gethashcode

Is it ok to call GetHashCode as a method to test equality from inside the Equals override? For example, is this code acceptable? public class Class1 { public string A { get; set; } public string B { get; set; } public override bool Equals(object obj) { Class1 other = obj as Class1; return other != null && other.GetHashCode() == this.GetHashCode(); } public override int GetHashCode() { int result = 0; result = (result ^ 397) ^ (A == null ? 0 : A.GetHashCode()); result = (result ^ 397) ^ (B == null ? 0 : B.GetHashCode()); return result; } } The others are right; your equality operation is broken

Everytime I write some data class, I usually spend so much time writing the IEquatable implementation. The last class I wrote was something like: public class Polygon { public Point[] Vertices { get; set; } } Implementing IEquatable was exaustive. Surely C#3.0/LINQ helps a lot, but the vertices can be shifted and/or in the reverse order, and that adds a lot of complexity to the Equals method. After many unit tests, and corresponding implementation, I gave up, and changed my application to accept only triangles, which IEquatable implementation required only 11 unit tests to be fully covered.

My understanding is that you're typically supposed to use xor with GetHashCode() to produce an int to identify your data by its value (as opposed to by its reference). Here's a simple example: class Foo { int m_a; int m_b; public int A { get { return m_a; } set { m_a = value; } } public int B { get { return m_b; } set { m_b = value; } } public Foo(int a, int b) { m_a = a; m_b = b; } public override int GetHashCode() { return A ^ B; } public override bool Equals(object obj) { return this.GetHashCode() == obj.GetHashCode(); } } The idea being, I want to compare one instance of Foo to another

Testing the Equals method is pretty much straight forward (as far as I know). But how on earth do you test the GetHashCode method? Test that two distinct objects which are equal have the same hash code (for various values). Check that non-equal objects give different hash codes, varying one aspect/property at a time. While the hash codes don't have to be different, you'd be really unlucky to pick different values for properties which happen to give the same hash code unless you've got a bug. Gallio/MbUnit v3.2 comes with convenient contract verifiers which are able to test your implementation

Possible Duplicate: Can I depend on the values of GetHashCode() to be consistent? If I use the Object.GetHashCode() method across two systems/framework versions, am I guaranteed to get the same value for the same input? In other words, does its value make a good key for persistent data? Note: I don't care about collisions in this problem. As a bonus, am I guaranteed to get the same value in Mono vs. Microsoft .Net? No. Other questions? :-) The algorithms used aren't published nor they are in the Ecma standard. I'll quote from the MSDN String.GetHashCode (I think that this example is good

I am using Entity Framework in my application. I implemented with the partial class of an entity the IEquatable<T> interface: Partial Class Address : Implements IEquatable(Of Address) 'Other part generated Public Overloads Function Equals(ByVal other As Address) As Boolean _ Implements System.IEquatable(Of Address).Equals If ReferenceEquals(Me, other) Then Return True Return AddressId = other.AddressId End Function Public Overrides Function Equals(ByVal obj As Object) As Boolean If obj Is Nothing Then Return MyBase.Equals(obj) If TypeOf obj Is Address Then Return Equals(DirectCast(obj, Address

The solution I propose involves quite a bit of code, but you can just copy it all and past it in a VS test solution assuming you have SqLite installed, and you should be able to run the tests yourself. As I have been struggling with the object identity versus object equality and database identity problem using Nhibernate, I have read various posts. However, I could not get a clear picture of how to properly set up object identity in conjunction with collections. Basically, the big problem, as I got, is that once an object is added to a collection it's identity (as derived by the GetHashCode)