# C# Filename, DateTime

A filename can store the date and time. This clearly identifies the file in a unique way. There are many ways of accomplishing this. Some are clearer to read and easier to modify than others. We include the date, or the date and the time.

Dates and required filenames
You can use DateTime formatting for filenames.

Date:              11/18/2008 5:04:44 PM
Required filename: text-2008-11-18_05-04-44-PM.bin

Date:              11/23/2004 12:05:47 AM
Required filename: text-2004-11-23-12_05-47-AM.bin

## Example

We use the current date and time as the name for a file on the file system. We call the string.Format method, and combine it with DateTime.Now, for a method that outputs the correct string based on the date and time.

DateTime.Nowstring.Format
Program that uses date as filename: C#

using System;
using System.IO;

class Program
{
static void Main()
{
//
// Write file containing the date with BIN extension
//
string n = string.Format("text-{0:yyyy-MM-dd_hh-mm-ss-tt}.bin",
DateTime.Now);
File.WriteAllText(n, "aaa");
}
}

Description of format string
"text-{0:yyyy-MM-dd_hh-mm-ss-tt}.bin"

text-      The first part of the output required
{0:        Indicates that this is a string placeholder
The zero indicates the index of the parameters inserted here
yyyy-      Prints the year in four digits followed by a dash
This has a "year 10000" problem
MM-        Prints the month in two digits
dd_        Prints the day in two digits followed by an underscore
hh-        Prints the hour in two digits
mm-        Prints the minute, also in two digits
ss-        As expected, it prints the seconds
tt         Prints AM or PM depending on the time of day

## Environment.SpecialFolder

You can use this code with Environment.SpecialFolder. You can encapsulate the logic for this in a property accessor. In other words, you can use a property to easily get the complete filename.

Property
Program that filename property: C#

using System;
using System.IO;

class Program
{
static string SpecialFileName
{
get
{
// A
return string.Format("{0}{1}text-{2:yyyy-MM-dd_hh-mm-ss-tt}.bin",
// B
Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments),
// C
Path.DirectorySeparatorChar,
// D
DateTime.Now);
}
}

static void Main()
{
Console.WriteLine(SpecialFileName);
}
}

Output

C:\Users\Sam\Documents\text-2008-11-18_05-23-13-PM.bin

This example uses a format string in part A. This format string has three placeholders: first, the directory name we get from SpecialFolder, then the separator char, and finally the specially formatted DateTime.

Also:The code specifies that the file name starts with text and ends with ".bin".

Next:It gets the My Documents folder path. This line uses the SpecialFolder.MyDocuments enum value to get the path.

Environment

Finally, it uses Path.DirectorySeparatorChar. On Windows, this is the backslash. You could just use \\ in the string instead. It uses DateTime.Now. As expected, this returns the local time.

## Separators

You can change the string separators by replacing the underscores and dashes with any other character, excluding some special ones. I haven't found many uses for separators other than dashes, underscores, and spaces.

Note:Filenames cannot have some characters.
This is explored further in a separate article.

Reserved Filenames

## Summary

We generated file names containing the current date. This is useful for "data dumps" and log files where daily files are written. Never use random names except when you need temporary files. It is often better to append the date.

Tip:If you have to write files frequently, using the "ss" field can help—this writes seconds.

C#: Time