JSON formatter in C#?

You could also use the Newtonsoft.Json library for this and call SerializeObject with the Formatting.Indented enum -

var x = JsonConvert.SerializeObject(jsonString, Formatting.Indented);

Documentation: Serialize an Object

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Update -

Just tried it again. Pretty sure this used to work - perhaps it changed in a subsequent version or perhaps i'm just imagining things. Anyway, as per the comments below, it doesn't quite work as expected. These do, however (just tested in linqpad). The first one is from the comments, the 2nd one is an example i found elsewhere in SO -

void Main()
{
    //Example 1
    var t = "{\"x\":57,\"y\":57.0,\"z\":\"Yes\"}";
    var obj = Newtonsoft.Json.JsonConvert.DeserializeObject(t); 
    var f = Newtonsoft.Json.JsonConvert.SerializeObject(obj, Newtonsoft.Json.Formatting.Indented);
    Console.WriteLine(f);

    //Example 2
    JToken jt = JToken.Parse(t);
    string formatted = jt.ToString(Newtonsoft.Json.Formatting.Indented);
    Console.WriteLine(formatted);

    //Example 2 in one line -
    Console.WriteLine(JToken.Parse(t).ToString(Newtonsoft.Json.Formatting.Indented));
}

I updated the old version, now it should support unquoted values such as integers and booleans.

I refactored the previous version and got the final version: The code is shorter and cleaner. Only require one extension method. The most important: fixed some bugs.

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);
        }
    }
}