Can Json.NET populate readonly fields in a class?

Question

I haven\'t seen much information about Json.NET supporting deserializing objects with readonly fields. I do notice that the .NET DataContract and DataMember att

Answer 1

afai can see changing a field to readonly results in a null value after deserialization. I had a working sample for another question (modified as shown below), and that's the behaviour I see.

public class NameAndId
{
    public string name;
    public int id; 
}

public class Data
{
    public NameAndId[] data;
}

public class Target
{
    public string id;
    public readonly NameAndId from;
    public DateTime updated_time;
    public readonly string message;
    public Data likes;
}

public class Program
{
    static void Main(string[] args)
    {
        string json = File.ReadAllText(@"c:\temp\json.txt");
        Target newTarget = JsonConvert.DeserializeObject<Target>(json);
    }
}

Answer 2

Not the most elegant solution, but you can extend the DefaultConstractResolver to do it:

public class ContractResolver : DefaultContractResolver
{
    protected override JsonProperty CreateProperty(MemberInfo member, MemberSerialization memberSerialization)
    {
        var property = base.CreateProperty(member, memberSerialization);
        property.Writable = CanSetMemberValue(member, true);
        return property;
    }

    public static bool CanSetMemberValue(MemberInfo member, bool nonPublic)
    {
        switch (member.MemberType)
        {
            case MemberTypes.Field:
                var fieldInfo = (FieldInfo)member;

                return nonPublic || fieldInfo.IsPublic;
            case MemberTypes.Property:
                var propertyInfo = (PropertyInfo)member;

                if (!propertyInfo.CanWrite)
                    return false;
                if (nonPublic)
                    return true;
                return (propertyInfo.GetSetMethod(nonPublic) != null);
            default:
                return false;
        }
    }
}

I have just remove one little check from the CanSetMemberValue method. Unfortunately it's neither virtual nor an instance method, so I had to override CreateProperty as well.

Answer 3

This can be done now. Declare your properties using the JsonProperty attribute, and ensure that they have a protected set declared:

[JsonProperty("Name")] public string Name {get; protected set;}

This didn't work for me when using only a get, but works perfectly with the protected set.