I like the simple solution.

DateTime minDate = DateTime.MaxValue;
DateTime maxDate = DateTime.MinValue;
foreach (string dateString in StringDates)
{
    DateTime date = DateTime.Parse(dateString);
    if (date < minDate)
        minDate = date;
    if (date > maxDate)
        maxDate = date;
}

Convert them to DateTime using ParseExact (or TryParseExact) and then use Linq to get the Min and Max:

List<DateTime> dates = StringDates
   .Select(x => DateTime.ParseExact(x, "dd.MM.yyyy", null))
   .ToList();
DateTime minDate = dates.Min();
DateTime maxDate = dates.Max();

Note that in your example the list was already sorted in ascending order. If you can guarantee that this will always be the case and you want better performance, you could just take the first and last element which would be O(1) instead of O(n). But the above is safer so even if your listed probably will always be sorted, you probably shouldn t make this optimization unless you actually need it, just in case one day the list doesn t come in sorted order.

use linq!:

    var list = new List<DateTime>();
    list.Add(new DateTime(2010, 1, 1));
    list.Add(new DateTime(2008, 1, 1));
    list.Add(new DateTime(2009, 1, 1));
    Console.WriteLine(list.Max(date => date));
    Console.WriteLine(list.Min(date => date));

Via Linq, you can do:

from row in StringDates
group row by true into r 
select new { 
    min = r.Min(z => z), 
    max = r.Max(z => z) 
}

This is similar to @Jeffrey s answer, but instead of parsing each date, it first finds the min and max dates comparing its string values, and then it parses the values at the end.

// This method handles the date comparisons
private int WeirdComparer(string strDate1, string strDate2)
{
    int res = string.Compare(strDate1, 6, strDate2, 6, 4);
    if (res == 0)
        res = string.Compare(strDate1, 3, strDate2, 3, 2);
    if (res == 0)
        res = string.Compare(strDate1, 0, strDate2, 0, 2);
    return res;
}

public void FindMinAndMaxDates(IList<string> strDates, out DateTime minDate, out DateTime maxDate)
{
    string min = "99.99.9999";
    string max = "00.00.0000";
    foreach (string strDate in strDates)
    {
        if (WeirdComparer(strDate, min) < 0)
            min = strDate;
        if (WeirdComparer(strDate, max) > 0)
            max = strDate;
    }
    minDate = DateTime.ParseExact(min, "dd.MM.yyyy", null);
    maxDate = DateTime.ParseExact(max, "dd.MM.yyyy", null);
}

I know this isn t a direct answer to your question, but could help others if they come here looking for something similar.

I ran across this issue today while trying to find just the max date from a list of objects. Sometimes there won t be a value in the date for the objects so I had to figure out how to use a try parse with my linq.

From a combination of what Mark used here I came up with this to solve my problem

        List<string> stringDates = new List<string>();
        stringDates.Add("Not a date");
        stringDates.Add("2015-01-01");
        stringDates.Add("2015-10-10");
        stringDates.Add("2015-9-10");
        stringDates.Add("123");

        DateTime sapInvoiceDate;
        DateTime tempDate = DateTime.MinValue;
        sapInvoiceDate = stringDates.Select(x => DateTime.TryParse(x, out tempDate)).Select(x => tempDate).Max();