C# – Making a property deserialize but not serialize with json.net

cjson.net

We have some configuration files which were generated by serializing C# objects with Json.net.

We'd like to migrate one property of the serialised class away from being a simple enum property into a class property.

One easy way to do this, would be to leave the old enum property on the class, and arrange for Json.net to read this property when we load the config, but not to save it again when we next serialize the object. We'll deal with generating the new class from the old enum separately.

Is there any simple way to mark (e.g. with attributes) a property of a C# object, so that Json.net will ignore it ONLY when serializing, but attend to it when deserializing?

Best Answer

There are actually several fairly simple approaches you can use to achieve the result you want.

Let's assume, for example, that you have your classes currently defined like this:

class Config
{
    public Fizz ObsoleteSetting { get; set; }
    public Bang ReplacementSetting { get; set; }
}

enum Fizz { Alpha, Beta, Gamma }

class Bang
{
    public string Value { get; set; }
}

And you want to do this:

string json = @"{ ""ObsoleteSetting"" : ""Gamma"" }";

// deserialize
Config config = JsonConvert.DeserializeObject<Config>(json);

// migrate
config.ReplacementSetting = 
    new Bang { Value = config.ObsoleteSetting.ToString() };

// serialize
json = JsonConvert.SerializeObject(config);
Console.WriteLine(json);

To get this:

{"ReplacementSetting":{"Value":"Gamma"}}

Approach 1: Add a ShouldSerialize method

Json.NET has the ability to conditionally serialize properties by looking for corresponding ShouldSerialize methods in the class.

To use this feature, add a boolean ShouldSerializeBlah() method to your class where Blah is replaced with the name of the property that you do not want to serialize. Make the implementation of this method always return false.

class Config
{
    public Fizz ObsoleteSetting { get; set; }

    public Bang ReplacementSetting { get; set; }

    public bool ShouldSerializeObsoleteSetting()
    {
        return false;
    }
}

Note: if you like this approach but you don't want to muddy up the public interface of your class by introducing a ShouldSerialize method, you can use an IContractResolver to do the same thing programmatically. See Conditional Property Serialization in the documentation.

Approach 2: Manipulate the JSON with JObjects

Instead of using JsonConvert.SerializeObject to do the serialization, load the config object into a JObject, then simply remove the unwanted property from the JSON before writing it out. It's just a couple of extra lines of code.

JObject jo = JObject.FromObject(config);

// remove the "ObsoleteSetting" JProperty from its parent
jo["ObsoleteSetting"].Parent.Remove();

json = jo.ToString();

Approach 3: Clever (ab)use of attributes

  1. Apply a [JsonIgnore] attribute to the property that you do not want to be serialized.
  2. Add an alternate, private property setter to the class with the same type as the original property. Make the implementation of that property set the original property.
  3. Apply a [JsonProperty] attribute to the alternate setter, giving it the same JSON name as the original property.

Here is the revised Config class:

class Config
{
    [JsonIgnore]
    public Fizz ObsoleteSetting { get; set; }

    [JsonProperty("ObsoleteSetting")]
    private Fizz ObsoleteSettingAlternateSetter
    {
        // get is intentionally omitted here
        set { ObsoleteSetting = value; }
    }

    public Bang ReplacementSetting { get; set; }
}
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