System.Text.Json JsonConverter String array to single string

Question

I would like to convert the following JSON. As you can see, the array has a single entry. I would like to convert it into a simple string.

{
    "@timestamp": [
        "2022-05-24T01:53:32.600Z"
    ],
    "site.siteCode": [
        "ZAR"
    ],
    "username": [
        "QR02159T1"
    ]
}

I tried to create a JsonConverter with System.Text.Json but it never call despite adding in the converter.

My Converter:

public class ArrayToSingleConverter : JsonConverter<string>
{
    public override string Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        if (reader.TokenType == JsonTokenType.StartArray)
        {
            // Proper array, we can deserialize from this token onwards.
            // return JsonSerializer.Deserialize<List<string>>(ref reader, options);
        }

        throw new NotImplementedException();
    }

    public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }
}

The model:

public class Ad4p 
{
    [JsonPropertyName("site.siteCode")]
    [JsonConverter(typeof(ArrayToSingleConverter))]
    public string Sitecode { get; set; }

    [JsonPropertyName("username")]
    [JsonConverter(typeof(ArrayToSingleConverter))]
    public string Username { get; set; }

    [JsonPropertyName("@timestamp")]
    [JsonConverter(typeof(ArrayToSingleConverter))]
    public string Timestamp { get; set; }
}

The implementation:

_jsonOptions.Converters.Add(new ArrayOrObjectJsonConverter<string>());
var ad4pList = JsonSerializer.Deserialize<Ad4p>(json, _jsonOptions);

But I have the following error:

Exception thrown: 'System.InvalidOperationException' in System.Text.Json.dll: 'The converter specified on 'CrawlerPowerBi.Models.Ad4p.Sitecode' is not compatible with the type 'System.String'.'

How to convert an array of strings or objects into a single string or object? Thanks.

Presumably you need to override CanConvert for your JsonConverter. https://learn.microsoft.com/en-us/dotnet/api/system.text.json.serialization.jsonconverter-1.canconvert?view=net-6.0. Has ArrayOrObjectJsonConverter you register anything todo with it? — Ralf, May 25 '22 at 09:06

Yong Shun · Accepted Answer · 2022-05-25T09:49:19.937

After some attempts and referring to a similar example Support round trip for Stack,

JsonSerializerOptions _jsonOptions = new JsonSerializerOptions();
var ad4pList = JsonSerializer.Deserialize<Ad4p>(json, _jsonOptions);

public class ArrayToSingleConverter : JsonConverter<string>
{

    public override string Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        if (reader.TokenType != JsonTokenType.StartArray)
        {
            throw new JsonException();
        }

        List<string> list = new List<string>();
        
        reader.Read();
        
        while (reader.TokenType != JsonTokenType.EndArray)
        {
            list.Add(JsonSerializer.Deserialize<string>(ref reader, options));

            reader.Read();
        }
        
        return list.FirstOrDefault();
    }

    public override void Write(Utf8JsonWriter writer, string value, JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }
}

Supported generic

public class ArrayToSingleConverter<T> : JsonConverter<T>
{

    public override T Read(ref Utf8JsonReader reader, Type typeToConvert, JsonSerializerOptions options)
    {
        if (reader.TokenType != JsonTokenType.StartArray)
        {
            throw new JsonException();
        }

        List<T> list = new List<T>();
        
        reader.Read();
        
        while (reader.TokenType != JsonTokenType.EndArray)
        {
            list.Add(JsonSerializer.Deserialize<T>(ref reader, options));

            reader.Read();
        }
        
        return list.FirstOrDefault();
    }

    public override void Write(Utf8JsonWriter writer, T value, JsonSerializerOptions options)
    {
        throw new NotImplementedException();
    }
}

public class Ad4p
{
    [JsonPropertyName("site.siteCode")]
    [JsonConverter(typeof(ArrayToSingleConverter<string>))]
    public string Sitecode { get; set; }

    [JsonPropertyName("username")]
    [JsonConverter(typeof(ArrayToSingleConverter<string>))]
    public string Username { get; set; }

    [JsonPropertyName("@timestamp")]
    [JsonConverter(typeof(ArrayToSingleConverter<string>))]
    public string Timestamp { get; set; }
}

Demo on .NET Fiddle

Alternative:

reader.Read();
        
while (reader.TokenType != JsonTokenType.EndArray)
{
    list.Add(JsonSerializer.Deserialize<T>(ref reader, options));

    reader.Read();
}

can be replaced with:

while (reader.Read() && reader.TokenType != JsonTokenType.EndArray)
{
    list.Add(JsonSerializer.Deserialize<T>(ref reader, options));
}

As both aimed to read the JSON value until EndArray was found.

Demo on .NET Fiddle (Alternative)

Thanks a lot. I succeedeed with Newtonsoft but I prefer native code. So your solution is the best. — araqiel, May 31 '22 at 06:03

AhuraMazda · Answer 2 · 2023-06-22T10:11:35.277

If you want to automatically convert the string array to a single string using a custom JSON converter in C#, you can create a custom converter class that inherits from JsonConverter provided by the Newtonsoft.Json library. Here's an example:

using Newtonsoft.Json;
using Newtonsoft.Json.Linq;
using System;

public class StringArrayConverter : JsonConverter
{
    private string _delimiter = " ; ";
    public override bool CanConvert(Type objectType)
    {
        return objectType == typeof(string[]);
    }

    public override object ReadJson(JsonReader reader, Type objectType, object existingValue, JsonSerializer serializer)
    {
        JToken token = JToken.Load(reader);

        if (token.Type == JTokenType.Array)
        {
              string[] arrayString = token.ToObject<string[]>();

              return string.Join(_delimiter, arrayString);
        }

        return null;
    }

    public override void WriteJson(JsonWriter writer, object value, JsonSerializer serializer)
    {
        if (value is string[] stringArray)
        {
            string convertedString = string.Join(", ", stringArray);
            writer.WriteValue(convertedString);
        }
    }
}

// Define the class representing your JSON object
public class MyJsonObject
{
    [JsonConverter(typeof(StringArrayConverter))]
    public string[] Data { get; set; }
}

class Program
{
    static void Main(string[] args)
    {
        // Example JSON object
        string json = @"{
            ""data"": [""string1"", ""string2"", ""string3""]
        }";

        // Deserialize the JSON object
        MyJsonObject jsonObject = JsonConvert.DeserializeObject<MyJsonObject>(json);

        // Access the data property
        string dataArray = jsonObject.Data;

        // Convert the array to a string
        string convertedString = dataArray;  // The converter automatically converted the array to a string

        Console.WriteLine(convertedString);
    }
}

In this example, the StringArrayConverter class is a custom converter that handles the conversion of the string array to a string and vice versa.

The CanConvert method checks if the objectType is a string array. The ReadJson method is responsible for deserializing the JSON array into a string array. The WriteJson method converts the string array to a single string before writing it as JSON.

In the MyJsonObject class, the Data property is annotated with the [JsonConverter(typeof(StringArrayConverter))] attribute to apply the custom converter.

When you deserialize the JSON object using JsonConvert.DeserializeObject, the string array is automatically converted to a single string during the deserialization process.

In the Main method, dataArray is an array of strings, but convertedString is automatically assigned the concatenated string value due to the custom converter.

Ensure that you have the Newtonsoft.Json NuGet package installed in your project for this code to work.

Not: This is ChatGPT answer and it solves my problem. I made a little bit change to work properly.

https://meta.stackoverflow.com/questions/421831/temporary-policy-generative-ai-e-g-chatgpt-is-banned - A lot of people were/are bulk posting answers that they haven't verified. It's good that you've been able to verify this one, but it's a fairly blanket policy because it was being misused — DavidW, Jun 22 '23 at 11:44

System.Text.Json JsonConverter String array to single string

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