Develop Reference

Two objects being equal not regarded as distinct by LINQ

I'm comparing two objects of type Triangle and apparently they are deemed equal (I implemented my custom GetHaschCode as well as Equal method and operator).
List<Triangle> triangles = ...;
bool same = triangles[0] == triangles[1];
// same is true
However, when I go Distinct() on that list, it keeps all the elements (which sound to me like it's comparing by reference and not by my custom conditions). Is it so and what can I do about it?
int countBefore = triangles.Count();
int countAfter = triangles.Distinct().Count();
bool same = countBefore == countAfter;
// same is true, again
I'm missing something fairly obvious, am I not?

You can do this in one of two ways...
Either as Andrew says implement IEquatable
public class Triangle : IEquatable<Triangle>
{
bool IEquatable<Triangle>.Equals(Triangle other)
{
return Equals(other);
}
public override bool Equals(object obj)
{
//...
}
public override int GetHashCode()
{
//...
}
}
Or you could create another class that implements IEqualityComparer(T) and pass that into the Distinct method call.
public class TriangleComparer : IEqualityComparer<Triangle>
{
public bool Equals(Triangle x, Triangle y)
{
return x.Equals(y);
}
public int GetHashCode(Triangle obj)
{
return obj.GetHashCode();
}
}

You need to implement the IEquatable(T) interface in your Triangle class.
From the Enumerable.Distinct() documentation:
The default equality comparer, Default, is used to compare values of
the types that implement the IEquatable generic interface. To
compare a custom data type, you need to implement this interface and
provide your own GetHashCode and Equals methods for the type.

Overriding equals in C# interface implementation

I have a class that implements an interface, such as this:
interface IInterface
{
string PropA { get; }
string PropB { get; }
}
class AClass : IInterface
{
string PropA { get; protected set; }
string PropB { get; protected set; }
}
Equality is determined based on PropA and PropB. When overriding the Equals method for AClass, should I attempt to cast obj to AClass, like this:
public override bool Equals(object obj)
{
AClass other = obj as AClass;
return other != null
&& AClass.PropA == other.PropA
&& AClass.PropB == PropB;
}
Or should I attempt to cast obj to IInterface, like this:
public override bool Equals(object obj)
{
IInterface other = obj as IInterface;
return other != null
&& AClass.PropA == other.PropA
&& AClass.PropB == PropB;
}

You could do whichever you want. The two aren't the same, functionally, but which is "right" for you is something that we can't answer. If I have a BClass class that implements the same interface, and it has the same values for both properties, should it be equal to your AClass object? If yes, do the latter, if not, do the former.
Personally, I would find the latter concerning. Generally I find that if a class is going to implement its own personal definition of equality, other classes shouldn't be equal to it. One main reason is that it's preferable if equality is symetric. That is to say aclass.Equals(bclass) should return the same thing as bclass.Equals(aclass). Getting that behavior when you don't restrict equality to the same type is...hard. It requires cooperation of all related classes.
If you have some compelling reason to be comparing IInterface implementations in which they might be different underlying classes but still both be "equal", I'd personally prefer to create an IEqualityComparer<IInterface> that defines equality for that interface. This would the be separate from the definition of equality for either of the two implementing classes.

Decide how you need it to function.
Resharper implementation:
class AClass : IInterface, IEquatable<AClass>
{
public bool Equals(AClass other)
{
if (ReferenceEquals(null, other)) return false;
if (ReferenceEquals(this, other)) return true;
return string.Equals(this.PropA, other.PropA) && string.Equals(this.PropB, other.PropB);
}
public override bool Equals(object obj)
{
if (ReferenceEquals(null, obj)) return false;
if (ReferenceEquals(this, obj)) return true;
if (obj.GetType() != typeof (AClass)) return false;
return Equals((AClass)obj);
}
public override int GetHashCode()
{
unchecked
{
return ((this.PropA != null ? this.PropA.GetHashCode() : 0) * 397) ^ (this.PropB != null ? this.PropB.GetHashCode() : 0);
}
}
public string PropA { get; protected set; }
public string PropB { get; protected set; }
}

If the purpose of the interface is to hide from consumers the fact that two equivalent objects might be of different classes, it may be a good idea to define a struct which holds a single private field of that interface type and chains to the appropriate methods of the interfaces. Use of such a struct should generally be essentially as efficient as using a variable of the interface type (the main exception would be if the struct ends up being boxed), but would shield client code from seeing the actual type of thing implementing the interface.
For example, one might have a interfaces IReadableMatrix<T> and IImmutableMatrix<T>, and corresponding structures ReadableMatrix<T> and ImmutableMatrix<T> with read-only members int Height, int Width, and T this[int row, int column], and ImmutableMatrix<T> AsImmutable();. Code which uses an ImmutableMatrix<double> shouldn't care how it's stored; it would be entirely possible that two instances of ImmutableMatrix might hold references to different implementations of IImmutableMatrix<T> which report identical content in every cell, but stored things entirely differently. One might be an instance of ArrayBackedMatrix<double>, which holds a 12x12 array that happens to hold zeroes in every element other than those on the diagonal, while the other might be a DiagonalMatrix<double>, and use a 12-item array that only stores things on the diagonal (and returns zero in response to a request for any other element). The use of different types to store the array data should be an implementation detail and not exposed to the client.
One slight detail of using a struct to wrap the arrays is that reference-type fields of a default-value structure will be null, but the structure itself will not. It may thus be desirable to either have the struct implement an IsNull property which returns true if the backing field is null, or else have the other struct members check whether the backing field is null and, if so, behave as an empty 0x0 matrix.

Comparing two instances of a 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

Distinct() returns duplicates with a user-defined type

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?

C# Queue problem

Suppose I have a class
XYNode
{
protected int mX;
protected int mY;
}
and a queue
Queue<XyNode> testQueue = new Queue<XYNode>();
I want to check if a node with that specific x and y coordinate is already in the queue.
The following obviously doesn't work :
testQueue.Contains(new XYNode(testX, testY))
because even if a node with those coordinates is in the queue, we're testing against a different XYNode object so it will always return false.
What's the right solution ?

The simplest way is to override Equals so that one XYNode knows whether it's equal to another XYNode. You should override GetHashCode() at the same time, and possibly also implement IEquatable<XYNode> to allow a strongly-typed equality comparison.
Alternatively, you could write an IEqualityComparer<XYNode> implementation to compare any two nodes and return whether or not they're the same - and then pass that into the call to the appropriate overload of the Contains extension method defined in Enumerable (assuming you're using .NET 3.5).
Further things to consider:
Could you use private fields instead of protected ones?
Could your class be sealed?
Could your class be immutable?
Should your class perhaps be a struct instead? (Judgement call...)
Should you overload the == and != operators?

To illustrate Jon Skeet's ... original ... answer:
class XYNode {
protected int mX;
protected int mY;
public override bool Equals(Object obj) {
if (obj == null || this.GetType() != obj.GetType()) { return false; }
XYNode otherNode = (XYNode)obj;
return (this.mX == other.mX) && (this.mY == other.mY);
}
}
This is a pretty simplistic solution. There are a lot of additional factors to consider, which Jon has already mentioned.

You could simply iterate with a foreach and check the X and Y of your XYNode on every element.