How to safely convert a string containing escaped JSON to valid JSON?

This is basically JSON encoded as a JSON string - after doctoring the end of your string very slightly, as per comments. It's not too hard to handle that in Json.NET, using JToken.Parse to effectively unescape first, then parsing the result:

using System;
using System.IO;
using Newtonsoft.Json.Linq;

class Program
{
    static void Main(string[] args)
    {
        string text = File.ReadAllText("test.json");
        JToken token = JToken.Parse(text);
        JObject json = JObject.Parse((string) token);
        Console.WriteLine(json);
    }
}

Output:

{
  "SomeResponse": {
    "FIrstAttribute": 8,
    "SecondAttribute": "On",
    "ThirdAttribute": {
      "Id": 2,
      "FirstName": "Okkie",
      "Name": "Bokkie",
      "Street": "",
      "StreetNumber": null,
      "PostCode": "",
      "City": "",
      "Country": ""
    }
  }
}

That should be fine even with data containing backslashes, as I'd expect the backslashes to be encoded once again - but it would be worth double-checking that.

By using Newtonsoft.Json, here is an example:

String json="{\"SomeResponse\":{\"FIrstAttribute\":8,\"SecondAttribute\":\"On\",\"ThirdAttribute\":{\"Id\":2,\"FirstName\":\"Okkie\",\"Name\":\"Bokkie\",\"Street\":\"\",\"StreetNumber\":null,\"PostCode\":\"\",\"City\":\"\",\"Country\":\"}}}";     

dynamic result = JsonConvert.DeserializeObject(json);