fix: use factory method for IJsonPatchWriter to ensure thread safety

src/JsonPatchGenerator/DefaultJsonPatchGenerator.cs

@@ -11,12 +11,12 @@ namespace Firebend.JsonPatch;
 public class DefaultJsonPatchGenerator : IJsonPatchGenerator
 {
     private readonly IJsonDiffDetector _diffDetector;
-    private readonly IJsonPatchWriter _writer;
+    private readonly Func<IJsonPatchWriter> _writerFactory;
     private readonly IJsonDiffSettingsProvider _settings;
-    public DefaultJsonPatchGenerator(IJsonDiffDetector diffDetector, IJsonPatchWriter writer, IJsonDiffSettingsProvider settings)
+    public DefaultJsonPatchGenerator(IJsonDiffDetector diffDetector, Func<IJsonPatchWriter> writerFactory, IJsonDiffSettingsProvider settings)
     {
         _diffDetector = diffDetector;
-        _writer = writer;
+        _writerFactory = writerFactory;
         _settings = settings;
     }
 
@@ -40,27 +40,31 @@ public JsonPatchDocument<T> Generate<T>(T original, T modified)
 
         var diffs = _diffDetector.DetectChanges(original, modified);
 
+        if (!diffs.Any()) { return new(); }
+
+        var writer = _writerFactory();
+
         foreach (var jsonDiff in diffs)
         {
             switch (jsonDiff.Change)
             {
                 case JsonChange.Unknown:
                     break;
                 case JsonChange.Add:
-                    _writer.WriteAdd(jsonDiff.Path, jsonDiff.Value);
+                    writer.WriteAdd(jsonDiff.Path, jsonDiff.Value);
                     break;
                 case JsonChange.Replace:
-                    _writer.WriteReplace(jsonDiff.Path, jsonDiff.Value);
+                    writer.WriteReplace(jsonDiff.Path, jsonDiff.Value);
                     break;
                 case JsonChange.Remove:
-                    _writer.WriteRemove(jsonDiff.Path);
+                    writer.WriteRemove(jsonDiff.Path);
                     break;
                 default:
                     throw new ArgumentOutOfRangeException();
             }
         }
 
-        var patchJson = _writer.Finish();
+        var patchJson = writer.Finish();
 
         if (string.IsNullOrWhiteSpace(patchJson))
         {

src/JsonPatchGenerator/Extensions/ServiceCollectionExtensions.cs

@@ -46,7 +46,10 @@ public static IServiceCollection AddJsonPatchGenerator(this IServiceCollection c
 
         collection.TryAddSingleton<IJsonDiffSettingsProvider>(new JsonDiffSettingsProvider(settings));
         collection.TryAddTransient<IJsonPatchWriter, JsonPatchWriter>();
-        collection.TryAddTransient<IJsonPatchGenerator, DefaultJsonPatchGenerator>();
+        collection.TryAddTransient<IJsonPatchGenerator>(sp =>
+            new DefaultJsonPatchGenerator(sp.GetRequiredService<IJsonDiffDetector>(),
+                sp.GetRequiredService<IJsonPatchWriter>,
+                sp.GetRequiredService<IJsonDiffSettingsProvider>()));
         collection.TryAddTransient<IJsonDiffDetector, JsonDiffDetector>();
 
         return collection;

tests/JsonPatchGenerator.Tests/JsonPatchGeneratorTests.cs

@@ -12,6 +12,7 @@
 using Newtonsoft.Json;
 using Newtonsoft.Json.Converters;
 using Newtonsoft.Json.Serialization;
+using System.Threading.Tasks;
 
 // ReSharper disable UnusedAutoPropertyAccessor.Local
 
@@ -620,6 +621,41 @@ public void Json_Patch_Generator_Should_Handle_Multiple_Generations()
             patchWithNoChanges.Operations.Should().BeEmpty();
         }
 
+        [TestMethod]
+        public async Task Json_Patch_Document_Generator_Should_Be_Thread_Safe()
+        {
+            //arrange
+            var generator = CreateGenerator();
+
+            var a1 = new Agent { FirstName = "A" };
+            var b1 = new Agent { FirstName = "B" };
+
+            var a2 = new Agent { LastName = "C" };
+            var b2 = new Agent { LastName = "D" };
+
+            //act
+            var task1 = Task.Run(() => generator.Generate(a1, b1));
+            var task2 = Task.Run(() => generator.Generate(a2, b2));
+
+            await Task.WhenAll(task1, task2);
+
+            //assert
+            var patch1 = await task1;
+            var patch2 = await task2;
+
+            patch1.Operations.Should().HaveCount(1);
+            var op1 = patch1.Operations[0];
+            op1.op.Should().Be("replace");
+            op1.path.Should().Be("/FirstName");
+            op1.value.Should().Be("B");
+
+            patch2.Operations.Should().HaveCount(1);
+            var op2 = patch2.Operations[0];
+            op2.op.Should().Be("replace");
+            op2.path.Should().Be("/LastName");
+            op2.value.Should().Be("D");
+        }
+
         private static JsonSerializerSettings CreateCustomSettings()
         {
             var serializerSettings = new JsonSerializerSettings