How to Handle Missing Fields During JSON Deserialization in Json.NET?

How to handle missing fields during JSON deserialization in Json.NET

When deserializing JSON objects using Json.NET, it is critical to handle missing fields correctly. By default, Json.NET ignores missing properties and returns default values, which may cause errors during subsequent processing.

Consider the following code:

using Newtonsoft.Json;

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

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

        var goodObj = JsonConvert.DeserializeObject<MyJsonObjView>(correctData);
        var badObj = JsonConvert.DeserializeObject<MyJsonObjView>(wrongData);

        Console.WriteLine(goodObj.MyJsonInt); // 输出 42
        Console.WriteLine(badObj.MyJsonInt); // 输出 0
    }
}

This code attempts to deserialize a JSON object into a MyJsonObjView object. The correct JSON contains the "MyJsonInt" attribute, while the incorrect JSON does not. The output is as follows:

<code>42
0</code>

As you can see, the code fails silently when deserializing the wrong JSON, returning default values ​​for missing properties. To avoid such behavior we can use the MissingMemberHandling setting.

JsonSerializerSettings settings = new JsonSerializerSettings();
settings.MissingMemberHandling = MissingMemberHandling.Error;

By setting MissingMemberHandling to Error we instruct Json.NET to throw an exception when a missing property is encountered during deserialization.

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

Now, if we deserialize the wrong JSON, we will get an exception:

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

This exception clearly indicates that deserialization failed due to a missing attribute, allowing us to handle the error appropriately.

By setting the MissingMemberHandling attribute, we can more effectively control the behavior of Json.NET when dealing with missing JSON attributes, thereby avoiding potential errors and improving the robustness of our code.

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