Newtonsoft.Json serialization returns empty json object

You could also decorate your class to serialize all members you want without having to specify [JsonProperty] for each of them.

[JsonObject(MemberSerialization.OptOut)]
public class Catagory {
    ...
}

The MemberSerialization enum allows you to specify what members you want to serialize:

• MemberSerialization.OptOut: All public members are serialized.
• MemberSerialization.OptIn: Only members marked with JsonPropertyAttribute or DataMemberAttribute are serialized.
• MemberSerialization.Fields: All public and private members are serialized.

By default, NewtonSoft.Json will only serialize public members, so make your fields public:

public class Catagory
{
    public int catagoryId;
    public string catagoryNameHindi;
    public string catagoryNameEnglish;
    public List<Object> subCatagories;

    public Catagory(int Id, string NameHindi, string NameEng, List<Object> l)
    {
        this.catagoryId = Id;
        this.catagoryNameHindi = NameHindi;
        this.catagoryNameEnglish = NameEng;
        this.subCatagories = l;
    }
}

If for some reason you really don't want to make your fields public, you can instead decorate them with the JsonPropertyAttribute to allow them to be serialized and deserialized:

[JsonProperty]
int catagoryId;

This attribute also allows specifying other options, such as specifying the property name to use when serializing/deserializing:

[JsonProperty("categoryId")]
int Category;