checking two object instances to see if they are the same

Question

i need to compare two objects but compare a number of their properties in one hit. this is not for sorting, but instead to confirm whether anything has changed; as one is the old saved instance, and the second is a newly imported instance of the same thing

i assume this is best served by writing a custom comparer. just am a bit confused as to whether to do IComparer, or IComparable, or what tbh.

thanks

nat

Answer 1

If you only have a single definition of equality for your class, you don't really need to implement any interface: simply override the Equalsmethod. Best practice though, would be to implement IEquatable<T>and to override GetHashCode sensibly (if you don't override the hash-code, equality will misbehave when collection classes, LINQ methods etc. use it as a pre-condition for equality). Here's a sample implementation:

public class Person : IEquatable<Person>
{
    public string Name { get; set; }
    public int Age { get; set; }

    public override int GetHashCode()
    {
        return (Name == null ? 0 : Name.GetHashCode()) ^ Age;
    }

    public override bool Equals(object obj)
    {
        return Equals(obj as Person);
    }

    public bool Equals(Person other)
    {
        return other != null && other.Name == Name && other.Age == Age;
    }
}

This will allow you to do:

Person savedPerson = ...
Person importedPerson = ...

bool hasChanged = !savedPerson.Equals(importedPerson);

---

If, on the other hand, you do have lots of different definitions of equality for different circumstances, your best bet would be to write up different IEqualityComparer<T>implementations. Here's a sample implementation:

public class AgeComparer : IEqualityComparer<Person>
{
    public bool Equals(Person x, Person y)
    {
        return (x == null || y == null) ? x == y : x.Age == y.Age;
    }

    public int GetHashCode(Person obj)
    {
        return obj == null ? 0 : obj.Age;
    }
}  

In this case, the check will look like:

Person savedPerson = ...
Person importedPerson = ...
IEqualityComparer<Person> comparer = ...

bool hasChanged = !comparer.Equals(savedPerson, importedPerson);