This was forked to add support for custom EqualityComparer<T>.

C# algorithm for diffing two lists of objects. The implementation and tests are heavily based on Google's Diff-Match-Patch - that's a 3rd party mirror of the code that was originally hosted here.

I've removed all the patching/merging stuff and anything specific to text, so what remains is generalised to support comparing two IReadOnlyList<T> where T can be any type comparable with EqualityComparer<T>.Default, e.g. one overriding Equals.

My purpose for this is to be able to compare two lists of objects, A and B, and get a short description of how to modify A to make it the same as B.

There is a single public method to call in the ListDiff namespace, ListDiff.Compare:

using static ListDiff.ListDiff;

var diff = Compare(list1, list2);

The returned sequence is of Diff objects, like this:

public class Diff<T>
{
    public Operation Operation { get; }       
    public IReadOnlyList<T> Items { get; set; }

Finally, Operations are:

public enum Operation
{
    Delete, Insert, Equal
}

That is the complete API. So for example:

using System;
using static ListDiff.ListDiff;

namespace DiffThing
{
    class Program
    {
        static void Main(string[] args)
        {
            var diffs = Compare(
                "The quick brown fox jumped over the dog".ToCharArray(),
                "The quack brown animal jumped the lazy dog".ToCharArray());

            foreach (var diff in diffs)
            {
                Console.WriteLine(diff);
            }
        }
    }
}

This prints:

Diff(Equal,"The qu")
Diff(Delete,"i")
Diff(Insert,"a")
Diff(Equal,"ck brown ")
Diff(Delete,"fox")
Diff(Insert,"animal")
Diff(Equal," jumped ")
Diff(Delete,"over ")
Diff(Equal,"the")
Diff(Insert," lazy")
Diff(Equal," dog")