Fix delayed converter resolution for nullables (#6453)

NinoFloris · NinoFloris · commit 87e8b6ad600f · 2026-03-12T06:59:44.000+01:00
(cherry picked from commit 013e771)
diff --git a/src/Npgsql/Internal/Converters/ArrayConverter.cs b/src/Npgsql/Internal/Converters/ArrayConverter.cs
@@ -619,45 +619,40 @@ protected override PgConverter<T> CreateConverter(PgConverterResolution effectiv
     protected override PgConverterResolution? GetEffectiveResolution(T? values, PgTypeId? expectedEffectivePgTypeId)
     {
         PgConverterResolution? resolution = null;
-        if (values is null)
+        switch (values)
         {
-            resolution = EffectiveTypeInfo.GetDefaultResolution(expectedEffectivePgTypeId);
-        }
-        else
-        {
-            switch (values)
-            {
-                case TElement[] array:
-                    foreach (var value in array)
-                    {
-                        var result = EffectiveTypeInfo.GetResolution(value, resolution?.PgTypeId ?? expectedEffectivePgTypeId);
-                        resolution ??= result;
-                    }
-                    break;
-                case List<TElement> list:
-                    foreach (var value in list)
-                    {
-                        var result = EffectiveTypeInfo.GetResolution(value, resolution?.PgTypeId ?? expectedEffectivePgTypeId);
-                        resolution ??= result;
-                    }
-                    break;
-                case IList<TElement> list:
-                    foreach (var value in list)
-                    {
-                        var result = EffectiveTypeInfo.GetResolution(value, resolution?.PgTypeId ?? expectedEffectivePgTypeId);
-                        resolution ??= result;
-                    }
-                    break;
-                case Array array:
-                    foreach (var value in array)
-                    {
-                        var result = EffectiveTypeInfo.GetResolutionAsObject(value, resolution?.PgTypeId ?? expectedEffectivePgTypeId);
-                        resolution ??= result;
-                    }
-                    break;
-                default:
-                    throw new NotSupportedException();
-            }
+            case TElement[] array:
+                foreach (var value in array)
+                {
+                    var result = EffectiveTypeInfo.GetResolution(value, resolution?.PgTypeId ?? expectedEffectivePgTypeId);
+                    resolution ??= result;
+                }
+                break;
+            case List<TElement> list:
+                foreach (var value in list)
+                {
+                    var result = EffectiveTypeInfo.GetResolution(value, resolution?.PgTypeId ?? expectedEffectivePgTypeId);
+                    resolution ??= result;
+                }
+                break;
+            case IList<TElement> list:
+                foreach (var value in list)
+                {
+                    var result = EffectiveTypeInfo.GetResolution(value, resolution?.PgTypeId ?? expectedEffectivePgTypeId);
+                    resolution ??= result;
+                }
+                break;
+            case Array array:
+                foreach (var value in array)
+                {
+                    var result = EffectiveTypeInfo.GetResolutionAsObject(value, resolution?.PgTypeId ?? expectedEffectivePgTypeId);
+                    resolution ??= result;
+                }
+                break;
+            case null:
+                break;
+            default:
+                throw new NotSupportedException();
         }
 
         return resolution;
diff --git a/src/Npgsql/Internal/Converters/NullableConverter.cs b/src/Npgsql/Internal/Converters/NullableConverter.cs
@@ -50,7 +50,7 @@ sealed class NullableConverterResolver<T>(PgResolverTypeInfo effectiveTypeInfo)
         => new NullableConverter<T>(effectiveResolution.GetConverter<T>());
 
     protected override PgConverterResolution? GetEffectiveResolution(T? value, PgTypeId? expectedEffectivePgTypeId)
-        => value is null
-            ? EffectiveTypeInfo.GetDefaultResolution(expectedEffectivePgTypeId)
-            : EffectiveTypeInfo.GetResolution(value.GetValueOrDefault(), expectedEffectivePgTypeId);
+        => value is { } inner
+            ? EffectiveTypeInfo.GetResolution(inner, expectedEffectivePgTypeId)
+            : null;
 }
diff --git a/test/Npgsql.Tests/Types/DateTimeTests.cs b/test/Npgsql.Tests/Types/DateTimeTests.cs
@@ -488,6 +488,18 @@ await AssertType(datasource,
             "timestamp with time zone[]",
             NpgsqlDbType.TimestampTz | NpgsqlDbType.Array);
 
+        // Make sure delayed converter resolution works when null precedes a non-null value.
+        // We expect the resolution of null values to not lock in the default type timestamp.
+        // This would cause the subsequent non-null value to fail to convert, as it requires timestamptz.
+        await AssertType(datasource,
+            new DateTime?[]
+            {
+                null,
+                new DateTime(1998, 4, 12, 13, 26, 38, DateTimeKind.Utc)
+            },
+            @"{NULL,""1998-04-12 15:26:38+02""}",
+            "timestamp with time zone[]");
+
         await AssertType(datasource,
             new DateTime?[]
             {
