NH-3884 - Default DateTime type can not be overridden

src/NHibernate.Test/TypesTest/ChangeDefaultTypeClass.cs

@@ -0,0 +1,11 @@
+using System;
+
+namespace NHibernate.Test.TypesTest
+{
+	public class ChangeDefaultTypeClass
+	{
+		public int Id { get; set; }
+
+		public DateTime NormalDateTimeValue { get; set; } = DateTime.Today;
+	}
+}

src/NHibernate.Test/TypesTest/ChangeDefaultTypeClass.hbm.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="utf-8" ?>
+
+<hibernate-mapping xmlns="urn:nhibernate-mapping-2.2" default-lazy="false">
+
+  <class
+    name="NHibernate.Test.TypesTest.ChangeDefaultTypeClass, NHibernate.Test"
+    table="bc_datetime"
+  >
+
+    <id name="Id" column="id">
+      <generator class="assigned"/>
+    </id>
+
+    <property name="NormalDateTimeValue"/>
+  </class>
+</hibernate-mapping>

src/NHibernate.Test/TypesTest/ChangeDefaultTypeFixture.cs

@@ -0,0 +1,85 @@
+using System;
+using System.Collections.Generic;
+using System.Reflection;
+using NHibernate.Cfg;
+using NHibernate.Engine;
+using NHibernate.Impl;
+using NHibernate.Type;
+using NUnit.Framework;
+
+namespace NHibernate.Test.TypesTest
+{
+	/// <summary>
+	/// TestFixtures for changing a default .Net type.
+	/// </summary>
+	[TestFixture]
+	public class ChangeDefaultTypeFixture : TypeFixtureBase
+	{
+		protected override string TypeName => "ChangeDefaultType";
+
+		private IType _originalDefaultDateTimeType;
+		private IType _testDefaultDateTimeType;
+
+		protected override void Configure(Configuration configuration)
+		{
+			_originalDefaultDateTimeType = TypeFactory.GetDefaultTypeFor(typeof(DateTime));
+			Assert.That(_originalDefaultDateTimeType, Is.Not.Null);
+			_testDefaultDateTimeType = NHibernateUtil.DateTime.Equals(_originalDefaultDateTimeType)
+				? (IType) NHibernateUtil.DateTimeNoMs
+				: NHibernateUtil.DateTime;
+			TypeFactory.RegisterType(typeof(DateTime), _testDefaultDateTimeType, TypeFactory.EmptyAliases);
+			base.Configure(configuration);
+		}
+
+		protected override void DropSchema()
+		{
+			base.DropSchema();
+			if (_originalDefaultDateTimeType != null)
+				TypeFactory.RegisterType(typeof(DateTime), _originalDefaultDateTimeType, TypeFactory.EmptyAliases);
+		}
+
+		[Test]
+		public void DefaultType()
+		{
+			Assert.That(TypeFactory.GetDefaultTypeFor(typeof(DateTime)), Is.EqualTo(_testDefaultDateTimeType));
+		}
+
+		[Test]
+		public void PropertyType()
+		{
+			Assert.That(
+				Sfi.GetClassMetadata(typeof(ChangeDefaultTypeClass))
+				   .GetPropertyType(nameof(ChangeDefaultTypeClass.NormalDateTimeValue)),
+				Is.EqualTo(_testDefaultDateTimeType));
+		}
+
+		[Test]
+		public void GuessType()
+		{
+			Assert.That(NHibernateUtil.GuessType(typeof(DateTime)), Is.EqualTo(_testDefaultDateTimeType));
+		}
+
+		[Test]
+		public void ParameterType()
+		{
+			var namedParametersField = typeof(AbstractQueryImpl)
+				.GetField("namedParameters", BindingFlags.Instance | BindingFlags.NonPublic);
+			Assert.That(namedParametersField, Is.Not.Null, "Missing internal field");
+
+			using (var s = OpenSession())
+			{
+				// Query where the parameter type cannot be deducted from compared entity property
+				var q = s.CreateQuery($"from {nameof(ChangeDefaultTypeClass)} where :date1 = :date2 or :date1 = :date3")
+				         .SetParameter("date1", DateTime.Now)
+				         .SetDateTime("date2", DateTime.Now)
+				         .SetDateTimeNoMs("date3", DateTime.Now);
+
+				var namedParameters = namedParametersField.GetValue(q) as Dictionary<string, TypedValue>;
+				Assert.That(namedParameters, Is.Not.Null, "Unable to retrieve parameters internal field");
+				Assert.That(namedParameters["date1"].Type, Is.EqualTo(_testDefaultDateTimeType));
+				Assert.That(namedParameters["date2"].Type, Is.EqualTo(NHibernateUtil.DateTime));
+				Assert.That(namedParameters["date3"].Type, Is.EqualTo(NHibernateUtil.DateTimeNoMs));
+			}
+		}
+	}
+}

src/NHibernate/Type/TypeFactory.cs

@@ -6,6 +6,7 @@
 using System.Xml;
 using System.Xml.Linq;
 using NHibernate.Bytecode;
+using NHibernate.Cfg;
 using NHibernate.Classic;
 using NHibernate.SqlTypes;
 using NHibernate.UserTypes;
@@ -33,8 +34,9 @@ private enum TypeClassification
 			PrecisionScale
 		}
 
+		public static readonly string[] EmptyAliases = System.Array.Empty<string>();
+
 		private static readonly INHibernateLogger _log = NHibernateLogger.For(typeof(TypeFactory));
-		private static readonly string[] EmptyAliases= System.Array.Empty<string>();
 		private static readonly char[] PrecisionScaleSplit = { '(', ')', ',' };
 		private static readonly char[] LengthSplit = { '(', ')' };
 
@@ -95,7 +97,15 @@ private enum TypeClassification
 
 		private delegate NullableType NullableTypeCreatorDelegate(SqlType sqlType);
 
-		private static void RegisterType(System.Type systemType, IType nhibernateType, IEnumerable<string> aliases)
+		/// <summary>
+		/// <para>Defines which NHibernate type should be chosen by default for handling a given .Net type.</para>
+		/// <para>This must be done before any operation on NHibernate, including building its
+		/// <see cref="Configuration" /> and building session factory. Otherwise the behavior will be undefined.</para>
+		/// </summary>
+		/// <param name="systemType">The .Net type.</param>
+		/// <param name="nhibernateType">The NHibernate type.</param>
+		/// <param name="aliases">The additional aliases to map to the type. Use <see cref="EmptyAliases"/> if none.</param>
+		public static void RegisterType(System.Type systemType, IType nhibernateType, IEnumerable<string> aliases)
 		{
 			var typeAliases = new List<string>(aliases);
 			typeAliases.AddRange(GetClrTypeAliases(systemType));