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Convert string into a valid JSON in c#

In the code snippet below, the JSON string in the commented out jsonString variable is valid while the uncommented out one causes JObject.Parse to throw a JsonReaderException with the message:
After parsing a value an unexpected character was encountered: e. Path 'Key', line 1, position 15.
var jsonString = "{\"Key\":\"Value \"extra\" \"}";
//var jsonString = "{\"Key\":\"Value \\\"extra\\\" \"}";
JObject.Parse(jsonString);
Are there any methods available in Newtonsoft.Json or elsewhere that can transform a JSON string to make it valid?

No, because NewtonSoft cannot guess what you want. E.g. is extra a new key and did you just ommit a comma or is it part of the previous value, or is it just something that can be ignored. It would be better to have the thing you are consuming the json from construct valid json.

Using Regex might help you to resolve the existing JSON you have. If you can control how subsequent JSON is generated, you really should fix it at that point.
This solution counts the value as existing from the first " after a "key":, through to the last " before a , or a }, and then it reserializes the value to ensure that it is correctly escaped. If it finds ",, it expects it to be followed by another key ("key":). This is in an attempt to avoid red herrings (i.e. {"key": "test "," value"}) which might otherwise confuse it.
private static string FixJson(string json)
{
var regex = new Regex("\"(?<key>.*?)\"\\W?:\\W?\"(?<value>.*?)\"(?=,\".*?\"\\W?:|}$)");
return regex.Replace(json, new MatchEvaluator(m => {
var key = m.Groups["key"].Value;
var val = m.Groups["value"].Value;
return string.Format("\"{0}\":{1}", key, JsonConvert.SerializeObject(val));
}));
}
Disclaimer: It's a regular expression, it's not foolproof, and if your JSON is more broken than you have indicated, it will probably spit out broken JSON, or incorrect values, so use it at your own risk.
Try it online

How to read JSON data?

Is this json data format?
string json = {"answer":"Line 1","mark": 1},{"answer":"Line 3","mark": 1}
I try below code but it only working with one param. ex: {"answer":"Line 1","mark": 1}. I try split json string but it isn't best way.
JObject jObject = JObject.Parse(json );
string asw = jObject["answer"].ToString();
int mark = (int)jObject["mark"];
txtAnswer.Text = asw + "////" + mark + "\n";

This is a very basic JSON question which any number of tutorials could've answered for you.
Is it valid JSON ? No, and JSONLint could've told you that.
How do you read it in ?
First, wrap your JSON in square brackets so it's valid.
Then, define a class to store the records in:
public class Something
{
public string answer { get; set; }
public string mark { get; set; }
}
And finally, use JSON.Net to convert your string into a list of these records.
string json = "[{\"answer\":\"Line 1\",\"mark\": 1},{\"answer\":\"Line 3\",\"mark\": 1}]";
List<Something> records = JsonConvert.DeserializeObject<List<Something>>(json); // JSON.Net
foreach (Something record in records)
{
System.Diagnostics.Trace.WriteLine(string.Format("Answer: {0}, Mark: {1}", record.answer, record.mark));
}
Easy as that.

Is this json data format?
string json = {"answer":"Line 1","mark": 1},{"answer":"Line 3","mark": 1}
Nope, what you've got there doesn't look like valid C# or JSON. Try putting it inside a JSON array, then inside a proper string:
string json = "[{\"answer\":\"Line 1\",\"mark\": 1},{\"answer\":\"Line 3\",\"mark\": 1}]";
(Hope I've got all the escaping right there.)
That's the C# escaped equivalent of the following JSON:
[{"answer":"Line 1","mark": 1}, {"answer":"Line 3","mark": 1}]
Then read up on JObject.Parse()for more info.

Yes it is json format. But There are multiple objects. You are not looping through it. One way could be
dynamic dynJson = JsonConvert.DeserializeObject(json);
foreach (var item in dynJson)
{
Console.WriteLine("{0} {1}\n", item.answer, item.mark);
}

Is "[]" valid JSON?

I'm having troubles de-serializing this JSON string using JSON.NET (note the quotes):
"[]"
Depending on which JSON validation website you go to, this is valid JSON (jsonlint for example says it is).
The JSON.NET code:
void Main()
{
string json = "\"[]\"";
var x = JsonConvert.DeserializeObject<User[]>(json);
Console.WriteLine(x);
}
// Define other methods and classes here
public class User
{
public string Id { get; set; }
public int Age { get; set; }
}
The exception
Error converting value "[]" to type 'UserQuery+User[]'. Path '', line 1, position 4.
Is there a way of forcing JSON.NET to parse this?

Part 1: Is "[]" valid JSON?
There are several documents and standards on JSON, and hundreds of parsers; and some of them suppose that JSON can only be object {} or an array [], but some allow single values like strings, numbers to be used as JSON.
Read this article, it widely describes this problem.
What is the minimum valid JSON?
This dispute on JSON validity is another question. In your case, it doesn't matter, because...
Part 2: why your code isn't working.
Even if we allow non-objects \ non-arrays to be valid JSON, then your JSON represents a single string equal to "[]". It could be anything else, not brackets, it is not an array notation, but just two symbols "[" and "]".
However, you try to parse this JSON as an array of objects, which will anyway result into error.
In other words, even if it is a valid JSON, then it is a valid JSON string, not JSON array.
var str1 = JSON.parse("\"[]\""),
str2 = JSON.parse("\"could be anything else, not brackets\""),
arr = JSON.parse("[]");
console.log(typeof str1);
console.log(typeof str2);
console.log(typeof arr);
var str1_s = JSON.stringify([]);
console.log("Valid JSON of an empty array: " + str1_s);
var arr_s = JSON.stringify("[]");
console.log("Partly valid JSON of a string '[]': " + arr_s);
Part 3: what should you do
The best idea - stop using invalid JSON as input. Tell whoever gave you this JSON that it is invalid JSON array and you cannot use it. You would be able to deserialize a JSON into your array of User if it was correct just like you use it:
string json = "[]";
var x = JsonConvert.DeserializeObject<User[]>(json);
Console.WriteLine(x);
If this JSON is provided from 3rd party services and you can do nothing about that, then you need to tidy it up and make it valid. Yeah, unfortunately, sometimes it happens.
How? It depends on what is your value when there ARE objects (users).
It may be a JSON-serialized JSON-string (double-serialized) like this, and then you need to deserialize a string, and then deserialize an array.
Or it can just have two odd quotes in the beginning and the end, and you can just remove them.

It is valid JSON, but the deserializer failes because the datatypes do not match.
"[]"
Is a string, so the deserializer wants to serialize it to a string.
[]
Is an empty array. So, in short, this should work:
string json = "[]";
var x = JsonConvert.DeserializeObject<User[]>(json);
Console.WriteLine(x);

Json get array object from JSON Object C#

I have this json string passed to my webapi
string jsonstring = "{\"datamodel\": \"[{\"K1\":\"V1\",\"K2\":\"V2\"}]\"}";
I may have more than on pair of (K,V) inside. How do i parse this in C# ?
I thought i could first convert my string to a JObject and get the key for datamodel from that and then use JArray to parse the K,V. But its throwing a jsonreader exception on the first line of code here
JObject my_obj = JsonConvert.DeserializeObject<JObject>(jsonstring.ToString());
and then do this..
JObject data = my_obj["datamodel"].Value<JObject>();

First of all, the JSON string you are posting is not valid. Given your comment, you can clean up the quotes before and after the square brackets using this snippet:
string jsonstring = "{\"datamodel\": \"[{\"K1\":\"V1\",\"K2\":\"V2\"}]\"}";;
string jsonstringCleaned = jsonstring.Replace("\"[", "[").Replace("]\"", "]");
var my_obj = JsonConvert.DeserializeObject<JObject>(jsonstringCleaned);

The code is right, but the exception you are getting is related to the formatting of your JSON string. If you put valid JSON in this code, it should work as expected.

There are \" missing around V1 in your JSON string.
It should look like this:
string jsonstring = "{\"datamodel\": \"[{\"K1\":\"V1\",\"K2\":\"V2\"}]\"}";

First always make sure that you have a valid Json string. A simple way to do that is paste it into a Json to C# converter tool, such as this one: http://json2csharp.com/
It may be simpler and more readable to use single quotes within your Json string if that is an option, as it avoids the need to escape the double quotes:
string jsonstring = "{'datamodel': [{'K1':'V1','K2':'V2'}]}"
Now we deserialize the object and get the JArray. There is no need to call the ToString() on the JSON jsonstring string.
var my_obj = JsonConvert.DeserializeObject<JObject>(jsonstring);
var data = (JArray)my_obj["datamodel"];
A better and more concise way to accomplish the same result could be to just use JObject.Parse. We can accomplish the same result with just one line of code.
var data = (JArray)JObject.Parse(jsonstring)["datamodel"];

Cast string representation to Json

I have a string in this representation
{
transaction_id = 120,
transaction_shortname = 120. AUTO
}
It is not a Json representation i want to know if there is a simple way to transform it to Json representation like this:
{
"transaction_id": "120",
"transaction_shortname": "120. AUTO"
}
After that i can do the following to get a Transaction object:
JObject j = JObject.Parse("{\"transaction_id\": \"120\",\"transaction_shortname\": \"120. AUTO\"}");
transaction ttttt = JsonConvert.DeserializeObject<transaction>(j.ToString());

No, this can't be converted to JSON automatically, you need to parse the format you have manually. And I don't know any language which supports this syntax.
However, if you're absolutely sure there won't be some complex cases like quoted strings and "=" and "\"" in values, you can just apply regex:
Regex.Replace(
source.Replace("\r\n", "\n"),
#"(\n\s*)([^\n]*?)\s*=\s*([^\n]*?)([,\n])",
"$1\"$2\": \"$3\"$4")

The excerpt you've given qualifies as HJSON, and so can be parsed by any HJSON library. https://hjson.org/

Thanks for your response,
Lets say i have an object
object j ;
it's base is
{
transaction_id = 120,
transaction_shortname = 120. AUTO
}
I ended by doing the following :
transaction t = JsonConvert.DeserializeObject<transaction>(JsonConvert.SerializeObject(j));