Fix NullReferenceException and InvalidCastException in SystemClassWithMembersAndTypesRecord

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

@@ -95,7 +95,9 @@ SerializationRecord Create<T>(T value) where T : unmanaged
         bool HasMember(string name, int order, PrimitiveType primitiveType)
             => classInfo.MemberNames.TryGetValue(name, out int memberOrder)
             && memberOrder == order
-            && ((PrimitiveType)memberTypeInfo.Infos[order].AdditionalInfo!) == primitiveType;
+            && memberTypeInfo.Infos[order].BinaryType == BinaryType.Primitive
+            && memberTypeInfo.Infos[order].AdditionalInfo is PrimitiveType pt
+            && pt == primitiveType;
     }
 
     internal override (AllowedRecordTypes allowed, PrimitiveType primitiveType) GetNextAllowedRecordType()

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

@@ -632,4 +632,26 @@ public void SurrogateCharacters(bool array)
 
         Assert.Throws<SerializationException>(() => NrbfDecoder.Decode(stream));
     }
+
+    [Fact]
+    public void FuzzerInput_SystemClassInsteadOfPrimitive_DateTime()
+    {
+        // This test reproduces the InvalidCastException found by the fuzzer
+        // where dateData member has BinaryType.SystemClass instead of BinaryType.Primitive
+        byte[] input = Convert.FromBase64String("AAGAAAD/////AQAAAAAAAAAEAQAAAA9TeXN0ZW0uRGF0ZVRpbWUCAAAABXRpY2tzCGRhdGVEYXRhAAMJEAAAYF9qYWdnZWQGBQoKCgoHAQD3/wABAAAAAgAtAAAGCgoKCgs=");
+
+        using MemoryStream stream = new MemoryStream(input);
+        Assert.Throws<SerializationException>(() => NrbfDecoder.Decode(stream));
+    }
+
+    [Fact]
+    public void FuzzerInput_MissingAdditionalInfo_DateTime()
+    {
+        // This test reproduces the NullReferenceException found by the fuzzer
+        // where BinaryType.String has null AdditionalInfo
+        byte[] input = Convert.FromBase64String("AAEAAMz/////AQAAAAAAAAAEAQAAAA9TeXN0ZW0uRGF0ZVRpbWUCAAAABXRpY2tzCGRhdGVEYXRhAAEJEAAAYF+5MtwIAABgX7k2sLCwsLCwsLCwsLCwsLCwsLCwsNHcCAk=");
+
+        using MemoryStream stream = new MemoryStream(input);
+        Assert.Throws<SerializationException>(() => NrbfDecoder.Decode(stream));
+    }
 }