using System;

using System.Collections.Generic;

using System.Globalization;

using System.Linq;

public class Program {

  private static string ConvertToStandard(string value) {

    if (DateTime.TryParseExact(value,

          new string[] { "yyyy/M/d", "d/M/yyyy", "M-d-yyyy", "yyyyMMdd"},

          CultureInfo.InvariantCulture,

          DateTimeStyles.AssumeLocal,

          out var date))

      return date.ToString("yyyyMMdd");

    else

      return value;

  } 

  public static List<string> TransformDateFormat(List<string> dates) {

    if (dates == null)

      throw new ArgumentNullException(nameof(dates));   

    return dates.Select(s => ConvertToStandard(s)).ToList();

  } 

  public static void Main() {

    List<string> original = new List<string>() {

      "2010/02/20", "19/12/2016", "11-18-2012", "20130720",

    };

    Console.Write(String.Join(", ", TransformDateFormat(original)));  

  }

}