How can I create a JsonPatchDocument from comparing two c# objects?

Question

Given I have two c# objects of the same type, I want to compare them to create a JsonPatchDocument.

I have a StyleDetail class defined like this:

public class StyleDetail     {         public string Id { get; set; }         public string Code { get; set; }         public string Name { get; set; }         public decimal OriginalPrice { get; set; }         public decimal Price { get; set; }         public string Notes { get; set; }         public string ImageUrl { get; set; }         public bool Wishlist { get; set; }         public List<string> Attributes { get; set; }         public ColourList Colours { get; set; }         public SizeList Sizes { get; set; }         public ResultPage<Style> Related { get; set; }         public ResultPage<Style> Similar { get; set; }         public List<Promotion> Promotions { get; set; }         public int StoreStock { get; set; }         public StyleDetail()         {             Attributes = new List<string>();             Colours = new ColourList();             Sizes = new SizeList();             Promotions = new List<Promotion>();         }     } 

if I have two StyleDetail objects

StyleDetail styleNew = db.GetStyle(123); StyleDetail styleOld = db.GetStyle(456); 

I now want to create a JsonPatchDocument so I can send the differences to my REST API... How to do this??

JsonPatchDocument patch = new JsonPatchDocument(); // Now I want to populate patch with the differences between styleNew and styleOld - how? 

in javascript, there is a library to do this https://www.npmjs.com/package/rfc6902

Calculate diff between two objects:

rfc6902.createPatch({first: 'Chris'}, {first: 'Chris', last: 'Brown'});

[ { op: 'add', path: '/last', value: 'Brown' } ] 

but I am looking for a c# implementation

Answer 1

Let's abuse the fact that your classes are serializable to JSON! Here's a first attempt at a patch creator that doesn't care about your actual object, only about the JSON representation of that object.

public static JsonPatchDocument CreatePatch(object originalObject, object modifiedObject) {     var original = JObject.FromObject(originalObject);     var modified = JObject.FromObject(modifiedObject);      var patch = new JsonPatchDocument();     FillPatchForObject(original, modified, patch, "/");      return patch; }  static void FillPatchForObject(JObject orig, JObject mod, JsonPatchDocument patch, string path) {     var origNames = orig.Properties().Select(x => x.Name).ToArray();     var modNames = mod.Properties().Select(x => x.Name).ToArray();      // Names removed in modified     foreach (var k in origNames.Except(modNames))     {         var prop = orig.Property(k);         patch.Remove(path + prop.Name);     }      // Names added in modified     foreach (var k in modNames.Except(origNames))     {         var prop = mod.Property(k);         patch.Add(path + prop.Name, prop.Value);     }      // Present in both     foreach (var k in origNames.Intersect(modNames))     {         var origProp = orig.Property(k);         var modProp = mod.Property(k);          if (origProp.Value.Type != modProp.Value.Type)         {             patch.Replace(path + modProp.Name, modProp.Value);         }         else if (!string.Equals(                         origProp.Value.ToString(Newtonsoft.Json.Formatting.None),                         modProp.Value.ToString(Newtonsoft.Json.Formatting.None)))         {             if (origProp.Value.Type == JTokenType.Object)             {                 // Recurse into objects                 FillPatchForObject(origProp.Value as JObject, modProp.Value as JObject, patch, path + modProp.Name +"/");             }             else             {                 // Replace values directly                 patch.Replace(path + modProp.Name, modProp.Value);             }         }            } } 

Usage:

var patch = CreatePatch(     new { Unchanged = new[] { 1, 2, 3, 4, 5 }, Changed = "1", Removed = "1" },     new { Unchanged = new[] { 1, 2, 3, 4, 5 }, Changed = "2", Added = new { x = "1" } });  // Result of JsonConvert.SerializeObject(patch) [   {     "path": "/Removed",     "op": "remove"   },   {     "value": {       "x": "1"     },     "path": "/Added",     "op": "add"   },   {     "value": "2",     "path": "/Changed",     "op": "replace"   } ]