I am trying to make several operations use the same comparison method but I have been unable to get all of them to use it. In the following code I put a breakpoint to check which of the operations are using it. My results show that while List.Contains() does use it, List.Distinct() and "==" do not.
How can I make all 3 of them use my equality comparison?
namespace ConsoleApplication3
{
class Program
{
static void Main(string[] args)
{
List<TestObject> list1 = new List<TestObject>();
list1.Add(new TestObject("test1"));
list1.Add(new TestObject("test1"));
List<TestObject> list2 = list1.Distinct().ToList();
if (list1[0] == list1[1]) { }
if (list2.Contains(list1[0])){ }
}
}
class TestObject: IEquatable<TestObject>
{
string name;
public TestObject(string name)
{
this.name = name;
}
public bool Equals(TestObject other)
{
return this.name == other.name;
}
}
}
Thank you
For IEnumerable<T>, Distinct uses the default IEquatableComparer<T> comparer (unless you pass your own comparer). Internally this relies on GetHashCode to perform equality checks, where as Contains uses Equals hence why they don't yield the same results.
It's generally recommended you override both Equals and GetHashCode when implementing your own custom equality checks anyway, see Guidelines for Overloading Equals() and == Operator.
== is normally used to determine whether two references are the same. If you want to change this behavior for your class then you need to override the == operator as per the instructions here:
https://msdn.microsoft.com/en-US/library/ms173147(v=vs.80).aspx
Related
One of the great advantages is supposed to be value based/structural equality, but how do I get that to work with collection properties?
Concrete simple example:
public record Something(string Id);
public record Sample(List<Something> something);
With the above records I would expect the following test to pass:
[Fact]
public void Test()
{
var x = new Sample(new List<Something>() {
new Something("x1")
});
var y = new Sample(new List<Something>() {
new Something("x1")
});
Assert.Equal(x, y);
}
I understand that it is because of List being a reference type, but does it exist a collection that implements value based comparison? Basically I would like to do a "deep" value based comparison.
Records don't do this automatically, but you can implement the Equals method yourself:
public record Sample(List<Something> something) : IEquatable<Sample>
{
public virtual bool Equals(Sample? other) =>
other != null &&
Enumerable.SequenceEqual(something, other.something);
}
But note that GetHashCode should be overridden to be consistent with Equals. See also implement GetHashCode() for objects that contain collections
I have an object
public class Foo
{
public string A{ get; set; }
public string B{ get; set; }
}
I am comparing the return value from the SUT when it returns null for A and B like this.
Assert.That(returnValue, Is.EqualTo(new Foo { A = null, B = null}));
This did not work, so I tried
Assert.That(returnValue, Is.SameAs(new Foo { A = null, B = null}));
This didn't work either.
I get a message like
Expected: same as <Namespace+Foo>
But was: <Namespace+Foo>
What am I doing wrong?
From nunit documentation,
When checking the equality of user-defined classes, NUnit makes use of the Equals override on the expected object. If you neglect to override Equals, you can expect failures non-identical objects. In particular, overriding operator== without overriding Equals has no effect.
You can however supply your own comparer to test if the values are equal for the properties.
If the default NUnit or .NET behavior for testing equality doesn't meet your needs, you can supply a comparer of your own through the Using modifier. When used with EqualConstraint, you may supply an IEqualityComparer, IEqualityComparer, IComparer, IComparer; or Comparison as the argument to Using.
Assert.That( myObj1, Is.EqualTo( myObj2 ).Using( myComparer ) );
So in this case your comparer would be
public class FooComparer : IEqualityComparer
{
public bool Equals(Foo x, Foo y)
{
if (x.A == y.A && x.B == y.B)
{
return true;
}
return false;
}
public int GetHashCode(Foo inst)
{
return inst.GetHashCode();
}
}
Nunit is comparing the two objects objects by reference so it is showing the two objects are not equal. You will have to override the Equals method in your object class.
I have a class like this
public class TestData
{
public string Name {get;set;}
public string type {get;set;}
public List<string> Members = new List<string>();
public void AddMembers(string[] members)
{
Members.AddRange(members);
}
}
I want to know if it is possible to directly compare to instances of this class to eachother and find out they are exactly the same? what is the mechanism? I am looking gor something like if(testData1 == testData2) //Do Something And if not, how to do so?
You should implement the IEquatable<T> interface on your class, which will allow you to define your equality-logic.
Actually, you should override the Equals method as well.
public class TestData : IEquatable<TestData>
{
public string Name {get;set;}
public string type {get;set;}
public List<string> Members = new List<string>();
public void AddMembers(string[] members)
{
Members.AddRange(members);
}
// Overriding Equals member method, which will call the IEquatable implementation
// if appropriate.
public override bool Equals( Object obj )
{
var other = obj as TestData;
if( other == null ) return false;
return Equals (other);
}
public override int GetHashCode()
{
// Provide own implementation
}
// This is the method that must be implemented to conform to the
// IEquatable contract
public bool Equals( TestData other )
{
if( other == null )
{
return false;
}
if( ReferenceEquals (this, other) )
{
return true;
}
// You can also use a specific StringComparer instead of EqualityComparer<string>
// Check out the specific implementations (StringComparer.CurrentCulture, e.a.).
if( EqualityComparer<string>.Default.Compare (Name, other.Name) == false )
{
return false;
}
...
// To compare the members array, you could perhaps use the
// [SequenceEquals][2] method. But, be aware that [] {"a", "b"} will not
// be considerd equal as [] {"b", "a"}
return true;
}
}
One way of doing it is to implement IEquatable<T>
public class TestData : IEquatable<TestData>
{
public string Name {get;set;}
public string type {get;set;}
public List<string> Members = new List<string>();
public void AddMembers(string[] members)
{
Members.AddRange(members);
}
public bool Equals(TestData other)
{
if (this.Name != other.Name) return false;
if (this.type != other.type) return false;
// TODO: Compare Members and return false if not the same
return true;
}
}
if (testData1.Equals(testData2))
// classes are the same
You can also just override the Equals(object) method (from System.Object), if you do this you should also override GetHashCode see here
There are three ways objects of some reference type T can be compared to each other:
With the object.Equals method
With an implementation of IEquatable<T>.Equals (only for types that implement IEquatable<T>)
With the comparison operator ==
Furthermore, there are two possibilities for each of these cases:
The static type of the objects being compared is T (or some other base of T)
The static type of the objects being compared is object
The rules you absolutely need to know are:
The default for both Equals and operator== is to test for reference equality
Implementations of Equals will work correctly no matter what the static type of the objects being compared is
IEquatable<T>.Equals should always behave the same as object.Equals, but if the static type of the objects is T it will offer slightly better performance
So what does all of this mean in practice?
As a rule of thumb you should use Equals to check for equality (overriding object.Equals as necessary) and implement IEquatable<T> as well to provide slightly better performance. In this case object.Equals should be implemented in terms of IEquatable<T>.Equals.
For some specific types (such as System.String) it's also acceptable to use operator==, although you have to be careful not to make "polymorphic comparisons". The Equals methods, on the other hand, will work correctly even if you do make such comparisons.
You can see an example of polymorphic comparison and why it can be a problem here.
Finally, never forget that if you override object.Equals you must also override object.GetHashCode accordingly.
I see many good answers here but just in case you want the comparison to work like
if(testData1 == testData2) // DoSomething
instead of using Equals function you can override == and != operators:
public static bool operator == (TestData left, TestData right)
{
bool comparison = true; //Make the desired comparison
return comparison;
}
public static bool operator != (TestData left, TestData right)
{
return !(left == right);
}
You can override the equals method and inside it manually compare the objects
Also take a look at Guidelines for Overloading Equals() and Operator ==
You will need to define the rules that make object A equal to object B and then override the Equals operator for this type.
http://msdn.microsoft.com/en-us/library/ms173147(v=vs.80).aspx
First of all equality is difficult to define and only you can define as to what equality means for you
Does it means members have same value
Or they are pointing to same location.
Here is a discussion and an answer here
What is "Best Practice" For Comparing Two Instances of a Reference Type?
Implement the IEquatable<T> interface. This defines a generalized method that a value type or class implements to create a type-specific method for determining equality of instances. More information here:
http://msdn.microsoft.com/en-us/library/ms131187.aspx
I'm trying to write a Linq query which returns an array of objects, with unique values in their constructors. For integer types, Distinct returns only one copy of each value, but when I try creating my list of objects, things fall apart. I suspect it's a problem with the equality operator for my class, but when I set a breakpoint, it's never hit.
Filtering out the duplicate int in a sub-expression solves the problem, and also saves me from constructing objects that will be immediately discarded, but I'm curious why this version doesn't work.
UPDATE: 11:04 PM Several folks have pointed out that MyType doesn't override GetHashCode(). I'm afraid I oversimplified the example. The original MyType does indeed implement it. I've added it below, modified only to put the hash code in a temp variable before returning it.
Running through the debugger, I see that all five invocations of GetHashCode return a different value. And since MyType only inherits from Object, this is presumably the same behavior Object would exhibit.
Would I be correct then to conclude that the hash should instead be based on the contents of Value? This was my first attempt at overriding operators, and at the time, it didn't appear that GetHashCode needed to be particularly fancy. (This is the first time one of my equality checks didn't seem to work properly.)
class Program
{
static void Main(string[] args)
{
int[] list = { 1, 3, 4, 4, 5 };
int[] list2 =
(from value in list
select value).Distinct().ToArray(); // One copy of each value.
MyType[] distinct =
(from value in list
select new MyType(value)).Distinct().ToArray(); // Two objects created with 4.
Array.ForEach(distinct, value => Console.WriteLine(value));
}
}
class MyType
{
public int Value { get; private set; }
public MyType(int arg)
{
Value = arg;
}
public override int GetHashCode()
{
int retval = base.GetHashCode();
return retval;
}
public override bool Equals(object obj)
{
if (obj == null)
return false;
MyType rhs = obj as MyType;
if ((Object)rhs == null)
return false;
return this == rhs;
}
public static bool operator ==(MyType lhs, MyType rhs)
{
bool result;
if ((Object)lhs != null && (Object)rhs != null)
result = lhs.Value == rhs.Value;
else
result = (Object)lhs == (Object)rhs;
return result;
}
public static bool operator !=(MyType lhs, MyType rhs)
{
return !(lhs == rhs);
}
}
You need to override GetHashCode() in your class. GetHashCode must be implemented in tandem with Equals overloads. It is common for code to check for hashcode equality before calling Equals. That's why your Equals implementation is not getting called.
Your suspicion is correct,it is the equality which currently just checks the object references. Even your implementation does not do anything extra, change it to this:
public override bool Equals(object obj)
{
if (obj == null)
return false;
MyType rhs = obj as MyType;
if ((Object)rhs == null)
return false;
return this.Value == rhs.Value;
}
In you equality method you are still testing for reference equality, rather than semantic equality, eg on this line:
result = (Object)lhs == (Object)rhs
you are just comparing two object references which, even if they hold exactly the same data, are still not the same object. Instead, your test for equality needs to compare one or more properties of your object. For instance, if your object had an ID property, and objects with the same ID should be considered semantically equivalent, then you could do this:
result = lhs.ID == rhs.ID
Note that overriding Equals() means you should also override GetHashCode(), which is another kettle of fish, and can be quite difficult to do correctly.
You need to implement GetHashCode().
It seems that a simple Distinct operation can be implemented more elegantly as follows:
var distinct = items.GroupBy(x => x.ID).Select(x => x.First());
where ID is the property that determines if two objects are semantically equivalent. From the confusion here (including that of myself), the default implementation of Distinct() seems to be a little convoluted.
I think MyType needs to implement IEquatable for this to work.
The other answers have pretty much covered the fact that you need to implement Equals and GetHashCode correctly, but as a side note you may be interested to know that anonymous types have these values implemented automatically:
var distinct =
(from value in list
select new {Value = value}).Distinct().ToArray();
So without ever having to define this class, you automatically get the Equals and GetHashCode behavior you're looking for. Cool, eh?
I have a class, show below, which is used as a key in a Dictionary<ValuesAandB, string>
I'm having issues when trying to find any key within this dictionary, it never finds it at all. As you can see, I have overridden Equals and GetHashCode.
To look for the key I'm using
ValuesAandB key = new ValuesAandB(A,B);
if (DictionaryName.ContainsKey(key)) {
...
}
Is there anything else that I'm missing? Can anyone point out what I'm doing wrong?
private class ValuesAandB {
public string valueA;
public string valueB;
// Constructor
public ValuesAandB (string valueAIn, string valueBIn) {
valueA = valueAIn;
valueB = ValueBIn;
}
public class EqualityComparer : IEqualityComparer<ValuesAandB> {
public bool Equals(ValuesAandB x, ValuesAandB y) {
return ((x.valueA.Equals(y.valueA)) && (x.valueB.Equals(y.valueB)));
}
public int GetHashCode(ValuesAandB x) {
return x.valueA.GetHashCode() ^ x.valueB.GetHashCode();
}
}
}
And before anyone asks, yes the values are in the Dictionary!
You have not overridden Equals and GetHashCode. You have implemented a second class which can serve as an EqualityComparer. If you don't construct the Dictionary with the EqualityComparer, it will not be used.
The simplest fix would be to override GetHashCode and Equals directly rather than implementing a comparer (comparers are generally only interesting when you need to supply multiple different comparison types (case sensitive and case insensitive for example) or when you need to be able to perform comparisons on a class which you don't control.
I had this problem, turns out the dictionary was comparing referances for my key, not the values in the object.
I was using a custom Point class as keys. I overrode the ToString() and the GetHashCode() methods and viola, key lookup worked fine.
It looks like you're comparing two strings. Iirc, when using .Equals(), you are comparing the reference of the strings, not the actual contents. To implement an EqualityComparer that works with strings, you would want to use the String.Compare() method.
public class EqualityComparer : IEqualityComparer<ValuesAandB>
{
public bool Equals(ValuesAandB x, ValuesAandB y)
{
return ((String.Compare(x.valueA,y.valueA) == 0) &&
(String.Compare(x.valueB, y.valueB) == 0));
}
// gethashcode stuff here
}
I could be a little off with the code, that should get you close...