json add new object to existing json file C#

Question

I'm trying to automate the addition of new objects to an existing JSON file. I looked all around the web but only found adding data and stuff but not a whole object. This is how the file that I want to edit looks:

[
    {"id":"123","name":"carl"}
]

and I want to go to

[
     {"id":"123","name":"carl"},
     {"id":"1234","name":"carl2"}
]

Thank you for all your answers but I don't think everyone completely understands what i mean I have tried some of the answers but then I get this:

[
    "{\"id\":\"123\",\"name\":\"carl\"}"
]"{\"id\":\"1234\",\"name\":\"carl2\"}"

and I want everything in between the [].

Deserialize to a c# data type (or `JObject`, or `dynamic`, etc.), add a new item to the list, then serialize back to a string? — crashmstr, Oct 12 '15 at 12:28

score 18 · Accepted Answer · answered Oct 12 '15 at 12:32

18

If you use json.NET you can simply deserialize and serialize the json.

var list = JsonConvert.DeserializeObject<List<Person>>(myJsonString);
list.Add(new Person(1234,"carl2");
var convertedJson = JsonConvert.SerializeObject(list, Formatting.Indented);

answered Oct 12 '15 at 12:32

Michael Mairegger

6,833
28
41

Does this actually maintain the order? – David Pine Jul 17 '19 at 17:26
Well, since order is not guaranteed in JSON, it really shouldn't matter. – PRMan Feb 14 '20 at 21:27
@PRMan JSON preserves the order of array items, but not that of dictionary items. – Akash Agarwal Sep 21 '20 at 00:49
2

Is this a good option to do if you have a large Json file? – Ahune ajé o ahe Dec 03 '20 at 10:39

score 8 · Answer 2 · edited Oct 12 '15 at 12:52

Using Json.Net

//load from file
var initialJson = "[{\"id\":\"123\",\"name\":\"carl\"}]";

var array = JArray.Parse(initialJson);

var itemToAdd = new JObject();
itemToAdd["id"] = 1234;
itemToAdd["name"] = "carl2";
array.Add(itemToAdd);

var jsonToOutput = JsonConvert.SerializeObject(array, Formatting.Indented);

//save to file here

Using this method doesn't require strongly typed objects

You could replace this bit:

//load from file
var initialJson = "[{\"id\":\"123\",\"name\":\"carl\"}]";

With

var initialJson = File.ReadAllText(@"c:\myjson.json")

To load the json from a text file

LMK · Answer 3 · 2019-07-04T21:40:36.373

A better performing solution than serializing/deserializing what may be a large file would be to open a FileStream, seek 1 character before the end, then serialize and write your new object into the array, then write a closing bracket. See this question C# How to delete last 2 characters from text file and write into the same line at the end my text?, I'll copy the code here - you already know how to serialize your object and encode it into bytes.

using(var fs = new FileStream("file.json")) {    
    fs.Seek(-1,SeekOrigin.End);        
    fs.Write(mySerializedJSONObjAsBytes,0,mySerializedJSONObjAsBytes.Length); // include a leading comma character if required
    fs.Write(squareBracketByte, 0, 1);
    fs.SetLength(fs.Position); //Only needed if new content may be smaller than old
}

Sorry haven't tested any of that, it's off the top of my head. Pro-tip: wrap FileStream in a StreamWriter so can write strings directly.

This is the best answer since all the others deserialize/serialize the whole json file only to add one element to the list, which is unnecessary and will impact performance if the file is big and/or you need to add new elements frequently. — Canovas, Jun 22 '22 at 21:50

Arthur Rey · Answer 4 · 2019-10-02T15:09:55.273

2

You could create a method:

public string AddObjectsToJson<T>(string json, List<T> objects)
{
    List<T> list = JsonConvert.DeserializeObject<List<T>>(json);
    list.AddRange(objects);
    return JsonConvert.SerializeObject(list);
}

Then use it like this:

string baseJson = "[{\"id\":\"123\",\"name\":\"carl\"}]";
List<Person> personsToAdd = new List<Person>() { new Person(1234,"carl2") };

string updatedJson = AddObjectsToJson(baseJson, personsToAdd);

edited Oct 02 '19 at 15:09

answered Oct 12 '15 at 12:57

Arthur Rey

2,990
3
19
42

2

Since you are trying to add a List of objects to a List, you should use AddRange() instead of Add() – Abhishek Dec 05 '16 at 07:49

sajadre · Answer 5 · 2021-03-09T05:49:26.127

1

this would be a sample for you:

var list = JsonConvert.DeserializeObject<List<Example>>(json);
    Example example = new Example();
    example.name = "Product2";
    example.id="1";
    list.Add(example);
    
    string output = JsonConvert.SerializeObject(list);
    
    public class Example
    {
        public string id {get;set;}
        public string name { get; set; }
        
    }

edited Mar 09 '21 at 05:49

answered Mar 06 '21 at 06:32

sajadre

1,141
2
15
30

user20342903 · Answer 6 · 2022-10-26T19:55:46.330

I have been looking for a solution to this exact question for a week now. I finally figured out how to append it to the existing json array and not add it as a solo object after the array. Make sure you are using WriteAllText and not AppendAllText. Here is what I did:

JArray array = JsonConvert.DeserializeObject<JArray (jsonFile);

JObject obj = new JObject();
obj.Add("ID", "123");
obj.Add("Name", "Brian");

array.Add(obj);

var jsonToOutput = JsonConvert.SerializeObject(array, Formatting.Indented);
File.WriteAllText("fileName.json", jsonToOutput);

json add new object to existing json file C#

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