Serialize nullable nullable types (#465)

.gitignore

@@ -8,3 +8,4 @@ BenchmarkDotNet.Artifacts/
 Parquet.sln.DotSettings.user
 launchSettings.json
 .DS_Store
+*.user

src/Parquet.Test/Serialisation/ParquetSerializerTest.cs

@@ -162,6 +162,13 @@
             public double? ParentMeterValue { get; set; }
 
             public decimal? ParentResolution { get; set; }
+
+            public DateTime? ParentTime { get; set; }
+
+            public Interval? ParentInterval { get; set; }
+
+            public TimeSpan? ParentTimeSpan { get; set; }
+
         }
 
         [Fact]
@@ -172,8 +179,12 @@
                 EventName = i % 2 == 0 ? "on" : "off",
                 MeterValue = i,
                 ParentId = (i % 4 == 0) ? null : i,
+                ExternalId = Guid.NewGuid(),
                 ParentMeterValue = (i % 5 == 0) ? null : (double?)i,
-                ParentResolution = (i % 6 == 0) ? null : (decimal?)i
+                ParentResolution = (i % 6 == 0) ? null : (decimal?)i,
+                ParentTime = (i % 5 == 0) ? null : DateTime.UtcNow.AddSeconds(i),
+                ParentInterval = (i % 5 == 0) ? null : new Interval(i, i, i),
+                ParentTimeSpan = (i % 5 == 0) ? null : TimeSpan.FromSeconds(i)
             }).ToList();
 
             await Compare(data);
@@ -186,8 +197,13 @@
                 ["Timestamp"] = DateTime.UtcNow.AddSeconds(i),
                 ["EventName"] = i % 2 == 0 ? "on" : "off",
                 ["MeterValue"] = (double)i,
                 ["ParentId"] = (i % 4 == 0) ? null : i,
-                ["ExternalId"] = Guid.NewGuid()
+                ["ExternalId"] = Guid.NewGuid(),
+                ["ParentMeterValue"] = (i % 5 == 0) ? null : (double?)i,
+                ["ParentResolution"] = (i % 6 == 0) ? null : (decimal?)i,
+                ["ParentTime"] = (i % 5 == 0) ? null : DateTime.UtcNow.AddSeconds(i),
+                ["ParentInterval"] = (i % 5 == 0) ? null : new Interval(i, i, i),
+                ["ParentTimeSpan"] = (i % 5 == 0) ? null : TimeSpan.FromSeconds(i)
             }).ToList();
 
             await DictCompare<NullableRecord>(data);
@@ -380,7 +396,7 @@
         public async Task List_Structs_Serde_Dict() {
             var data = Enumerable.Range(0, 1_000).Select(i => new Dictionary<string, object> {
                 ["PersonId"] = i,
                 ["Comments"] = i % 2 == 0 ? "none" : null,
                 ["Addresses"] = Enumerable.Range(0, 4).Select(a => new Dictionary<string, object> {
                     ["City"] = "Birmingham",
                     ["Country"] = "United Kingdom"

src/Parquet/Serialization/TypeExtensions.cs

@@ -127,7 +127,7 @@ private static Field ConstructDataField(string name, string propertyName, Type t
             Field r;
             bool? isNullable = member == null
                 ? null
-                : member.IsRequired ? false : null;
+                : member.IsRequired ? false : IsNullable(t);
 
             if(t == typeof(DateTime) || t == typeof(DateTime?)) {
                 ParquetTimestampAttribute? tsa = member?.TimestampAttribute;
@@ -165,6 +165,14 @@ private static Field ConstructDataField(string name, string propertyName, Type t
             return r;
         }
 
+        static bool IsNullable(Type type) {
+            if(!type.IsValueType)
+                return true; // ref-type
+            if(Nullable.GetUnderlyingType(type) != null)
+                return true; // Nullable<T>
+            return false; // value-type
+        }
+
         private static MapField ConstructMapField(string name, string propertyName,
             Type tKey, Type tValue,
             bool forWriting) {