サンプルコード

indentJson.cs

using System.IO;
using System.Text;
using System.Runtime.Serialization.Json;
using System.Xml;

class Example
{
    public static void Main(string[] args)
    {
        string json = "[{\"color\":\"red\",\"value\":\"#f00\"}," +
                       "{\"color\":\"green\",\"value\":\"#0f0\"}," +
                       "{\"color\":\"blue\",\"value\":\"#00f\"}," +
                       "{\"color\":\"cyan\",\"value\":\"#0ff\"}," +
                       "{\"color\":\"magenta\",\"value\":\"#f0f\"}," +
                       "{\"color\":\"yellow\",\"value\":\"#ff0\"}," +
                       "{\"color\":\"black\",\"value\":\"#000\"}]";
        string indented = convertToIndentedJson(json);
        System.Console.WriteLine("in:");
        System.Console.WriteLine(json);
        System.Console.WriteLine();
        System.Console.WriteLine("out:");
        System.Console.WriteLine(indented);
    }

    private static string convertToIndentedJson(string json)
    {
        byte[] buffer = Encoding.UTF8.GetBytes(json);
        using (MemoryStream stream = new MemoryStream())
        using (XmlDictionaryWriter writer = JsonReaderWriterFactory.CreateJsonWriter(stream, Encoding.UTF8, true, true))
        using (XmlDictionaryReader reader = JsonReaderWriterFactory.CreateJsonReader(buffer, XmlDictionaryReaderQuotas.Max))
        {
            writer.WriteNode(reader, true);
            writer.Flush();
            return Encoding.UTF8.GetString(stream.ToArray());
        }
    }
}

コンパイル用のバッチファイル

indentJson.cs.bat

@setlocal
@pushd "%~dp0"
@for %%1 in ("%~n0") do @set "BASENAME=%%~n1"
@for /d %%1 in (%SystemRoot%\Microsoft.NET\Framework\v*)   do @if exist "%%~1\csc.exe" set "CSC=%%~1\csc.exe"
@for /d %%1 in (%SystemRoot%\Microsoft.NET\Framework64\v*) do @if exist "%%~1\csc.exe" set "CSC64=%%~1\csc.exe"
"%CSC%"   /target:exe /platform:x86 /optimize+ /warnaserror+ /out:"%BASENAME%.exe"   "%~n0"
"%CSC64%" /target:exe /platform:x64 /optimize+ /warnaserror+ /out:"%BASENAME%64.exe" "%~n0"
@popd
@endlocal

実行結果

>indentJson.exe
in:
[{"color":"red","value":"#f00"},{"color":"green","value":"#0f0"},{"color":"blue","value":"#00f"},{"color":"cyan","value":"#0ff"},{"color":"magenta","value":"#f0f"},{"color":"yellow","value":"#ff0"},{"color":"black","value":"#000"}]

out:
[
  {
    "color": "red",
    "value": "#f00"
  },
  {
    "color": "green",
    "value": "#0f0"
  },
  {
    "color": "blue",
    "value": "#00f"
  },
  {
    "color": "cyan",
    "value": "#0ff"
  },
  {
    "color": "magenta",
    "value": "#f0f"
  },
  {
    "color": "yellow",
    "value": "#ff0"
  },
  {
    "color": "black",
    "value": "#000"
  }
]

ポイント

CreateJsonWriter()の第4引数がindentなので、こいつへtrueを指定すること