Class ValueComparer<T>

Namespace

Microsoft.EntityFrameworkCore.ChangeTracking

Assembly

Microsoft.EntityFrameworkCore.dll

Specifies custom value snapshotting and comparison for CLR types that cannot be compared with Equals(object, object) and/or need a deep copy when taking a snapshot. For example, arrays of primitive types will require both if mutation is to be detected.

public class ValueComparer<T> : ValueComparer, IEqualityComparer, IEqualityComparer<object>, IEqualityComparer<T>

Type Parameters

T

The type.

Inheritance

object

ValueComparer

ValueComparer<T>

Implements

IEqualityComparer

IEqualityComparer<object>

IEqualityComparer<T>

Derived

ArrayStructuralComparer<TElement>

CaseInsensitiveValueComparer

GeometryValueComparer<TGeometry>

ListComparer<TElement>

NullableValueTypeListComparer<TElement>

ObjectListComparer<TElement>

Inherited Members

ValueComparer.HashCodeAddMethod

ValueComparer.ToHashCodeMethod

ValueComparer.BoolIdentity

ValueComparer.ExtractEqualsBody(Expression, Expression)

ValueComparer.ExtractHashCodeBody(Expression)

ValueComparer.ExtractSnapshotBody(Expression)

ValueComparer.Add(HashCode, int)

ValueComparer.CreateDefault(Type, bool)

ValueComparer.CreateDefault<T>(bool)

object.GetType()

object.MemberwiseClone()

object.ToString()

object.Equals(object)

object.ReferenceEquals(object, object)

object.GetHashCode()

Extension Methods

ValueComparerExtensions.ToNullableComparer(ValueComparer?, Type)

ValueComparerExtensions.IsDefault(ValueComparer)

Remarks

Snapshotting is the process of creating a copy of the value into a snapshot so it can later be compared to determine if it has changed. For some types, such as collections, this needs to be a deep copy of the collection rather than just a shallow copy of the reference.

See EF Core value comparers for more information and examples.

Constructors

ValueComparer(bool)

Creates a new ValueComparer<T> with a default comparison expression and a shallow copy for the snapshot.

public ValueComparer(bool favorStructuralComparisons)

Parameters

favorStructuralComparisons bool

If true, then EF will use IStructuralEquatable if the type implements it. This is usually used when byte arrays act as keys.

ValueComparer(Expression<Func<T?, T?, bool>>, Expression<Func<T, int>>)

Creates a new ValueComparer<T> with the given comparison expression. A shallow copy will be used for the snapshot.

public ValueComparer(Expression<Func<T?, T?, bool>> equalsExpression, Expression<Func<T, int>> hashCodeExpression)

Parameters

equalsExpression Expression<Func<T, T, bool>>

The comparison expression.

hashCodeExpression Expression<Func<T, int>>

The associated hash code generator.

ValueComparer(Expression<Func<T?, T?, bool>>, Expression<Func<T, int>>, Expression<Func<T, T>>)

Creates a new ValueComparer<T> with the given comparison and snapshotting expressions.

public ValueComparer(Expression<Func<T?, T?, bool>> equalsExpression, Expression<Func<T, int>> hashCodeExpression, Expression<Func<T, T>> snapshotExpression)

Parameters

equalsExpression Expression<Func<T, T, bool>>

The comparison expression.

hashCodeExpression Expression<Func<T, int>>

The associated hash code generator.

snapshotExpression Expression<Func<T, T>>

The snapshot expression.

Remarks

Snapshotting is the process of creating a copy of the value into a snapshot so it can later be compared to determine if it has changed. For some types, such as collections, this needs to be a deep copy of the collection rather than just a shallow copy of the reference.

Properties

EqualsExpression

The comparison expression.

public virtual Expression<Func<T?, T?, bool>> EqualsExpression { get; }

Property Value

Expression<Func<T, T, bool>>

HashCodeExpression

The hash code expression.

public virtual Expression<Func<T, int>> HashCodeExpression { get; }

Property Value

Expression<Func<T, int>>

SnapshotExpression

The snapshot expression.

public virtual Expression<Func<T, T>> SnapshotExpression { get; }

Property Value

Expression<Func<T, T>>

Remarks

Snapshotting is the process of creating a copy of the value into a snapshot so it can later be compared to determine if it has changed. For some types, such as collections, this needs to be a deep copy of the collection rather than just a shallow copy of the reference.

Type

The type.

public override Type Type { get; }

Property Value

Type

Methods

CreateDefaultEqualsExpression()

Creates an expression for equality.

protected static Expression<Func<T?, T?, bool>> CreateDefaultEqualsExpression()

Returns

Expression<Func<T, T, bool>>

The equality expression.

CreateDefaultHashCodeExpression(bool)

Creates an expression for generating a hash code.

protected static Expression<Func<T, int>> CreateDefaultHashCodeExpression(bool favorStructuralComparisons)

Parameters

favorStructuralComparisons bool

If true, then IStructuralEquatable is used if the type implements it.

Returns

Expression<Func<T, int>>

The hash code expression.

CreateDefaultSnapshotExpression(bool)

Creates an expression for creating a snapshot of a value.

protected static Expression<Func<T, T>> CreateDefaultSnapshotExpression(bool favorStructuralComparisons)

Parameters

favorStructuralComparisons bool

Returns

Expression<Func<T, T>>

The snapshot expression.

Equals(object?, object?)

Compares the two instances to determine if they are equal.

public override bool Equals(object? left, object? right)

Parameters

left object

The first instance.

right object

The second instance.

Returns

bool

true if they are equal; false otherwise.

Equals(T?, T?)

Compares the two instances to determine if they are equal.

public virtual bool Equals(T? left, T? right)

Parameters

left T

The first instance.

right T

The second instance.

Returns

bool

true if they are equal; false otherwise.

GetHashCode(object?)

Returns the hash code for the given instance.

public override int GetHashCode(object? instance)

Parameters

instance object

The instance.

Returns

int

The hash code.

GetHashCode(T)

Returns the hash code for the given instance.

public virtual int GetHashCode(T instance)

Parameters

instance T

The instance.

Returns

int

The hash code.

Snapshot(object?)

Creates a snapshot of the given instance.

public override object? Snapshot(object? instance)

Parameters

instance object

The instance.

Returns

object

The snapshot.

Remarks

Snapshotting is the process of creating a copy of the value into a snapshot so it can later be compared to determine if it has changed. For some types, such as collections, this needs to be a deep copy of the collection rather than just a shallow copy of the reference.

Snapshot(T)

Creates a snapshot of the given instance.

public virtual T Snapshot(T instance)

Parameters

instance T

The instance.

Returns

T

The snapshot.

Remarks

Snapshotting is the process of creating a copy of the value into a snapshot so it can later be compared to determine if it has changed. For some types, such as collections, this needs to be a deep copy of the collection rather than just a shallow copy of the reference.