Append TimeStamp to a File Name C#

DateTime.Now.ToString("yyyyMMddHHmmssfff")

or string.Format

string.Format("{0:yyyy-MM-dd_hh-mm-ss-tt}",DateTime.Now);


There are following custom format specifiers y (year), M (month), d (day), h (hour 12), H (hour 24), m (minute), s (second), f (second fraction), F (second fraction, trailing zeroes are trimmed), t (P.M or A.M) and z (time zone).


With Extension Method

Usage:
string result = "myfile.txt".AppendTimeStamp();
//test20160604234625642.txt

public static class MyExtensions
{
    public static string AppendTimeStamp(this string fileName)
    {
        return string.Concat(
            Path.GetFileNameWithoutExtension(fileName),
            DateTime.Now.ToString("yyyyMMddHHmmssfff"),
            Path.GetExtension(fileName)
            );
    }
}