C#, DateTime.TryParseExact, parse datetime

datetime_tryParse.cs

private static DateTime? ParseDateTime(string str, string fmt)
{
    DateTime dtParsed;
    if (DateTime.TryParseExact(str, fmt, null, System.Globalization.DateTimeStyles.None, out dtParsed))
    {
        return dtParsed;
    }
    else
    {
        return null;
    }
}

//---------------------
// apply
string date = "20200408";
string time = "1716";
DateTime? dtParsed = ParseDateTime($"{date}{time}", "yyyyMMddHHmm");
Console.WriteLine($"{dtParsed}");

TryParseTwDate.cs

/// <summary>
/// 解析民國年日期。只支援三種格式：九碼 YYY/MM/DD、YYY-MM-DD，或七碼 YYYMMDD。
/// </summary>
public static DateTime? ParseTwDate(string str)
{
  if (String.IsNullOrWhiteSpace(str))
    return null;

  string twdate9 = null;
  if (Regex.IsMatch(str, @"^\d{3}\d{2}\d{2}$")) // 七碼:YYYMMDD 
    twdate9 = str.Substring(0, 3) + "/" + str.Substring(3, 2) + "/" + str.Substring(5, 2);
  else if (Regex.IsMatch(str, @"^\d{3}-\d{2}-\d{2}$")) // 九碼:YYY-MM-DD 
    twdate9 = str.Replace("-", "/");
  else if (Regex.IsMatch(str, @"^\d{3}\/\d{2}\/\d{2}$")) // 九碼:YYY/MM/DD 
    twdate9 = str;
  else
    throw new ApplicationException($"民國年日期格式錯誤！輸入字串：{str}。");

  var twCulture = new System.Globalization.CultureInfo("zh-TW") { 
    DateTimeFormat = { 
      Calendar = new TaiwanCalendar() 
    } 
  };

  DateTime dt;
  if (!DateTime.TryParseExact(twdate9, "yyy\\/MM\\/dd", twCulture, System.Globalization.DateTimeStyles.None, out dt))
    throw new ApplicationException($"民國年日期格式錯誤！輸入字串：{str}。");

  return new Nullable<DateTime>(dt);
}

/// <summary>
/// 解析民國年日期。只支援三種格式：九碼 YYY/MM/DD、YYY-MM-DD，或七碼 YYYMMDD。
/// </summary>
public static bool TryParseTwDate(string str, out DateTime? dt)
{
  try
  {
    dt = ParseTwDate(str);
    return true;
  }
  catch
  {
    dt = null;
    return false;
  }
}