Simple JSON pretty printing in C#, as a string extension.

prettyprint.cs

public static class StringExtensions {

    //Returns the string, if JSON, as a pretty printed string.
    public static string PrettyPrint(this string d)
    {
        IList<string> actionLines = d.SplitAndKeepDelimiters('{', '}', ',');
        string s = "";

        //Write per line, leave space between each action.
        int tabDepth = 0;
        for (int i = 0; i < actionLines.Count; i++)
        {
            //Modifying tab depth for removal.
            if (actionLines[i] == "}") { tabDepth--; }

            //Adding tab depth.
            s += new string(' ', tabDepth * 4);

            //Adding text contents.
            s += actionLines[i];

            //Checking if a line ending is required.
            if (i != actionLines.Count - 1 && actionLines[i + 1] != ",")
            {
                s += "\r\n";
            }

            //Modifying tab depth for addition.
            if (actionLines[i] == "{") { tabDepth++; }
        }

        return s;
    }
    
    //You also need this, but you can replace this with Regex if you want. Just less performant.
    public static IList<string> SplitAndKeepDelimiters(this string s, params char[] delimiters)
        {
            var parts = new List<string>();
            if (!string.IsNullOrEmpty(s))
            {
                int iFirst = 0;
                do
                {
                    int iLast = s.IndexOfAny(delimiters, iFirst);
                    if (iLast >= 0)
                    {
                        if (iLast > iFirst)
                            parts.Add(s.Substring(iFirst, iLast - iFirst)); //part before the delimiter
                        parts.Add(new string(s[iLast], 1));//the delimiter
                        iFirst = iLast + 1;
                        continue;
                    }

                    //No delimiters were found, but at least one character remains. Add the rest and stop.
                    parts.Add(s.Substring(iFirst, s.Length - iFirst));
                    break;

                } while (iFirst < s.Length);
            }

            return parts;
        }
    
}