c# compare the data in two object models

Question

I have a dialog, when spawned it gets populated with the data in an object model. At this point the data is copied and stored in a "backup" object model. When the user has finished making their changes, and click "ok" to dismiss the dialog, I need a quick way of comparing the backup object model with the live one - if anything is changed I can create the user a new undo state.

I don't want to have to go and write comparison function for every single class in the object model if possible.

If I serialised both object models and they were identical but stored in different memory locations would they be equal? Does some simple way exist to compare two serialised object models?

Answer 1

I didn't bother with a hash string but just a straight Binary serialisation works wonders. When the dialog opens serialise the object model.

BinaryFormatter formatter = new BinaryFormatter();
m_backupStream = new MemoryStream();
formatter.Serialize(m_backupStream,m_objectModel);

Then if the user adds to the object model using available controls (or not). When the dialog closes you can compare to the original serialisation with a new one - this for me is how i decide whether or not an Undo state is required.

BinaryFormatter formatter = new BinaryFormatter();
MemoryStream liveStream = new MemoryStream();
formatter.Serialize(liveStream,m_objectModel);
byte[] streamOneBytes = liveStream.ToArray();
byte[] streamTwoBytes = m_backupStream.ToArray();
if(!CompareArrays(streamOneBytes, streamTwoBytes))
    AddUndoState();

And the compare arrays function incase anybody needs it - prob not the best way of comparing two arrays im sure.

private bool CompareArrays(byte[] a, byte[] b)
{
    if (a.Length != b.Length)
       return false;

    for (int i = 0; i < a.Length;i++)
    {
       if (a[i] != b[i])
        return false;
    }
    return true;
}

Answer 2

I'd say the best way is to implement the equality operators on all classes in your model (which is usually a good idea anyway if you're going to do comparisons).

class Book
{
    public string Title { get; set; }
    public string Author { get; set; }
    public ICollection<Chapter> Chapters { get; set; }

    public bool Equals(Book other)
    {
        if (ReferenceEquals(null, other)) return false;
        if (ReferenceEquals(this, other)) return true;
        return Equals(other.Title, Title) && Equals(other.Author, Author) && Equals(other.Chapters, Chapters);
    }

    public override bool Equals(object obj)
    {
        if (ReferenceEquals(null, obj)) return false;
        if (ReferenceEquals(this, obj)) return true;
        if (obj.GetType() != typeof (Book)) return false;
        return Equals((Book) obj);
    }

    public override int GetHashCode()
    {
        unchecked
        {
            int result = (Title != null ? Title.GetHashCode() : 0);
            result = (result*397) ^ (Author != null ? Author.GetHashCode() : 0);
            result = (result*397) ^ (Chapters != null ? Chapters.GetHashCode() : 0);
            return result;
        }
    }
}

This snippet is auto-generated by ReSharper, but you can use this as a basis. Basically you will have to extend the non overriden Equals method with your custom comparison logic.

For instance, you might want to use SequenceEquals from the Linq extensions to check if the chapters collection is equal in sequence.

Comparing two books will now be as simple as saying:

Book book1 = new Book();
Book book2 = new Book();

book1.Title = "A book!";
book2.Title = "A book!";

bool equality = book1.Equals(book2); // returns true

book2.Title = "A different Title";
equality = book1.Equals(book2); // returns false

Keep in mind that there's another way of implementing equality: the System.IEquatable, which is used by various classes in the System.Collections namespace for determining equality.

I'd say check that out as well and you're well on your way!

Answer 3

I understand your question to be how one can compare two objects for value equality (as opposed to reference equality) without prior knowledge of the types, such as if they implement IEquatable or override Equals.

To do this I recommend two options:

A. Use an all-purpose serialization class to serialize both objects and compare their value. For example I have a class called XmlSerializer that takes any object and serializes its public properties as an XML document. Two objects that have the same values and possibly the same reference will have the same values in this sense.

B. Using reflection, compare the values of all of the properties of both objects, like:

bool Equal(object a, object b)
{
    // They're both null.
    if (a == null && b == null) return true;
    // One is null, so they can't be the same.
    if (a == null || b == null) return false;
    // How can they be the same if they're different types?
    if (a.GetType() != b.GetType()) return false; 
    var Props = a.GetType().GetProperties();
    foreach(var Prop in Props)
    {
        // See notes *
        var aPropValue = Prop.GetValue(a) ?? string.Empty;
        var bPropValue = Prop.GetValue(b) ?? string.Empty;
        if(aPropValue.ToString() != bPropValue.ToString())
            return false;
    }
    return true;
}

Here we're assuming that we can easily compare the properties, like if they all implement IConvertible, or correctly override ToString. If that's not the case I would check if they implement IConvertible and if not, recursively call Equal() on the properties.

This only works if you're content with comparing public properties. Of course you COULD check private and protected fields and properties too, but if you know so little about the objects you're probably asking for trouble but doing so.