Force conversion of empty JSON array to Dictionary type

问题

The problem is, that json_encode() function in PHP leaves ambiguity for tools which are reading its output. In PHP both lists and dictionaries are same type of array.

echo json_encode([]); // []
echo json_encode(["5" => "something"]); // {"5": "something"}

In JSON.NET I want to force both [] and {"5": "something"} to convert to Dictionary<string, string> type. However it recognizes [] as prohibited structure for Dictionary and throws an Exception.

Can I quickly leave empty JSON arrays nulled or force them to convert to empty Dictionary type?

FINAL SOLUTION

I modified accepted answer to make it generic and reusable for other types.

public class DictionaryOrEmptyArrayConverter<T,F> : JsonConverter
{
    public override bool CanWrite { get { return false; } }
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(Dictionary<T, F>);
    }
    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        switch (reader.TokenType)
        {
            case JsonToken.StartArray:
                reader.Read();
                if (reader.TokenType == JsonToken.EndArray)
                    return new Dictionary<T, F>();
                else
                    throw new JsonSerializationException("Non-empty JSON array does not make a valid Dictionary!");
            case JsonToken.Null:
                return null;
            case JsonToken.StartObject:
                var tw = new System.IO.StringWriter();
                var writer = new JsonTextWriter(tw);
                writer.WriteStartObject();
                int initialDepth = reader.Depth;
                while (reader.Read() && reader.Depth > initialDepth)
                {
                    writer.WriteToken(reader);
                }
                writer.WriteEndObject();
                writer.Flush();
                return JsonConvert.DeserializeObject<Dictionary<T, F>>(tw.ToString());
            default:
                throw new JsonSerializationException("Unexpected token!");
        }
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        throw new NotImplementedException();
    }
}

You should use it with your JSON object as in example below:

public class Company
{
    public string industry_name { get; set; }

    [JsonConverter(typeof(DictionaryOrEmptyArrayConverter<int, Upgrade>))]
    public Dictionary<int, Upgrade> upgrades { get; set; }
}

public class Upgrade
{
    public int level { get; set; }
}

It allows you to quickly convert JSON string to objects with DeserializeObject method:

var result = JsonConvert.DeserializeObject<Company>(jsonString);

回答1:

You can do it with a custom JsonConverter, although I think this solution leaves a bit to be desired:

    private class DictionaryConverter : JsonConverter
    {
        public override bool CanWrite { get { return false; } }
        public override bool CanConvert(Type objectType)
        {
            return objectType == typeof(Dictionary<string, string>);
        }
        public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
        {
            if (reader.TokenType == JsonToken.StartArray)
            {
                reader.Read();
                if (reader.TokenType == JsonToken.EndArray)
                    return new Dictionary<string, string>();
                else
                    throw new JsonSerializationException("Non-empty JSON array does not make a valid Dictionary!");
            }
            else if (reader.TokenType == JsonToken.Null)
            {
                return null;
            }
            else if (reader.TokenType == JsonToken.StartObject)
            {
                Dictionary<string, string> ret = new Dictionary<string, string>();
                reader.Read();
                while (reader.TokenType != JsonToken.EndObject)
                {
                    if (reader.TokenType != JsonToken.PropertyName)
                        throw new JsonSerializationException("Unexpected token!");
                    string key = (string)reader.Value;
                    reader.Read();
                    if (reader.TokenType != JsonToken.String)
                        throw new JsonSerializationException("Unexpected token!");
                    string value = (string)reader.Value;
                    ret.Add(key, value);
                    reader.Read();
                }
                return ret;
            }
            else
            {
                throw new JsonSerializationException("Unexpected token!");
            }
        }

        public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
        {
            throw new NotImplementedException();
        }
    }

See this code in action at https://dotnetfiddle.net/zzlzH4

回答2:

Or you could preprocess the json text by removing all those empty array properties.

//Will not work with formatted json due to whitespace    
json = Regex.Replace(json, ",?\"[^\"]+\":[[]]", "");

来源：https://stackoverflow.com/questions/26725138/force-conversion-of-empty-json-array-to-dictionary-type

标签

json

json.net