How can i convert my dictionary
Dictionary<string,string> to JSON string 
and then again convert from JSON string to
Dictionary<string,string> in c#?
Try this using extension methods.
public static class Extensions
{
    public static string FromDictionaryToJson(this Dictionary<string, string> dictionary)
    {
        var kvs = dictionary.Select(kvp => string.Format("\"{0}\":\"{1}\"", kvp.Key, string.Concat(",", kvp.Value)));
        return string.Concat("{", string.Join(",", kvs), "}");
    }
    public static Dictionary<string, string> FromJsonToDictionary(this string json)
    {
        string[] keyValueArray = json.Replace("{", string.Empty).Replace("}", string.Empty).Replace("\"", string.Empty).Split(',');
        return keyValueArray.ToDictionary(item => item.Split(':')[0], item => item.Split(':')[1]);
    }
}
Here is how you would use them after.
    class Program
    {
        static void Main(string[] args)
        {
            Dictionary<string, string> dictss = new Dictionary<string, string>();
            dictss.Add("onekey", "oneval");
            dictss.Add("twokey", "twoval");
            dictss.Add("threekey", "threeval");
            dictss.Add("fourkey", "fourval");
            dictss.Add("fivekey", "fiveval");
            string jsonString = dictss.FromDictionaryToJson(); //call extension method
            Console.WriteLine(jsonString);
            Dictionary<string, string> dictss2 = jsonString.FromJsonToDictionary(); //call extension method
            foreach(KeyValuePair<string,string> kv in dictss2)
                Console.WriteLine(string.Format("key={0},value={1}", kv.Key, kv.Value));
        }
    }
Or using plain functions
        public string FromDictionaryToJson(Dictionary<string, string> dictionary)
        {
            var kvs = dictionary.Select(kvp => string.Format("\"{0}\":\"{1}\"", kvp.Key, string.Join(",", kvp.Value)));
            return string.Concat("{", string.Join(",", kvs), "}");
        }
        public Dictionary<string, string> FromJsonToDictionary(string json)
        {
            string[] keyValueArray = json.Replace("{", string.Empty).Replace("}", string.Empty).Replace("\"", string.Empty).Split(',');
            return keyValueArray.ToDictionary(item => item.Split(':')[0], item => item.Split(':')[1]);
        }
 
    
    