Added value comparer for arrays

* Multidimensional arrays not yet supported (#314)
* In some cases boxing will occur when comparing (and snapshotting) the
  elements (dotnet/efcore#11072)

Closes #305

src/EFCore.PG/Storage/Internal/Mapping/NpgsqlArrayTypeMapping.cs

@@ -23,7 +23,12 @@
 
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
+using System.Linq.Expressions;
 using System.Text;
+using JetBrains.Annotations;
+using Microsoft.EntityFrameworkCore.ChangeTracking;
+using Microsoft.EntityFrameworkCore.Migrations.Operations;
 
 namespace Microsoft.EntityFrameworkCore.Storage.Internal
 {
@@ -35,11 +40,13 @@ public sealed class NpgsqlArrayTypeMapping : RelationalTypeMapping
         /// Creates the default array mapping (i.e. for the single-dimensional CLR array type)
         /// </summary>
         internal NpgsqlArrayTypeMapping(RelationalTypeMapping elementMapping)
-            : this(elementMapping, elementMapping.ClrType.MakeArrayType())
-        {}
+            : this(elementMapping, elementMapping.ClrType.MakeArrayType()) {}
 
         internal NpgsqlArrayTypeMapping(RelationalTypeMapping elementMapping, Type arrayType)
-            : base(GenerateArrayTypeName(elementMapping.StoreType), arrayType)
+            : this(elementMapping, arrayType, CreateComparer(elementMapping, arrayType)) {}
+
+        NpgsqlArrayTypeMapping(RelationalTypeMapping elementMapping, Type arrayType, ValueComparer comparer)
+            : base(GenerateArrayTypeName(elementMapping.StoreType), arrayType, null, comparer)
         {
             ElementMapping = elementMapping;
         }
@@ -74,7 +81,7 @@ static string GenerateArrayTypeName(string elementTypeName)
         }
 
         public override RelationalTypeMapping Clone(string storeType, int? size)
-            => new NpgsqlArrayTypeMapping(ElementMapping);
+            => new NpgsqlArrayTypeMapping(ElementMapping, ClrType, Comparer);
 
         protected override string GenerateNonNullSqlLiteral(object value)
         {
@@ -95,5 +102,146 @@ protected override string GenerateNonNullSqlLiteral(object value)
             sb.Append("]");
             return sb.ToString();
         }
+
+        #region Value Comparison
+
+        static ValueComparer CreateComparer(RelationalTypeMapping elementMapping, Type arrayType)
+        {
+            Debug.Assert(arrayType.IsArray);
+            var elementType = arrayType.GetElementType();
+
+            // In .NET, single-dimensional arrays implement IList<T> and can therefore be accessed generically
+            // (i.e. efficiently). Multi-dimensional arrays don't, and can only be accessed non-generically via IList.
+
+            //if (!arrayType.IsAssignableFrom(typeof(IList<>).MakeGenericType(elementType)))
+            if (!typeof(IList<>).MakeGenericType(elementType).IsAssignableFrom(arrayType))
+                return null;   // TODO: Implement multi-dimensional array support (#314)
+
+            // We usee different comparer implementations based on whether we have a non-null element comparer,
+            // and if not, whether the element is IEquatable<TElem>
+
+            if (elementMapping.Comparer != null)
+                return (ValueComparer)Activator.CreateInstance(
+                    typeof(SingleDimComparerWithComparer<>).MakeGenericType(elementType), elementMapping);
+
+            if (typeof(IEquatable<>).MakeGenericType(elementType).IsAssignableFrom(elementType))
+                return (ValueComparer)Activator.CreateInstance(typeof(SingleDimComparerWithIEquatable<>).MakeGenericType(elementType));
+
+            // There's no custom comparer, and the element type doesn't implement IEquatable<TElem>. We have
+            // no choice but to use the non-generic Equals method.
+            return (ValueComparer)Activator.CreateInstance(typeof(SingleDimComparerWithEquals<>).MakeGenericType(elementType));
+        }
+
+        class SingleDimComparerWithComparer<TElem> : ValueComparer<IList<TElem>>
+        {
+            public SingleDimComparerWithComparer(RelationalTypeMapping elementMapping) : base(
+                (a, b) => Compare(a, b, elementMapping.Comparer.CompareFunc),
+                source => Snapshot(source, elementMapping.Comparer.SnapshotFunc)) {}
+
+            static bool Compare(IList<TElem> a, IList<TElem> b, Func<object, object, bool> elementComparer)
+            {
+                if (a.Count != b.Count)
+                    return false;
+
+                // Note: the following currently boxes every element access because ValueComparer isn't really
+                // generic (see https://github.com/aspnet/EntityFrameworkCore/issues/11072)
+                for (var i = 0; i < a.Count; i++)
+                    if (!elementComparer(a[i], b[i]))
+                        return false;
+
+                return true;
+            }
+
+            static IList<TElem> Snapshot(IList<TElem> source, Func<object, object> elementSnapshotFunc)
+            {
+                var snapshot = new TElem[source.Count];
+                // Note: the following currently boxes every element access because ValueComparer isn't really
+                // generic (see https://github.com/aspnet/EntityFrameworkCore/issues/11072)
+                for (var i = 0; i < source.Count; i++)
+                    snapshot[i] = (TElem)elementSnapshotFunc(source[i]);
+                return snapshot;
+            }
+        }
+
+        class SingleDimComparerWithIEquatable<TElem> : ValueComparer<IList<TElem>>
+            where TElem : IEquatable<TElem>
+        {
+            public SingleDimComparerWithIEquatable(): base(
+                (a, b) => Compare(a, b),
+                source => Snapshot(source)) {}
+
+            static bool Compare(IList<TElem> a, IList<TElem> b)
+            {
+                if (a.Count != b.Count)
+                    return false;
+
+                for (var i = 0; i < a.Count; i++)
+                {
+                    var elem1 = a[i];
+                    var elem2 = b[i];
+                    if (elem1 == null)
+                    {
+                        if (elem2 == null)
+                            continue;
+                        return false;
+                    }
+                    if (!elem1.Equals(elem2))
+                        return false;
+                }
+
+                return true;
+            }
+
+            static IList<TElem> Snapshot(IList<TElem> source)
+            {
+                var snapshot = new TElem[source.Count];
+                for (var i = 0; i < source.Count; i++)
+                    snapshot[i] = source[i];
+                return snapshot;
+            }
+        }
+
+        class SingleDimComparerWithEquals<TElem> : ValueComparer<IList<TElem>>
+        {
+            public SingleDimComparerWithEquals() : base(
+                (a, b) => Compare(a, b),
+                source => Snapshot(source)) {}
+
+            static bool Compare(IList<TElem> a, IList<TElem> b)
+            {
+                if (a.Count != b.Count)
+                    return false;
+
+                // Note: the following currently boxes every element access because ValueComparer isn't really
+                // generic (see https://github.com/aspnet/EntityFrameworkCore/issues/11072)
+                for (var i = 0; i < a.Count; i++)
+                {
+                    var elem1 = a[i];
+                    var elem2 = b[i];
+                    if (elem1 == null)
+                    {
+                        if (elem2 == null)
+                            continue;
+                        return false;
+                    }
+                    if (!elem1.Equals(elem2))
+                        return false;
+                }
+
+                return true;
+            }
+
+            static IList<TElem> Snapshot(IList<TElem> source)
+            {
+                var snapshot = new TElem[source.Count];
+                // Note: the following currently boxes every element access because ValueComparer isn't really
+                // generic (see https://github.com/aspnet/EntityFrameworkCore/issues/11072)
+                for (var i = 0; i < source.Count; i++)
+                    snapshot[i] = source[i];
+                return snapshot;
+            }
+        }
+
+        #endregion Value Comparison
     }
 }

test/EFCore.PG.Tests/Storage/NpgsqlTypeMappingTest.cs

@@ -163,6 +163,39 @@ public void GenerateSqlLiteral_returns_bit_literal()
         public void GenerateSqlLiteral_returns_array_literal()
             => Assert.Equal("ARRAY[3,4]", GetMapping(typeof(int[])).GenerateSqlLiteral(new[] {3, 4}));
 
+        [Fact]
+        public void ValueComparer_int_array()
+        {
+            // This exercises array's comparer when the element doesn't have a comparer, but it implements
+            // IEquatable<T>
+            var source = new[] { 2, 3, 4 };
+
+            var comparer = GetMapping(typeof(int[])).Comparer;
+            var snapshot = (int[])comparer.SnapshotFunc(source);
+            Assert.Equal(source, snapshot);
+            Assert.True(comparer.CompareFunc(source, snapshot));
+            snapshot[1] = 8;
+            Assert.False(comparer.CompareFunc(source, snapshot));
+        }
+
+        [Fact]
+        public void ValueComparer_hstore_array()
+        {
+            // This exercises array's comparer when the element has its own non-null comparer
+            var source = new[]
+            {
+                new Dictionary<string, string> { { "k1", "v1"} },
+                new Dictionary<string, string> { { "k2", "v2"} },
+            };
+
+            var comparer = GetMapping(typeof(Dictionary<string, string>[])).Comparer;
+            var snapshot = (Dictionary<string, string>[])comparer.SnapshotFunc(source);
+            Assert.Equal(source, snapshot);
+            Assert.True(comparer.CompareFunc(source, snapshot));
+            snapshot[1]["k2"] = "v8";
+            Assert.False(comparer.CompareFunc(source, snapshot));
+        }
+
         [Fact]
         public void GenerateSqlLiteral_returns_bytea_literal()
             => Assert.Equal(@"BYTEA E'\\xDEADBEEF'", GetMapping("bytea").GenerateSqlLiteral(new byte[] { 222, 173, 190, 239 }));
@@ -184,7 +217,7 @@ public void ValueComparer_hstore()
                 { "k1", "v1"},
                 { "k2", "v2"}
             };
-            
+
             var comparer = GetMapping("hstore").Comparer;
             var snapshot = (Dictionary<string, string>)comparer.SnapshotFunc(source);
             Assert.Equal(source, snapshot);