An EqualityComparer is not the way to go – it can only filter your result set in memory eg: var objects = yourResults.ToEnumerable().Distinct(yourEqualityComparer); You can use the GroupBy method to group by IDs and the First method to let your database only retrieve a unique entry per ID eg: var objects = yourResults.GroupBy(o => o.Id).Select(g … Read more
The grouping algorithm (and I think all LINQ methods) using an equality comparer always first compares hash codes and only executes Equals if two hash codes are equal. You can see that if you add tracing statements in the equality comparer: class PointComparer : IEqualityComparer<Point> { public bool Equals(Point a, Point b) { Console.WriteLine(“Equals: point … Read more
IEqualityComparer<T> is an interface for an object that performs the comparison on two objects of the type T. IEquatable<T> is for an object of type T so that it can compare itself to another of the same type.
A bit of background first… Every object in .NET has an Equals method and a GetHashCode method. The Equals method is used to compare one object with another object – to see if the two objects are equivalent. The GetHashCode method generates a 32-bit integer representation of the object. Since there is no limit to … Read more
There is no such comparer in .NET Framework, but you can create one: public class IEnumerableComparer<T> : IEqualityComparer<IEnumerable<T>> { public bool Equals(IEnumerable<T> x, IEnumerable<T> y) { return Object.ReferenceEquals(x, y) || (x != null && y != null && x.SequenceEqual(y)); } public int GetHashCode(IEnumerable<T> obj) { // Will not throw an OverflowException unchecked { return obj.Where(e … Read more
All types in .NET support the Object.Equals() method which, by default, compares two types for reference equality. However, sometimes, it also desirable to be able to compare two types for structural equality. The best example of this is arrays, which with .NET 4 now implement the IStructuralEquatable interface. This makes it possible to distinguish whether … Read more
Just in case there is no default implementation, this is my own: Edit by 280Z28: Rationale for using RuntimeHelpers.GetHashCode(object), which many of you probably haven’t seen before. 🙂 This method has two effects that make it the correct call for this implementation: It returns 0 when the object is null. Since ReferenceEquals works for null … Read more
Your GetHashCode implementation always returns the same value. Distinct relies on a good hash function to work efficiently because it internally builds a hash table. When implementing interfaces of classes it is important to read the documentation, to know which contract you’re supposed to implement.1 In your code, the solution is to forward GetHashCode to … Read more
LINQ Distinct is not that smart when it comes to custom objects. All it does is look at your list and see that it has two different objects (it doesn’t care that they have the same values for the member fields). One workaround is to implement the IEquatable interface as shown here. If you modify … Read more