var str = "05/12/2020";
if (!DateTime.TryParseExact(str, "dd/MM/yyyy", System.Globalization.CultureInfo.InvariantCulture, System.Globalization.DateTimeStyles.None, out var parsedDateTime))
{
throw new Exception("ouch! no pude parsear la fecha");
}
Console.WriteLine(parsedDateTime.ToString("yyyddMM"));