using System;
using System.Linq;
 
public class Test
{
	public static void Main()
	{
        var listStr = new[] {
        	"20170822_10", "20170821_1", "20170823_4", "20170821_10",
        	"20170822_11", "20170822_5", "20170822_2", "20170821_3",
        	"20170823_6", "20170823_21", "20170823_20", "20170823_2"
        };
        var sorted = listStr
            .OrderBy(s => s.Split('_')[0])
            .ThenBy(s => int.Parse(s.Split('_')[1]));
        foreach (var s in sorted) {
        	Console.WriteLine(s);
        }
	}
}

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