C#格式化JSON字符串

private string ConvertStringToJson(string str)
{
   //格式化json字符串
   JsonSerializer serializer = new JsonSerializer();
   TextReader tr = new StringReader(str);
   JsonTextReader jtr = new JsonTextReader(tr);
   object obj = serializer.Deserialize(jtr);
   if (obj != null)
   {
       StringWriter textWriter = new StringWriter();
       JsonTextWriter jsonWriter = new JsonTextWriter(textWriter)
       {
           Formatting = Formatting.Indented,
           Indentation = 4,
           IndentChar = ' '
       };
       serializer.Serialize(jsonWriter, obj);
       return textWriter.ToString();
   }
   else
   {
       return str;
   }         
}