I've noticed these two interfaces, and several associated classes, have been added in .NET 4. They seem a bit superfluous to me; I've read several blogs about them, but I still can't figure out what problem they solve that was tricky before .NET 4.
What use are IStructuralEquatable and IStructuralComparable?
C# in a nutshell book:
Because Array is a class, arrays are always (themselves)
reference types, regardless of the array’s element type. This means that the statementarrayB = arrayAresults in two variables that reference the same array. Similarly, two distinct arrays will always fail an equality test—unless you use a custom equality comparer. Framework 4.0 introduced one for the purpose of comparing elements in arrays which you can access via theStructuralComparisonstype.
object[] a1 = { "string", 123, true};
object[] a2 = { "string", 123, true};
Console.WriteLine(a1 == a2); // False
Console.WriteLine(a1.Equals(a2)); // False
IStructuralEquatable se1 = a1;
Console.WriteLine(se1.Equals(a2, StructuralComparisons.StructuralEqualityComparer)); // True
Console.WriteLine(StructuralComparisons.StructuralEqualityComparer.Equals(a1, a2)); // True
object[] a3 = {"string", 123, true};
object[] a4 = {"string", 123, true};
object[] a5 = {"string", 124, true};
IStructuralComparable se2 = a3;
Console.WriteLine(se2.CompareTo(a4, StructuralComparisons.StructuralComparer)); // 0
Console.WriteLine(StructuralComparisons.StructuralComparer.Compare(a3, a4)); // 0
Console.WriteLine(StructuralComparisons.StructuralComparer.Compare(a4, a5)); // -1
Console.WriteLine(StructuralComparisons.StructuralComparer.Compare(a5, a4)); // 1
F# started using them since .net 4. ( .net 2 is here)
These interfaces are crucial to F#
let list1 = [1;5;9]
let list2 = List.append [1;5] [9]
printfn "are they equal? %b" (list1 = list2)
list1.GetType().GetInterfaces().Dump()
