How to add a new object to an existing JSON file

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
– crashmstr, Commented Oct 12, 2015 at 12:28

Michael Mairegger · Accepted Answer · 2015-10-12 12:32:36Z

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, 2015 at 12:32

Michael Mairegger

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4 Comments

David Pine Over a year ago

Does this actually maintain the order?

PRMan Over a year ago

Well, since order is not guaranteed in JSON, it really shouldn't matter.

Akash Agarwal Over a year ago

@PRMan JSON preserves the order of array items, but not that of dictionary items.

Ahune ajé o ahe Over a year ago

Is this a good option to do if you have a large Json file?

HadiRj · Accepted Answer · 2015-10-12 12:52:44Z

7

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

edited Oct 12, 2015 at 12:52

HadiRj

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answered Oct 12, 2015 at 12:41

Alex

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Comments

LMK · Accepted Answer · 2019-07-04 21:40:36Z

4

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.

edited Jul 4, 2019 at 21:40

answered Jul 4, 2019 at 4:14

LMK

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1 Comment

Canovas Over a year ago

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.

Arthur Rey · Accepted Answer · 2019-10-02 15:09:55Z

1

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 2, 2019 at 15:09

answered Oct 12, 2015 at 12:57

Arthur Rey

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1 Comment

Abhishek Over a year ago

Since you are trying to add a List of objects to a List, you should use AddRange() instead of Add()

sajadre · Accepted Answer · 2021-03-09 05:49:26Z

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 9, 2021 at 5:49

answered Mar 6, 2021 at 6:32

sajadre

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Comments

TylerH · Accepted Answer · 2023-09-19 21:25:10Z

0

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);

edited Sep 19, 2023 at 21:25

TylerH

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answered Oct 26, 2022 at 19:54

user20342903

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