType.Double => new MemberPrimitiveTypedRecord<double>(reader.ReadDouble()),
             PrimitiveType.Decimal => new MemberPrimitiveTypedRecord<decimal>(decimal.Parse(reader.ReadString(), CultureInfo.InvariantCulture)),
-            PrimitiveType.DateTime => new MemberPrimitiveTypedRecord<DateTime>(Utils.BinaryReaderExtensions.CreateDateTimeFromData(reader.ReadInt64())),
+            PrimitiveType.DateTime => new MemberPrimitiveTypedRecord<DateTime>(Utils.BinaryReaderExtensions.CreateDateTimeFromData(reader.ReadUInt64())),
             // String is handled with a record, never on it's own
             _ => new MemberPrimitiveTypedRecord<TimeSpan>(new TimeSpan(reader.ReadInt64())),
         };

src/libraries/System.Formats.Nrbf/src/System/Formats/Nrbf/SystemClassWithMembersAndTypesRecord.cs

@@ -77,10 +77,10 @@ internal SerializationRecord TryToMapToUserFriendly()
         }
         else if (MemberValues.Count == 2
             && HasMember("ticks") && HasMember("dateData")
-            && MemberValues[0] is long value && MemberValues[1] is ulong
+            && MemberValues[0] is long && MemberValues[1] is ulong dateData
             && TypeNameMatches(typeof(DateTime)))
         {
-            return Create(Utils.BinaryReaderExtensions.CreateDateTimeFromData(value));
+            return Create(Utils.BinaryReaderExtensions.CreateDateTimeFromData(dateData));
         }
         else if(MemberValues.Count == 4
             && HasMember("lo") && HasMember("mid") && HasMember("hi") && HasMember("flags")

src/libraries/System.Formats.Nrbf/src/System/Formats/Nrbf/Utils/BinaryReaderExtensions.cs

@@ -3,14 +3,18 @@
 
 using System.Globalization;
 using System.IO;
+using System.Reflection;
 using System.Reflection.Metadata;
 using System.Runtime.CompilerServices;
 using System.Runtime.Serialization;
+using System.Threading;
 
 namespace System.Formats.Nrbf.Utils;
 
 internal static class BinaryReaderExtensions
 {
+    private static object? s_baseAmbiguousDstDateTime;
+
     internal static BinaryArrayType ReadArrayType(this BinaryReader reader)
     {
         byte arrayType = reader.ReadByte();
@@ -70,36 +74,67 @@ internal static object ReadPrimitiveValue(this BinaryReader reader, PrimitiveTyp
             PrimitiveType.Single => reader.ReadSingle(),
             PrimitiveType.Double => reader.ReadDouble(),
             PrimitiveType.Decimal => decimal.Parse(reader.ReadString(), CultureInfo.InvariantCulture),
-            PrimitiveType.DateTime => CreateDateTimeFromData(reader.ReadInt64()),
+            PrimitiveType.DateTime => CreateDateTimeFromData(reader.ReadUInt64()),
             _ => new TimeSpan(reader.ReadInt64()),
         };
 
-    // TODO: fix https://github.com/dotnet/runtime/issues/102826
     /// <summary>
     ///  Creates a <see cref="DateTime"/> object from raw data with validation.
     /// </summary>
-    /// <exception cref="SerializationException"><paramref name="data"/> was invalid.</exception>
-    internal static DateTime CreateDateTimeFromData(long data)
+    /// <exception cref="SerializationException"><paramref name="dateData"/> was invalid.</exception>
+    internal static DateTime CreateDateTimeFromData(ulong dateData)
     {
-        // Copied from System.Runtime.Serialization.Formatters.Binary.BinaryParser
-
-        // Use DateTime's public constructor to validate the input, but we
-        // can't return that result as it strips off the kind. To address
-        // that, store the value directly into a DateTime via an unsafe cast.
-        // See BinaryFormatterWriter.WriteDateTime for details.
+        ulong ticks = dateData & 0x3FFFFFFF_FFFFFFFFUL;
+        DateTimeKind kind = (DateTimeKind)(dateData >> 62);
 
         try
         {
-            const long TicksMask = 0x3FFFFFFFFFFFFFFF;
-            _ = new DateTime(data & TicksMask);
+            return ((uint)kind <= (uint)DateTimeKind.Local) ? new DateTime((long)ticks, kind) : CreateFromAmbiguousDst(ticks);
         }
         catch (ArgumentException ex)
         {
-            // Bad data
             throw new SerializationException(ex.Message, ex);
         }
 
-        return Unsafe.As<long, DateTime>(ref data);
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        static DateTime CreateFromAmbiguousDst(ulong ticks)
+        {
+            // There's no public API to create a DateTime from an ambiguous DST, and we
+            // can't use private reflection to access undocumented .NET Framework APIs.
+            // However, the ISerializable pattern *is* a documented protocol, so we can
+            // use DateTime's serialization ctor to create a zero-tick "ambiguous" instance,
+            // then keep reusing it as the base to which we can add our tick offsets.
+
+            if (s_baseAmbiguousDstDateTime is not DateTime baseDateTime)
+            {
+#pragma warning disable SYSLIB0050 // Type or member is obsolete
+                SerializationInfo si = new(typeof(DateTime), new FormatterConverter());
+                si.AddValue("ticks", 0L); // legacy value (serialized as long) - specify both just to be safe
+                si.AddValue("dateData", 0xC0000000_00000000UL); // new value (serialized as ulong)
+
+#if NET
+                baseDateTime = CallPrivateSerializationConstructor(si, new StreamingContext(StreamingContextStates.All));
+#else
+                ConstructorInfo ci = typeof(DateTime).GetConstructor(
+                    BindingFlags.Instance | BindingFlags.NonPublic,
+                    binder: null,
+                    new Type[] { typeof(SerializationInfo), typeof(StreamingContext) },
+                    modifiers: null);
+
+                baseDateTime = (DateTime)ci.Invoke(new object[] { si, new StreamingContext(StreamingContextStates.All) });
+#endif
+
+#pragma warning restore SYSLIB0050 // Type or member is obsolete
+                Volatile.Write(ref s_baseAmbiguousDstDateTime, baseDateTime); // it's ok if two threads race here
+            }
+
+            return baseDateTime.AddTicks((long)ticks);
+        }
+
+#if NET
+        [UnsafeAccessor(UnsafeAccessorKind.Constructor)]
+        extern static DateTime CallPrivateSerializationConstructor(SerializationInfo si, StreamingContext ct);
+#endif
     }
 
     internal static bool? IsDataAvailable(this BinaryReader reader, long requiredBytes)

src/libraries/System.Formats.Nrbf/tests/EdgeCaseTests.cs

@@ -1,4 +1,5 @@
-using System.IO;
+using System.Collections.Generic;
+using System.IO;
 using System.Runtime.Serialization.Formatters;
 using System.Runtime.Serialization.Formatters.Binary;
 using Microsoft.DotNet.XUnitExtensions;
@@ -103,4 +104,44 @@ public void FormatterTypeStyleOtherThanTypesAlwaysAreNotSupportedByDesign(Format
 
         Assert.Throws<NotSupportedException>(() => NrbfDecoder.Decode(ms));
     }
+
+    public static IEnumerable<object[]> CanReadAllKindsOfDateTimes_Arguments
+    {
+        get
+        {
+            yield return new object[] { new DateTime(1990, 11, 24, 0, 0, 0, DateTimeKind.Local) };
+            yield return new object[] { new DateTime(1990, 11, 25, 0, 0, 0, DateTimeKind.Utc) };
+            yield return new object[] { new DateTime(1990, 11, 26, 0, 0, 0, DateTimeKind.Unspecified) };
+        }
+    }
+
+    [Theory]
+    [MemberData(nameof(CanReadAllKindsOfDateTimes_Arguments))]
+    public void CanReadAllKindsOfDateTimes_DateTimeIsTheRootRecord(DateTime input)
+    {
+        using MemoryStream stream = Serialize(input);
+
+        PrimitiveTypeRecord<DateTime> dateTimeRecord = (PrimitiveTypeRecord<DateTime>)NrbfDecoder.Decode(stream);
+
+        Assert.Equal(input.Ticks, dateTimeRecord.Value.Ticks);
+        Assert.Equal(input.Kind, dateTimeRecord.Value.Kind);
+    }
+
+    [Serializable]
+    public class ClassWithDateTime
+    {
+        public DateTime Value;
+    }
+
+    [Theory]
+    [MemberData(nameof(CanReadAllKindsOfDateTimes_Arguments))]
+    public void CanReadAllKindsOfDateTimes_DateTimeIsMemberOfTheRootRecord(DateTime input)
+    {
+        using MemoryStream stream = Serialize(new ClassWithDateTime() { Value = input });
+
+        ClassRecord classRecord = NrbfDecoder.DecodeClassRecord(stream);
+
+        Assert.Equal(input.Ticks, classRecord.GetDateTime(nameof(ClassWithDateTime.Value)).Ticks);
+        Assert.Equal(input.Kind, classRecord.GetDateTime(nameof(ClassWithDateTime.Value)).Kind);
+    }
 }