How to convert list to json

Question

I have a function that connects to the database and executes a query and returns the data, now I am trying to call that function and convert the data to JSON

public List<string> getListItems()
        {
            AirportClass airport = new AirportClass();
            return JArray.Parse(airport.getListItems());
            return airport.getListItems();
        }

I keep getting this error 'The best overloaded method match for 'Newtonsoft.Json.Linq.JArray.Parse(string)' has some invalid arguments'

What am I doing wrong?

I have also tried the following:

public List<string> getListItems()
        {
            AirportClass airport = new AirportClass();
            JavaScriptSerializer JsonConvert = new JavaScriptSerializer();
            JsonConvert.Serialize(airport.getListItems());
            return JsonConvert;
        }

but this also gives me an error 'Cannot implicitly convert type 'JavaScriptSerializer' to Generic.List'

I also tried:

public List<string> getListItems()
        {
            AirportClass airport = new AirportClass();
            JsonSerializer JsonConvert = new JsonSerializer();
            JsonConvert.Serialize(getListItems());
            return airport.getListItems();
        }

but this gives me another error 'No overload for method 'Serialize' takes 1 arguments'

Solution

You are not saving the return of the Serialize result:

JsonConvert.Serialize(airport.getListItems());
return JsonConvert;

Corrected with correct return type. Converting from object to JSON produces a single string encoded as a JSON array, so your return type is string, not List<string>

public string getListItems() {
  AirportClass airport = new AirportClass();
  //removed JsonConvert declaration cause that would hide the JsonConvert class, don't name variables the same name as classes
  string result = JsonConvert.SerializeObject(airport.getListItems());
  return result;
}

It does not produce a List, to do that you'd need to serialize each item seperately, which is different from serializing a list. In that case you'd just use a loop to call Serialize on each item.

Comments

[Giannis Paraskevopoulos]

First of all there is no point in two returns.

[Ehsan Sajjad]

you are passing JArray.parse() list of string while it accepts string

[AaronLS]

Parse is the wrong method to use here. Parsing is for converting JSON to objects. You are trying to convert an object to JSON. You need to Serialize: james.newtonking.com/json/help/index.html#

[user3723240]

I still dont understand

[AaronLS]

@user3723240 Sorry my link was wrong, see this: james.newtonking.com/json/help/index.html?topic=html/…

[user3723240]

This gives me another error 'No overload for method 'Serialize' takes 1 arguments'

[user3723240]

JsonSerializer JsonConvert = new JsonSerializer(); string result = JsonConvert.Serialize(airport.getListItems()); return result;

[AaronLS]

Ah i copied from your code, should be SerializeObject from the documentation. Also you shouldn't be declaring a variable named JsonConvert, there is already a class named that in the framework.

[user3723240]

That got rid of one error, now I am getting a new one for return result - 'Cannot implicitly convert type 'string' to 'System.Collections.Generic.List<string>'

[AaronLS]

@user3723240 Can you update your question to show an example of the JSON string you are trying to produce? Such as in the documentation they show the result ["Starcraft","Halo","Legend of Zelda"]. Serializing to JSON produces a single string, but you are expecting a list of strings, so it is unclear if you want to serialize each object individually. You will need to take a step back and figure out what it is you are trying to accomplish.