DateTime DateTime Format DateTime.Now Environment File Property string.Format

Filename, `DateTime`

A file name can store the date and time. This clearly identifies the file in a unique way. There are many ways of accomplishing this.

Notes, file names

Some methods are clearer to read and easier to modify than others. We include the date, or the date and the time. A format string is provided.

An 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.

Result This method outputs the correct string based on the date and time. We use the string as a file name.

using System;
using System.IO;

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

text-      The first part of the output required
           Files will all start with text-
{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.

Part 1 This format string has 3 placeholders: the directory name we get from SpecialFolder, the separator char, and the DateTime.

Part 2 It gets the MyDocuments folder path. This line uses the SpecialFolder.MyDocuments enum value to get the path.

Part 3 The code uses Path.DirectorySeparatorChar. On Windows, this is the backslash—you could just use "\\" in the string instead.

Part 4 We get the current date and time by accessing the DateTime.Now property.

using System;
using System.IO;

class Program
{
    static string SpecialFileName
    {
        get
        {
            // Part 1: specify the format string.
            return string.Format("{0}{1}text-{2:yyyy-MM-dd_hh-mm-ss-tt}.bin",
                // Part 2: call GetFolderPath.
                Environment.GetFolderPath(Environment.SpecialFolder.MyDocuments),
                // Part 3: specify separator char.
                Path.DirectorySeparatorChar,
                // Part 4: get current time.
                DateTime.Now);
        }
    }

    static void Main()
    {
        Console.WriteLine(SpecialFileName);
    }
}
C:\Users\mount\OneDrive\Documents\text-2024-02-01_04-07-57-PM.bin

Notes, separators

You can change the string separators by replacing the underscores and dashes with any other character, excluding some special ones.

Note File names cannot have some characters. And some words are reserved on Windows—they cannot be file names.

A summary

We generated file names containing the current date. This is useful for "data dumps" and log files where daily files are written. It helps us organize output files.