using System;

using Newtonsoft.Json;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main(string[] args)

    {

        string json = @"[{ ""_id"":""xxx"", ""firstName"":""Brayann""}, { ""_id"":""yyy"", ""firstName"":""Peter""}]";

        var result = JsonConvert.DeserializeObject<List<RootListObject>>(json)

                                 .ToDictionary(x=> x.id, y=> y.firstName);

        Console.WriteLine(result["xxx"]);

    }

​

    public class RootListObject

    {

        [JsonProperty(PropertyName ="_id")]

        public string id { get; set; }

        public string firstName { get; set; }

    }

}