How to create JSON string in C#

Using Newtonsoft.Json makes it really easier:

Product product = new Product();
product.Name = "Apple";
product.Expiry = new DateTime(2008, 12, 28);
product.Price = 3.99M;
product.Sizes = new string[] { "Small", "Medium", "Large" };

string json = JsonConvert.SerializeObject(product);

Documentation: Serializing and Deserializing JSON

You could use the JavaScriptSerializer class, check this article to build an useful extension method.

Code from article:

namespace ExtensionMethods
{
    public static class JSONHelper
    {
        public static string ToJSON(this object obj)
        {
            JavaScriptSerializer serializer = new JavaScriptSerializer();
            return serializer.Serialize(obj);
        }

        public static string ToJSON(this object obj, int recursionDepth)
        {
            JavaScriptSerializer serializer = new JavaScriptSerializer();
            serializer.RecursionLimit = recursionDepth;
            return serializer.Serialize(obj);
        }
    }
}

Usage:

using ExtensionMethods;

...

List<Person> people = new List<Person>{
                   new Person{ID = 1, FirstName = "Scott", LastName = "Gurthie"},
                   new Person{ID = 2, FirstName = "Bill", LastName = "Gates"}
                   };


string jsonString = people.ToJSON();

Simlpe use of Newtonsoft.Json and Newtonsoft.Json.Linq libraries.

        //Create my object
        var my_jsondata = new
        {
            Host = @"sftp.myhost.gr",
            UserName = "my_username",
            Password = "my_password",
            SourceDir = "/export/zip/mypath/",
            FileName = "my_file.zip"
        };

        //Tranform it to Json object
        string json_data = JsonConvert.SerializeObject(my_jsondata);

        //Print the Json object
        Console.WriteLine(json_data);

        //Parse the json object
        JObject json_object = JObject.Parse(json_data);

        //Print the parsed Json object
        Console.WriteLine((string)json_object["Host"]);
        Console.WriteLine((string)json_object["UserName"]);
        Console.WriteLine((string)json_object["Password"]);
        Console.WriteLine((string)json_object["SourceDir"]);
        Console.WriteLine((string)json_object["FileName"]);