JsonHelper.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

// all credit goes to https://stackoverflow.com/a/6237866
// Authors:
//      https://stackoverflow.com/users/491243/john-woo
//      https://stackoverflow.com/users/267587/peter-long

class JsonHelper
{
    private const string INDENT_STRING = "    ";
    public static string FormatJson(string str)
    {
        var indent = 0;
        var quoted = false;
        var sb = new StringBuilder();
        for (var i = 0; i < str.Length; i++)
        {
            var ch = str[i];
            switch (ch)
            {
                case '{':
                case '[':
                    sb.Append(ch);
                    if (!quoted)
                    {
                        sb.AppendLine();
                        Enumerable.Range(0, ++indent).ForEach(item => sb.Append(INDENT_STRING));
                    }
                    break;
                case '}':
                case ']':
                    if (!quoted)
                    {
                        sb.AppendLine();
                        Enumerable.Range(0, --indent).ForEach(item => sb.Append(INDENT_STRING));
                    }
                    sb.Append(ch);
                    break;
                case '"':
                    sb.Append(ch);
                    bool escaped = false;
                    var index = i;
                    while (index > 0 && str[--index] == '\\')
                        escaped = !escaped;
                    if (!escaped)
                        quoted = !quoted;
                    break;
                case ',':
                    sb.Append(ch);
                    if (!quoted)
                    {
                        sb.AppendLine();
                        Enumerable.Range(0, indent).ForEach(item => sb.Append(INDENT_STRING));
                    }
                    break;
                case ':':
                    sb.Append(ch);
                    if (!quoted)
                        sb.Append(" ");
                    break;
                default:
                    sb.Append(ch);
                    break;
            }
        }
        return sb.ToString();
    }
}

static class Extensions
{
    public static void ForEach<T>(this IEnumerable<T> ie, Action<T> action)
    {
        foreach (var i in ie)
        {
            action(i);
        }
    }
}