How Can I Detect Undefined Properties During JSON Deserialization with Json.NET?

Using Json.NET to detect undefined properties after deserialization

In JSON-based applications, it is critical to handle deserialization errors efficiently. While the default behavior of Json.NET is to silently fall back to default values, there are situations where exceptions need to be thrown.

To meet this need, Json.NET provides MissingMemberHandling settings. By setting this property to Error, any attempt to deserialize a JSON property without a corresponding model property will trigger JsonSerializationException.

Consider the following code:

using Newtonsoft.Json;
using System;

namespace JsonFailTest
{
    [JsonObject(MemberSerialization.OptOut)]
    public class MyJsonObjView
    {
        [JsonProperty("MyJsonInt")]
        public int MyJsonInt { get; set; }
    }

    public static class Program
    {
        public static void Main(string[] args)
        {
            string correctData = @"{'MyJsonInt': 42}";
            string wrongData = @"{'SomeOtherProperty': 'fbe8c20b'}";

            var goodObj = JsonConvert.DeserializeObject<MyJsonObjView>(correctData);
            Console.WriteLine(goodObj.MyJsonInt);

            try
            {
                JsonSerializerSettings settings = new JsonSerializerSettings
                {
                    MissingMemberHandling = MissingMemberHandling.Error
                };

                var badObj = JsonConvert.DeserializeObject<MyJsonObjView>(wrongData, settings);
                Console.WriteLine(badObj.MyJsonInt);
            }
            catch (Exception ex)
            {
                Console.WriteLine($"{ex.GetType().Name}: {ex.Message}");
            }
        }
    }
}

With this modified code, deserialization of the wrongData object will now result in the following error:

<code>System.Runtime.Serialization.Json.JsonSerializationException: Could not find member 'SomeOtherProperty' on object of type 'MyJsonObjView'. Path 'SomeOtherProperty', line 3, position 33.</code>

This error message clearly identifies the property that caused the deserialization to fail, making debugging more efficient. By using the MissingMemberHandling setting, you can force strict attribute validation during deserialization to ensure data integrity.

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