Consider null values during comparison operations

Resolves #11

EFCore.SqlServer.HierarchyId.Abstractions/HierarchyId.cs

@@ -131,7 +131,12 @@ public override string ToString()
         /// <param name="hid2">The second node to compare.</param>
         /// <returns>True if <paramref name="hid1"/> and <paramref name="hid2"/> are equal; otherwise, false.</returns>
         public static bool operator ==(HierarchyId hid1, HierarchyId hid2)
-            => Unwrap(hid1).CompareTo(Unwrap(hid2)) == 0;
+        {
+            var sh1 = Unwrap(hid1);
+            var sh2 = Unwrap(hid2);
+
+            return sh1.IsNull == sh2.IsNull && sh1.CompareTo(sh2) == 0;
+        }
 
         /// <summary>
         /// Evaluates whether two nodes are unequal.
@@ -140,7 +145,12 @@ public override string ToString()
         /// <param name="hid2">The second node to compare.</param>
         /// <returns>True if <paramref name="hid1"/> and <paramref name="hid2"/> are unequal; otherwise, false.</returns>
         public static bool operator !=(HierarchyId hid1, HierarchyId hid2)
-            => Unwrap(hid1).CompareTo(Unwrap(hid2)) != 0;
+        {
+            var sh1 = Unwrap(hid1);
+            var sh2 = Unwrap(hid2);
+
+            return sh1.IsNull != sh2.IsNull || sh1.CompareTo(sh2) != 0;
+        }
 
         /// <summary>
         /// Evaluates whether one node is less than another.
@@ -149,7 +159,12 @@ public override string ToString()
         /// <param name="hid2">The second node to compare.</param>
         /// <returns>True if <paramref name="hid1"/> is less than <paramref name="hid2"/>; otherwise, false.</returns>
         public static bool operator <(HierarchyId hid1, HierarchyId hid2)
-            => Unwrap(hid1).CompareTo(Unwrap(hid2)) < 0;
+        {
+            var sh1 = Unwrap(hid1);
+            var sh2 = Unwrap(hid2);
+
+            return !sh1.IsNull && !sh2.IsNull && sh1.CompareTo(sh2) < 0;
+        }
 
         /// <summary>
         /// Evaluates whether one node is greater than another.
@@ -158,7 +173,12 @@ public override string ToString()
         /// <param name="hid2">The second node to compare.</param>
         /// <returns>True if <paramref name="hid1"/> is greather than <paramref name="hid2"/>; otherwise, false.</returns>
         public static bool operator >(HierarchyId hid1, HierarchyId hid2)
-            => Unwrap(hid1).CompareTo(Unwrap(hid2)) > 0;
+        {
+            var sh1 = Unwrap(hid1);
+            var sh2 = Unwrap(hid2);
+
+            return !sh1.IsNull && !sh2.IsNull && sh1.CompareTo(sh2) > 0;
+        }
 
         /// <summary>
         /// Evaluates whether one node is less than or equal to another.
@@ -167,7 +187,12 @@ public override string ToString()
         /// <param name="hid2">The second node to compare.</param>
         /// <returns>True if <paramref name="hid1"/> is less than or equal to <paramref name="hid2"/>; otherwise, false.</returns>
         public static bool operator <=(HierarchyId hid1, HierarchyId hid2)
-            => Unwrap(hid1).CompareTo(Unwrap(hid2)) <= 0;
+        {
+            var sh1 = Unwrap(hid1);
+            var sh2 = Unwrap(hid2);
+
+            return !sh1.IsNull && !sh2.IsNull && sh1.CompareTo(sh2) <= 0;
+        }
 
         /// <summary>
         /// Evaluates whether one node is greater than or equal to another.
@@ -176,12 +201,17 @@ public override string ToString()
         /// <param name="hid2">The second node to compare.</param>
         /// <returns>True if <paramref name="hid1"/> is greater than or equal to <paramref name="hid2"/>; otherwise, false.</returns>
         public static bool operator >=(HierarchyId hid1, HierarchyId hid2)
-            => Unwrap(hid1).CompareTo(Unwrap(hid2)) >= 0;
+        {
+            var sh1 = Unwrap(hid1);
+            var sh2 = Unwrap(hid2);
+
+            return !sh1.IsNull && !sh2.IsNull && sh1.CompareTo(sh2) >= 0;
+        }
 
         private static SqlHierarchyId Unwrap(HierarchyId value)
             => value?._value ?? SqlHierarchyId.Null;
 
         private static HierarchyId Wrap(SqlHierarchyId value)
             => value.IsNull ? null : new HierarchyId(value);
     }
-}
+}

EFCore.SqlServer.HierarchyId.Test/NullabilityTests.cs

@@ -0,0 +1,80 @@
+using System.Linq;
+using Xunit;
+
+namespace Microsoft.EntityFrameworkCore.SqlServer
+{
+    public class NullabilityTests
+    {
+        [Fact]
+        public void Null_against_null()
+        {
+            Assert.True((HierarchyId)null == (HierarchyId)null);
+            Assert.False((HierarchyId)null != (HierarchyId)null);
+            Assert.False((HierarchyId)null > (HierarchyId)null);
+            Assert.False((HierarchyId)null >= (HierarchyId)null);
+            Assert.False((HierarchyId)null < (HierarchyId)null);
+            Assert.False((HierarchyId)null <= (HierarchyId)null);
+        }
+
+        [Fact]
+        public void Null_against_nonNull()
+        {
+            var hid = HierarchyId.GetRoot();
+            Assert.False(hid == (HierarchyId)null);
+            Assert.False((HierarchyId)null == hid);
+
+            Assert.True(hid != (HierarchyId)null);
+            Assert.True((HierarchyId)null != hid);
+
+            Assert.False(hid > (HierarchyId)null);
+            Assert.False((HierarchyId)null > hid);
+
+            Assert.False(hid >= (HierarchyId)null);
+            Assert.False((HierarchyId)null >= hid);
+
+            Assert.False(hid < (HierarchyId)null);
+            Assert.False((HierarchyId)null < hid);
+
+            Assert.False(hid <= (HierarchyId)null);
+            Assert.False((HierarchyId)null <= hid);
+        }
+
+        [Fact]
+        public void NullOnly_aggregates_equalTo_null()
+        {
+            var hid = (HierarchyId)null;
+            var collection = new[] { (HierarchyId)null, (HierarchyId)null, };
+            var min = collection.Min();
+            var max = collection.Max();
+
+            Assert.True(hid == min);
+            Assert.True(min == hid);
+            Assert.False(hid != min);
+            Assert.False(min != hid);
+
+            Assert.True(hid == max);
+            Assert.True(max == hid);
+            Assert.False(hid != max);
+            Assert.False(max != hid);
+        }
+
+        [Fact]
+        public void Aggregates_including_nulls_equalTo_nonNull()
+        {
+            var hid = HierarchyId.GetRoot();
+            var collection = new[] { (HierarchyId)null, (HierarchyId)null, HierarchyId.GetRoot(), HierarchyId.GetRoot(), };
+            var min = collection.Min();
+            var max = collection.Max();
+
+            Assert.True(hid == min);
+            Assert.True(min == hid);
+            Assert.False(hid != min);
+            Assert.False(min != hid);
+
+            Assert.True(hid == max);
+            Assert.True(max == hid);
+            Assert.False(hid != max);
+            Assert.False(max != hid);
+        }
+    }
+}