Deserialize List<string> from Json to List<object>

You are going to have to write a converter for the UserGroup

Here is a simple converter based on what was described in the question

public class UserGroupJsonConverter : JsonConverter {    public override bool CanConvert(Type objectType) {        return typeof(UserGroup) == objectType;    }    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer) {        return new UserGroup((string)reader.Value);    }    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer) {        writer.WriteValue(((UserGroup)value).Code);    }}

And then update the User to be aware of the converter by setting the ItemConverterType of the JsonProperty attribute

[JsonObject(MemberSerialization.OptIn)]public class User {    public User(string code) {        this.Code = code;    }    [JsonProperty]    public string Code { get; set; }    private List<UserGroup> groups;    [JsonProperty("Groups", ItemConverterType = typeof(UserGroupJsonConverter))]    public List<UserGroup> Groups {        get {            if (groups == null)                groups = new List<UserGroup>();            return groups;        }    }}

This would now allow for the JSON in the example

{    "Code": "Admin",    "Groups":    [        "Administrator",        "Superuser",        "User"    ]}

to be deserialized as desired

var user = JsonConvert.DeserializeObject<User>(json);

and to be serialized back into the same format.

var json = JsonConvert.SerializeObject(user);

Administrator is a string, UserGroup is an object so this is a type mismatch. The valid JSON for the scenario laid out in your code is:

{    "Code": "Admin",    "Groups":    [        {            "Code":"Administrator"        },        {            "Code":"Superuser"        },        {            "Code":"User"        }    ]}