System.Text.Json.JsonSerializer can't deserialize readonly collection property

[chenyj796]

I have a Config class which contains a readonly property named Items.
I serialize it to json string, and then deserialize the json string back as below.
After run the Run() method, cfg2.Items is empty when cfg3.Items contains one item.

public class Config
{
    public HashSet<string> Items { get; } = new HashSet<string>();
}

public class ConfigTest
{
    public void Run()
    {
        var cfg = new Config();
        cfg.Items.Add("item1");
        var json = System.Text.Json.JsonSerializer.Serialize(cfg);
        var cfg2 = System.Text.Json.JsonSerializer.Deserialize<Config>(json);
        var cfg3 = Newtonsoft.Json.JsonConvert.DeserializeObject<Config>(json);
    }
}

[steveharter]

This is by design for collections that don't have a setter. To avoid issues with adding to pre-populated collections (that the serializer doesn't instantiate) the deserializer uses "replace" semantics which requires the collection to have a setter.

However if there is no setter (as in your case) https://github.com/dotnet/corefx/issues/39477 is tracking that feature.