Documentation for value comparers (#2192)

Fixes #1986

entity-framework/core/modeling/value-comparers.md

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+---
+title: Value Comparers - EF Core
+description: Using value comparers to control how EF Core compares property values 
+author: ajcvickers
+ms.date: 03/20/2020
+uid: core/modeling/value-comparers
+---
+
+# Value Comparers
+
+> [!NOTE]  
+> This feature is new in EF Core 3.0.
+
+> [!TIP]  
+> The code in this document can be found on GitHub as a [runnable sample](https://github.com/dotnet/EntityFramework.Docs/tree/master/samples/core/Modeling/ValueConversions/).
+
+## Background
+
+EF Core needs to compare property values when:
+
+* Determining whether a property has been changed as part of [detecting changes for updates](xref:core/saving/basic)
+* Determining whether two key values are the same when resolving relationships 
+
+This is handled automatically for common primitive types such as int, bool, DateTime, etc.
+
+For more complex types, choices need to be made as to how to do the comparison.
+For example, a byte array could be compared:
+
+* By reference, such that a difference is only detected if a new byte array is used
+* By deep comparison, such that mutation of the bytes in the array is detected
+
+By default, EF Core uses the first of these approaches for non-key byte arrays.
+That is, only references are compared and a change is detected only when an existing byte array is replaced with a new one.
+This is a pragmatic decision that avoids deep comparison of many large byte arrays when executing SaveChanges.
+But the common scenario of replacing, say, an image with a different image is handled in a performant way.
+
+On the other hand, reference equality would not work when byte arrays are used to represent binary keys.
+It's very unlikely that an FK property is set to the _same instance_ as a PK property to which it needs to be compared.
+Therefore, EF Core uses deep comparisons for byte arrays acting as keys.
+This is unlikely to have a big performance hit since binary keys are usually short.
+
+### Snapshots
+
+Deep comparisons on mutable types means that EF Core needs the ability to create a deep "snapshot" of the property value.
+Just copying the reference instead would result in mutating both the current value and the snapshot, since they are _the same object_.
+Therefore, when deep comparisons are used on mutable types, deep snapshotting is also required.
+
+## Properties with value converters
+
+In the case above, EF Core has native mapping support for byte arrays and so can automatically choose appropriate defaults.
+However, if the property is mapped through a [value converter](xref:core/modeling/value-conversions), then EF Core can't always determine the appropriate comparison to use.
+Instead, EF Core always uses the default equality comparison defined by the type of the property.
+This is often correct, but may need to be overridden when mapping more complex types.
+
+### Simple immutable classes
+
+Consider a property the uses a value converter to map a simple, immutable class.
+
+[!code-csharp[SimpleImmutableClass](../../../samples/core/Modeling/ValueConversions/MappingImmutableClassProperty.cs?name=SimpleImmutableClass)]
+
+[!code-csharp[ConfigureImmutableClassProperty](../../../samples/core/Modeling/ValueConversions/MappingImmutableClassProperty.cs?name=ConfigureImmutableClassProperty)]
+
+Properties of this type do not need special comparisons or snapshots because:
+* Equality is overridden so that different instances will compare correctly
+* The type is immutable, so there is no chance of mutating a snapshot value
+
+So in this case the default behavior of EF Core is fine as it is.
+
+### Simple immutable Structs
+
+The mapping for simple structs is also simple and requires no special comparers or snapshotting.
+
+[!code-csharp[SimpleImmutableStruct](../../../samples/core/Modeling/ValueConversions/MappingImmutableStructProperty.cs?name=SimpleImmutableStruct)]
+
+[!code-csharp[ConfigureImmutableStructProperty](../../../samples/core/Modeling/ValueConversions/MappingImmutableStructProperty.cs?name=ConfigureImmutableStructProperty)]
+
+EF Core has built-in support for generating compiled, memberwise comparisons of struct properties.
+This means structs don't need to have equality overridden for EF, but you may still choose to do this for [other reasons](/dotnet/csharp/programming-guide/statements-expressions-operators/how-to-define-value-equality-for-a-type).
+Also, special snapshotting is not needed since structs immutable and are always memberwise copied anyway.
+(This is also true for mutable structs, but [mutable structs should in general be avoided](/dotnet/csharp/write-safe-efficient-code).)
+
+### Mutable classes
+
+It is recommended that you use immutable types (classes or structs) with value converters when possible.
+This is usually more efficient and has cleaner semantics than using a mutable type.
+
+However, that being said, it is common to use properties of types that the application cannot change.
+For example, mapping a property containing a list of numbers: 
+
+[!code-csharp[ListProperty](../../../samples/core/Modeling/ValueConversions/MappingListProperty.cs?name=ListProperty)]
+
+The [`List<T>` class](/dotnet/api/system.collections.generic.list-1?view=netstandard-2.1):
+* Has reference equality; two lists containing the same values are treated as different.
+* Is mutable; values in the list can be added and removed.
+
+A typical value conversion on a list property might convert the list to and from JSON:
+
+[!code-csharp[ConfigureListProperty](../../../samples/core/Modeling/ValueConversions/MappingListProperty.cs?name=ConfigureListProperty)]
+
+This then requires setting a `ValueComparer<T>` on the property to force EF Core use correct comparisons with this conversion:
+
+[!code-csharp[ConfigureListPropertyComparer](../../../samples/core/Modeling/ValueConversions/MappingListProperty.cs?name=ConfigureListPropertyComparer)]
+
+> [!NOTE]  
+> The model builder ("fluent") API to set a value comparer has not yet been implemented.
+> Instead, the code above calls SetValueComparer on the lower-level IMutableProperty exposed by the builder as 'Metadata'.
+
+The `ValueComparer<T>` constructor accepts three expressions:
+* An expression for checking quality
+* An expression for generating a hash code
+* An expression to snapshot a value  
+
+In this case the comparison is done by checking if the sequences of numbers are the same.
+
+Likewise, the hash code is built from this same sequence.
+(Note that this is a hash code over mutable values and hence can [cause problems](https://ericlippert.com/2011/02/28/guidelines-and-rules-for-gethashcode/).
+Be immutable instead if you can.)
+
+The snapshot is created by cloning the list with ToList.
+Again, this is only needed if the lists are going to be mutated.
+Be immutable instead if you can. 
+
+> [!NOTE]  
+> Value converters and comparers are constructed using expressions rather than simple delegates.
+> This is because EF inserts these expressions into a much more complex expression tree that is then compiled into an entity shaper delegate.
+> Conceptually, this is similar to compiler inlining.
+> For example, a simple conversion may just be a compiled in cast, rather than a call to another method to do the conversion.    
+
+### Key comparers
+
+The background section covers why key comparisons may require special semantics.
+Make sure to create a comparer that is appropriate for keys when setting it on a primary, principal, or foreign key property.
+
+Use [SetKeyValueComparer](/dotnet/api/microsoft.entityframeworkcore.mutablepropertyextensions.setkeyvaluecomparer?view=efcore-3.1) in the rare cases where different semantics is required on the same property.
+
+> [!NOTE]  
+> SetStructuralComparer has been obsoleted in EF Core 5.0.
+> Use SetKeyValueComparer instead.
+
+### Overriding defaults
+
+Sometimes the default comparison used by EF Core may not be appropriate.
+For example, mutation of byte arrays is not, by default, detected in EF Core.
+This can be overridden by setting a different comparer on the property: 
+
+[!code-csharp[OverrideComparer](../../../samples/core/Modeling/ValueConversions/OverridingByteArrayComparisons.cs?name=OverrideComparer)]
+
+EF Core will now compare byte sequences and will therefore detect byte array mutations.

samples/core/Modeling/ValueConversions/MappingImmutableClassProperty.cs

@@ -0,0 +1,96 @@
+using System.Diagnostics;
+using System.Linq;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+
+namespace EFModeling.ValueConversions
+{
+    public class MappingImmutableClassProperty : Program
+    {
+        public void Run()
+        {
+            ConsoleWriteLines("Sample showing value conversions for a simple immutable class...");
+
+            using (var context = new SampleDbContext())
+            {
+                CleanDatabase(context);
+
+                ConsoleWriteLines("Save a new entity...");
+
+                var entity = new MyEntityType { MyProperty = new ImmutableClass(7) };
+                context.Add(entity);
+                context.SaveChanges();
+
+                ConsoleWriteLines("Change the property value and save again...");
+
+                // This will be detected and EF will update the database on SaveChanges
+                entity.MyProperty = new ImmutableClass(77);
+
+                context.SaveChanges();
+            }
+
+            using (var context = new SampleDbContext())
+            {
+                ConsoleWriteLines("Read the entity back...");
+
+                var entity = context.Set<MyEntityType>().Single();
+
+                Debug.Assert(entity.MyProperty.Value == 77);
+            }
+
+            ConsoleWriteLines("Sample finished.");
+        }
+
+        public class SampleDbContext : DbContext
+        {
+            private static readonly ILoggerFactory
+                Logger = LoggerFactory.Create(x => x.AddConsole()); //.SetMinimumLevel(LogLevel.Debug));
+
+            protected override void OnModelCreating(ModelBuilder modelBuilder)
+            {
+                #region ConfigureImmutableClassProperty
+                modelBuilder
+                    .Entity<MyEntityType>()
+                    .Property(e => e.MyProperty)
+                    .HasConversion(
+                        v => v.Value,
+                        v => new ImmutableClass(v));
+
+                #endregion
+            }
+
+            protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
+                => optionsBuilder
+                    .UseLoggerFactory(Logger)
+                    .UseSqlite("DataSource=test.db")
+                    .EnableSensitiveDataLogging();
+        }
+
+        public class MyEntityType
+        {
+            public int Id { get; set; }
+            public ImmutableClass MyProperty { get; set; }
+        }
+
+        #region SimpleImmutableClass
+        public sealed class ImmutableClass
+        {
+            public ImmutableClass(int value)
+            {
+                Value = value;
+            }
+
+            public int Value { get; }
+
+            private bool Equals(ImmutableClass other)
+                => Value == other.Value;
+
+            public override bool Equals(object obj)
+                => ReferenceEquals(this, obj) || obj is ImmutableClass other && Equals(other);
+
+            public override int GetHashCode()
+                => Value;
+        }
+        #endregion
+    }
+}

samples/core/Modeling/ValueConversions/MappingImmutableStructProperty.cs

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+using System.Diagnostics;
+using System.Linq;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.Extensions.Logging;
+
+namespace EFModeling.ValueConversions
+{
+    public class MappingImmutableStructProperty : Program
+    {
+        public void Run()
+        {
+            ConsoleWriteLines("Sample showing value conversions for a simple immutable struct...");
+
+            using (var context = new SampleDbContext())
+            {
+                CleanDatabase(context);
+
+                ConsoleWriteLines("Save a new entity...");
+
+                var entity = new EntityType { MyProperty = new ImmutableStruct(6) };
+                context.Add(entity);
+                context.SaveChanges();
+
+                ConsoleWriteLines("Change the property value and save again...");
+
+                // This will be detected and EF will update the database on SaveChanges
+                entity.MyProperty = new ImmutableStruct(66);
+
+                context.SaveChanges();
+            }
+
+            using (var context = new SampleDbContext())
+            {
+                ConsoleWriteLines("Read the entity back...");
+
+                var entity = context.Set<EntityType>().Single();
+
+                Debug.Assert(entity.MyProperty.Value == 66);
+            }
+
+            ConsoleWriteLines("Sample finished.");
+        }
+
+        public class SampleDbContext : DbContext
+        {
+            private static readonly ILoggerFactory
+                Logger = LoggerFactory.Create(x => x.AddConsole()); //.SetMinimumLevel(LogLevel.Debug));
+
+            protected override void OnModelCreating(ModelBuilder modelBuilder)
+            {
+                #region ConfigureImmutableStructProperty
+                modelBuilder
+                    .Entity<EntityType>()
+                    .Property(e => e.MyProperty)
+                    .HasConversion(
+                        v => v.Value,
+                        v => new ImmutableStruct(v));
+                #endregion
+            }
+
+            protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
+                => optionsBuilder
+                    .UseLoggerFactory(Logger)
+                    .UseSqlite("DataSource=test.db")
+                    .EnableSensitiveDataLogging();
+        }
+
+        public class EntityType
+        {
+            public int Id { get; set; }
+            public ImmutableStruct MyProperty { get; set; }
+        }
+
+        #region SimpleImmutableStruct
+        public readonly struct ImmutableStruct
+        {
+            public ImmutableStruct(int value)
+            {
+                Value = value;
+            }
+
+            public int Value { get; }
+        }
+        #endregion
+    }
+}