Use Newtonsoft.Json as serializer

HabitSharp.csproj

@@ -5,6 +5,7 @@
   </PropertyGroup>
 
   <ItemGroup>
+    <PackageReference Include="Newtonsoft.Json" Version="11.0.2" />
     <PackageReference Include="RestSharp" Version="106.3.1" />
   </ItemGroup>
 

HabiticaApi.cs

@@ -1,39 +1,48 @@
-using System;
-using System.Collections.Generic;
-using RestSharp;
-using HabitSharp.Managers;
-
-namespace HabitSharp
-{
-    public class HabiticaApi
-    {
-        const string BaseURL = "https://habitica.com/api/v3";
-
-        private readonly RestClient Client;
-
-        public readonly TagManager Tags;
-
-        public HabiticaApi(string UserID, string ApiKey) : this(Guid.Parse(UserID), Guid.Parse(ApiKey)) { }
-
-        public HabiticaApi(Guid UserID, Guid ApiKey)
-        {
-            this.Client = new RestClient(BaseURL);
-            this.Client.Authenticator = new HabiticaAuthenticator(UserID, ApiKey);
-            this.Tags = new TagManager(this);
-        }
-
-        public T Execute<T>(RestRequest request) where T : new() {
-            var response = Client.Execute<T>(request);
-            if (response.ErrorException != null) throw new ApplicationException(
-                "An error occured while running a Habitica API request. See inner exception for more info",
-                response.ErrorException);
-            return response.Data;
-        }
-
-        public Result<Dictionary<string, string>> GetModelPaths(string ModelName) {
-            var req = new RestRequest("models/{model}/paths");
-            req.AddUrlSegment("model", ModelName);
-            return this.Execute<Result<Dictionary<string, string>>>(req);
-        }
-    }
-}
+using System;
+using System.Collections.Generic;
+using RestSharp;
+using HabitSharp.Managers;
+using HabitSharp.Serialization;
+
+namespace HabitSharp
+{
+    public class HabiticaApi
+    {
+        const string BaseURL = "https://habitica.com/api/v3";
+
+        private readonly RestClient Client;
+
+        public readonly TagManager Tags;
+
+        public HabiticaApi(string UserID, string ApiKey) : this(Guid.Parse(UserID), Guid.Parse(ApiKey)) { }
+
+        public HabiticaApi(Guid UserID, Guid ApiKey)
+        {
+            this.Client = new RestClient(BaseURL);
+            this.Client.Authenticator = new HabiticaAuthenticator(UserID, ApiKey);
+
+            // Override JSON serialization
+            this.Client.AddHandler("application/json", NewtonsoftJsonSerializer.Default);
+            this.Client.AddHandler("text/json", NewtonsoftJsonSerializer.Default);
+            this.Client.AddHandler("text/x-json", NewtonsoftJsonSerializer.Default);
+            this.Client.AddHandler("text/javascript", NewtonsoftJsonSerializer.Default);
+            this.Client.AddHandler("*+json", NewtonsoftJsonSerializer.Default);
+
+            this.Tags = new TagManager(this);
+        }
+
+        public T Execute<T>(RestRequest request) where T : new() {
+            var response = Client.Execute<T>(request);
+            if (response.ErrorException != null) throw new ApplicationException(
+                "An error occured while running a Habitica API request. See inner exception for more info",
+                response.ErrorException);
+            return response.Data;
+        }
+
+        public Result<Dictionary<string, string>> GetModelPaths(string ModelName) {
+            var req = new RestRequest("models/{model}/paths");
+            req.AddUrlSegment("model", ModelName);
+            return this.Execute<Result<Dictionary<string, string>>>(req);
+        }
+    }
+}

Serialization/NewtonsoftJsonSerializer.cs

@@ -0,0 +1,52 @@
+using System.IO;
+using Newtonsoft.Json;
+using RestSharp.Serializers;
+using RestSharp.Deserializers;
+
+namespace HabitSharp.Serialization {
+    public class NewtonsoftJsonSerializer : ISerializer, IDeserializer
+    {
+        private Newtonsoft.Json.JsonSerializer serializer;
+
+        public NewtonsoftJsonSerializer(Newtonsoft.Json.JsonSerializer serializer) => this.serializer = serializer;
+
+        public string ContentType {
+            get => "application/json";
+            set { }
+        }
+
+        public string DateFormat { get; set; }
+
+        public string Namespace { get; set; }
+
+        public string RootElement { get; set; }
+
+        public string Serialize(object obj)
+        {
+            using (var stringWriter = new StringWriter())
+            {
+                using (var jsonTextWriter = new JsonTextWriter(stringWriter))
+                {
+                    serializer.Serialize(jsonTextWriter, obj);
+                    return stringWriter.ToString();
+                }
+            }
+        }
+
+        public T Deserialize<T>(RestSharp.IRestResponse response)
+        {
+            using (var stringReader = new StringReader(response.Content))
+            {
+                using (var jsonTextReader = new JsonTextReader(stringReader))
+                {
+                    return serializer.Deserialize<T>(jsonTextReader);
+                }
+            }
+        }
+
+        public static NewtonsoftJsonSerializer Default =>
+            new NewtonsoftJsonSerializer(new Newtonsoft.Json.JsonSerializer() {
+                NullValueHandling = NullValueHandling.Ignore,
+            });
+    }
+}