I have the following class in C# 10:
public sealed record Country : IEquatable<Country>
{
public string Language { get; set; } = "xx";
public bool Equals(Country? other)
{
if (other is null)
{
return false;
}
return Language == other.Language;
}
public override int GetHashCode() => Language.GetHashCode();
}
Which when compiling will bring the following compile time error:
Interface member 'bool System.IEquatable<Country?>.Equals(Country?)' is not implemented
Which is... a very weird error message for the type in question. I tried varying the nullability of the Country parameter, and I tried generating the "missing" method, which will generate the following method for my IDE:
public bool Equals(Country? other) => throw new NotImplementedException();
And then, of course, the error switches to
Member with the same signature is already declared
Generating the Equals() method directly will just override the existing method.
What is going on here? Not all of my colleagues have the same error message for this class, so it might just be something that has nothing to do with the class. (I think I'm the only one on Windows, for example.) dotnet build works, so it might just be a problem with Rider 2021.3.2, but that's the IDE my colleagues use as well.
Its related to your record usage, records already implement this interface and IEquatable is not needed
https://learn.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-9.0/records#equality-members
Quick test in Visual studio and its likely IDE issue.
Records implement equality automatically: that's one of the main advantages of using one.
We can see this in SharpLab. If we paste in:
public sealed record Country
{
public string Language { get; set; } = "xx";
}
and select "Results: C#", we can see that the generated code includes:
public override int GetHashCode()
{
return EqualityComparer<Type>.Default.GetHashCode(EqualityContract) * -1521134295 + EqualityComparer<string>.Default.GetHashCode(<Language>k__BackingField);
}
[System.Runtime.CompilerServices.NullableContext(2)]
public override bool Equals(object obj)
{
return Equals(obj as Country);
}
[System.Runtime.CompilerServices.NullableContext(2)]
public bool Equals(Country other)
{
return (object)this == other || ((object)other != null && EqualityContract == other.EqualityContract && EqualityComparer<string>.Default.Equals(<Language>k__BackingField, other.<Language>k__BackingField));
}
as well as definitions of the == and != operators.
See here.
If you do want to override equality in a record, then you do that as you have done, by specifying your own bool Equals(Counter? other) and int GetHashCode() methods. There's no need to specify that the record implements IEquatable<Country> however, because the compiler will automatically add this interface for records.
Note that it's good practice to put in a test for EqualityContract == other.EqualityContract. This is used to make sure that you're not comparing a record of type Country with one which is a subclass of Country:
public bool Equals(Country? other)
{
if (ReferenceEquals(this, other))
return true;
if (other is null || EqualityContract != other.EqualityContract)
return false;
return Language == other.Language;
}
public override int GetHashCode() => Language.GetHashCode();
See on SharpLab.
Related
I would like to override the Equals and GetHashCode of my EntityBase class to support checking equality based on reference (as by default) or by the entity key (in case the references not matching).
Here is the code:
public abstract class EntityBase
{
protected virtual object Keys { get { return this; } }
public override bool Equals(object obj)
{
if (Keys == this) return base.Equals(obj);
var entity = obj as EntityBase;
if (entity == null) return false;
var re = ReferenceEquals(entity, this);
return re || Equals(entity.Keys, Keys);
}
public override int GetHashCode()
{
if (Keys == this) return base.GetHashCode();
return base.GetHashCode() * 17 + (Keys?.GetHashCode() ?? 0);
}
}
Now in the derived class it can be like this:
public class Entity : EntityBase {
protected override object Keys {
get {
return SomeKeyProperty;
}
}
}
So I expect that it should work, but the BindingSource I'm using showed that it does not work, like this:
//the myEntity here is contained (as reference) in myBindingSource
var index = myBindingSource.IndexOf(myEntity);
The code above gives the correct result if I don't override Equals for my EntityBase class, but with that overriding, the result will be wrong, looks like it always tries to look for the item based on the Keys value. I don't really understand what's wrong here.
When debugging it does not even hit the breakpoint I set in the Equals method. The code just runs through the IndexOf call like as it's a black box.
Could you explain to me what's wrong here and give me some suggestion about a possible fix (or even let me know if what I want to achieve is possible).
Actually the code I posted runs just like what it should. I want that the reference equality should have higher priority, but really when finding an item (like IndexOf) in a collection, what comes first will be the one found (if the references does not match then keys will be used).
The reason it did not work as I expect because the 2 ones compared here both have Keys equal to 0. So if key-based equality is used, that case should be considered as unequal. I need to add another property to determine which value is considered as null or empty Keys. Here is the code working as I expected:
public abstract class EntityBase
{
protected virtual object Keys { get { return this; } }
protected virtual object EmptyKeys {get { return null;} }
public override bool Equals(object obj)
{
if (Keys == this) return base.Equals(obj);
var entity = obj as EntityBase;
if (entity == null) return false;
var re = ReferenceEquals(entity, this);
return re || GetType() == entity.GetType() && Equals(entity.Keys, Keys) && !Equals(Keys, EmptyKeys);
}
public override int GetHashCode()
{
if (Keys == this) return base.GetHashCode();
return base.GetHashCode() * 17 + (Keys?.GetHashCode() ?? 0);
}
}
In the derived class, we need to override the EmptyKeys to indicate which value is considered as empty, NOTE that for numeric key (like long, ...) the value for EmptyKeys should be of the exact type with Keys, otherwise the Equals won't work.
public class Entity : EntityBase {
protected override object Keys {
get {
return SomeKeyProperty;//suppose this is a long property
}
}
protected override object EmptyKeys {
get {
return 0L;//here 0 value should also be a long value.
}
}
}
After that adjusting, the code works expectedly.
I am currently trying to test that two C# objects are the same (not referencing the same object, but have the same values). Trying to use Assert.Equals gives me "Assert.Equals should not be used for Assertions". What should I use ?
Use Asset.AreEqual.
The Asset class is a child of the object class (like any class in C#) and when you call Asset.Equals you are actually calling the method object.Equals which is not what you want to do.
Programmers of NUnit probably put an exception in Asset.Equals to prevent people from making the mistake of using the wrong equality method.
for this, you should override Equals Method in Your Class
public class Student
{
public int Age { get; set; }
public double Grade { get; set; }
public override bool Equals(object obj)
{
if (obj == null)
{
return false;
}
if (!(obj is Student))
{
return false;
}
Student st = obj as Student;
return (st.Age == this.Age &&
st.Grade == this.Grade);
}
}
then you can write this Code in Your Test Class:
Assert.AreEqual(st1,st2);
Whenever I write a new class or struct that is likely to hold some data, that may need to be compared, I always implement IEquatable<T> as this provides the class/struct with a strongly typed .Equals(T other) method.
example:
public struct Radius : IEquatable<Radius>
{
public Int32 TopLeft { get; set; }
public Int32 TopRight { get; set; }
public Int32 BottomLeft { get; set; }
public Int32 BottomRight { get; set; }
public bool Equals(Radius other)
{
return this.TopLeft == other.TopLeft
&& this.TopRight == other.TopRight
&& this.BottomLeft == other.BottomLeft
&& this.BottomRight == other.BottomRight;
}
}
As well as providing an implementation for .Equals(Radius other), I should really override the default implementation too (.Equals(object obj))
I have two options here, and my question is, which of these implementations is better?
Option 1 is to use casting:
public override bool Equals(object obj)
{
return this.Equals((Radius)obj);
}
Option 2 is to use the "as" keyword:
public override bool Equals(object obj)
{
return this.Equals(obj as Radius);
}
My reason for asking this is, using casting will throw an exception if obj cannot be cast to Radius, whereas as will resolve to null if it cannot be cast, therefore it just checks this against null, without throwing an exception; So is it better to throw an exception, or to just return false?
EDIT: As it has been pointed out by quite a few fellow SO'ers, structs cannot be null, therefore the second option does not apply for a struct. Therefore another question springs to mind: Should the overridden implementation of .Equals(object obj) be identical for structs and classes?
The Equals() method must never throw an exception.
An object of a different type is merely unequal.
Quoting the documentation:
Implementations of Equals must not throw exceptions; they should always return a value. For example, if obj is null, the Equals method should return false instead of throwing an ArgumentNullException.
As #SLaks already mentioned Equals() should never throw.
In this special case i think a usage of the is operator combined with a cast should help you out:
public override bool Equals(object obj)
{
if(obj is Radius)
return Equals((Radius)obj);
return false;
}
In cases where you have a class you should simply use the as operator:
public override bool Equals(object obj)
{
return Equals(obj as MyObj);
}
public bool Equals(MyObj obj)
{
if(ReferenceEquals(obj, null))
return false;
// ToDo: further checks for equality.
}
My personal opinion would be to use the second option, or even check before hand if the object is a "Radius" and then return false so that the intent is more clear
I'm trying to implement DDD approach in my head.
So far I know that entity is unique and identified by combination of its attributes.
Entity is abstract class which will be implemented by other entity classes.
I know so far that Version property is used to manage concurrency.
Need help with rest of this class.
I'm in process of learning DDD so please describe your thoughts on this or share useful concrete theme links.
public abstract class Entity<T>
{
#region Properties
protected T _Id;
private int _Version;
#endregion
private static bool IsTransient(Entity<T> obj)
{
return (obj != null) &&
Equals(obj.Id, default(T));
}
private Type GetUnproxiedType()
{
return GetType();
}
public virtual T Id
{
get
{
return _Id;
}
protected set
{
_Id = value;
}
}
public virtual int Version
{
get
{
return _Version;
}
set
{
_Version = value;
}
}
public override bool Equals(object obj)
{
return Equals(obj as Entity<T>);
}
public virtual bool Equals(Entity<T> other)
{
if (other == null) return false;
if (ReferenceEquals(this, other)) return true;
if (!IsTransient(this) &&
!IsTransient(other) &&
Equals(Id, other.Id))
{
var otherType = other.GetUnproxiedType();
var thisType = GetUnproxiedType();
return (thisType.IsAssignableFrom(otherType)
|| otherType.IsAssignableFrom(thisType));
}
return false;
}
public override int GetHashCode()
{
if (Equals(Id, default(T)))
{
return base.GetHashCode();
}
return Id.GetHashCode();
}
public static bool operator ==(Entity<T> e1, Entity<T> e2)
{
return Equals(e1, e2);
}
public static bool operator !=(Entity<T> e1, Entity<T> e2)
{
return !Equals(e1, e2);
}
}
Updated
After some researching I'm concluded with this
Using Equals method will come handy when we try to compare two objects. For example imaging place in application where we have the same object in more than one instance. One from the database and one from the client. We need to find out if that object is identical and yet not the same.
For this we will be using Equals method.
An entity is not uniquely identified by the value of its attributes; An entity has an identity, but that does not mean that the identity is made up by the value of its attributes.
That would mean that, if you change one if the attributes of an entity, it's identity will change, and you do not want that behaviour.
(For instance, if your address changes, your identity doesn't change, you're still the same person, living at another address).
(I see that you've implemented identity/equality by using an Id property, which is a good thing).
Next to entities, you have value - objects. The identity of a value object is made up by the values of its attributes, and therefore, it is better that a value object is immutable, since, changing the value object would also mean that you change its identity.
But, what is your question ?
In NHibernate 3.0 Cookbook, there is a sample implementation for a base Entity type. The equals is implemented like this:
public abstract class Entity<TId>
{
public virtual TId Id { get; protected set; }
public override bool Equals(object obj)
{
return Equals(obj as Entity<TId>);
}
private static bool IsTransient(Entity<TId> obj)
{
return obj != null && Equals(obj.Id, default(TId));
}
private Type GetUnproxiedType()
{
return GetType();
}
public virtual bool Equals(Entity<TId> other)
{
if (other == null) return false;
if (ReferenceEquals(this, other)) return true;
if (!IsTransient(this) && !IsTransient(this) && Equals(Id, other.Id))
{
var otherType = other.GetUnproxiedType();
var thisType = GetUnproxiedType();
return thisType.IsAssignableFrom(otherType) ||
otherType.IsAssignableFrom(thisType);
}
return false;
}
}
The reason for the GetUnproxiedType() method is this: There is an abstract base class Product, a concrete class Book which inherits from Product and a dynamic proxy class ProductProxy used by NHibernate for lazy loading. If a ProductProxy representing a Book and a concrete Book have the same Ids, they should be treated as equal. However I don't really see why calling GetType() on a ProductProxy instance should return Product in this case, and how it helps. Any ideas?
I actually went ahead and wrote to the author of the book about this code. It turns out this is due to how the proxy wrapping works. Here is his response:
"If you don't understand how the proxy frameworks work, the idea can seem magical.
When NHibernate returns a proxy for the purposes of lazy loading, it returns a proxy instance inherited from the actual type. There are a few members we can access without forcing a load from the database. Among these are proxy's Id property or field, GetType(), and in some circumstances Equals() and GetHashCode(). Accessing any other member will force a load from the database.
When that happens, the proxy creates an internal instance. So, for example, a lazy loaded instance of Customer (CustomerProxy102987098721340978), when loaded, will internally create a new Customer instance with all of the data from the database. The proxy then does something like this:
public overrides string Name
{
get {
return _loadedInstance.Name;
}
set { _loadedInstance.Name = value; }
}
Incidentally, it's this overriding that requires everything to be virtual on entities that allow lazy loaded.
So, all calls to the Name property on the proxy are relayed to the internal Customer instance that has the actual data.
GetUnproxiedType() takes advantage of this. A simple call to GetType() on the proxy will return typeof(CustomerProxy02139487509812340). A call to GetUnproxiedType() will be relayed to the internal customer instance, and the internal customer instance will return typeof(Customer)."
With current (v5.x) NHibernate proxy factories (static or dynamic, static being available since v5.1), this pattern is actually broken. The v5 built-in proxy factories do not intercept private methods.
And I think that was already the case for v4 ones.
For this pattern to work with current built-in proxy factories, GetUnproxiedType should be virtual (so not private by the way, but protected).
Otherwise, use NHibernateUtil.GetClass, which is meant for this and does not rely on brittle tricks. Its documentation warns it will initialize the proxy by side effect, but anyway the GetUnproxiedType trick must do the same for working.
Of course using NHibernateUtil.GetClass means having a direct dependency to NHibernate in a domain model base class. But depending on an implementation trick specific to an external (from the domain viewpoint) library implementation is no better in my opinion.
Moreover, some changes may cause the GetUnproxiedType trick to be even more broken in the future, like some ideas for reducing the number of cases causing a proxy to get initialized when it could be avoided. (See here by example.)
If you really want a GetUnproxiedType method not depending on a direct NHibernate reference, I think the only theoretically "safe" solution is to have it abstract and overridden in each concrete entity class for yielding typeof(YourEntityClass). But in practice, it would be cumbersome and error-prone (bad copy-paste for creating a new entity, forgetting to change that method...), while the abstract part won't help in case some concrete entity classes are further specialized through inheritance.
Another trick could be, from the type obtained by GetType, to check to which assembly it belongs (the proxy type will not belong to one of your assemblies), for searching the first type in the hierarchy belonging to your domain model assembly(ies).
Note that if the proxy is a proxy of a base class, not of a concrete class, and your helper method is set as private, it will yield the base class type, without initializing the proxy. Performance-wise, this is better. While a virtual GetUnproxiedType simply returning GetType would return the concrete class type with current proxy factories, but it would also initialize the proxy.
We use NH 2 and this example did not work for us. (It FAILED to unproxy the type and left proxy type, see below).
It said that 2 entities with the same id are not equal, when one of them is proxy(of COrganization) and other is not(DOrganization).
When we had a hierarchy:
class Organization
class AOrganization : Organization
class COrganization : Organization
{
public virtual COrganization GetConcrete()
{
return null;
}
}
class DOrganization : COrganization
{
public virtual COrganization GetConcrete()
{
return this;
}
}
AOrganization aOrganization;
COrganization cOrganization;
contract = new CContract(aOrganization, cOrganization as COrganization); //(COrganization)(cOrganization.GetConcrete()),
So CContract has a field of type COrganization. With a setter
public class Contract: Entity <short>
{
public virtual COrganization COrganization
{
get { return cOrganization; }
protected internal set
{
if (cOrganization != null && value != cOrganization) // != calls ==, which calls Equals, which calls GetUnproxiedType()
throw new Exception("Changing organization is not allowed.");
}
cOrganization = value;
}
}
private COrganization cOrganization;
}
We constructed new Contract, its constructor set the COrganization field pointing to some organization. Then we called UnitOfWork.Commit, NH tried to set COrganization field again (with the same id), GetUnproxiedType worked incorrectly, new and old values were recognized as non-equal, and exception was thrown...
Here is the place where error showed up:
var otherType = other.GetUnproxiedType();
var thisType = GetUnproxiedType();
return thisType.IsAssignableFrom(otherType) ||
otherType.IsAssignableFrom(thisType);
In debugger: otherType == COrganizationProxy - GetUnproxiedType failed...
thisType == DOrganization
COrganizationProxy and DOrganization both inherit COrganization.
So they are not IsAssignableFrom for each other...
Why does this example work for you?
Maybe because we have NH 2.0 or 2.1?
Or because of simple "cOrganization as COrganization" instead of "(COrganization)(cOrganization.GetConcrete())"?
Or because we have implementation of ==, != and Equals not only in Entity, but in Organization too?
public abstract class Organization : Entity<int>
{
public override bool Equals(object obj)
{
return base.Equals(obj);
}
public override int GetHashCode()
{
return base.GetHashCode();
}
public static bool operator ==(Organization object1, Organization object2)
{
return AreEqual(object1, object2);
}
public static bool operator !=(Organization object1, Organization object2)
{
return AreNotEqual(object1, object2);
}
}
public abstract class Entity<TId>
{
public virtual TId Id { get; /*protected*/ set; }
public override bool Equals(object obj)
{
return Equals(obj as Entity<TId>);
}
private static bool IsTransient(Entity<TId> obj)
{
return obj != null &&
Equals(obj.Id, default(TId));
}
private Type GetUnproxiedType()
{
return GetType();
}
public virtual bool Equals(Entity<TId> other)
{
if (other == null)
return false;
if (ReferenceEquals(this, other))
return true;
if (!IsTransient(this) &&
!IsTransient(other) &&
Equals(Id, other.Id))
{
var otherType = other.GetUnproxiedType();
var thisType = GetUnproxiedType();
return thisType.IsAssignableFrom(otherType) ||
otherType.IsAssignableFrom(thisType);
}
return false;
}
public override int GetHashCode()
{
if (Equals(Id, default(TId)))
return base.GetHashCode();
return Id.GetHashCode();
}
/// This method added by me
/// For == overloading
protected static bool AreEqual<TEntity>(TEntity entity1, TEntity entity2)
{
if ((object)entity1 == null)
{
return ((object)entity2 == null);
}
else
{
return entity1.Equals(entity2);
}
}
/// This method added by me
/// For != overloading
protected static bool AreNotEqual<TEntity>(TEntity entity1, TEntity entity2)
{
return !AreEqual(entity1, entity2);
}
}