Checking If A Year Is A Leap Year in C# & .NET

A problem you may run into is checking whether a particular year is a leap year.

Many of us know a leap year is one that is divisible by 4.

What we might not know is that there are two other rules:

1. If the year is divisible by 100, it is not a leap year
2. If the year is divisible by 400, it is a leap year

The algorithm is as follows:

bool IsLeapYear(int year)
{
	return year % 400 == 0 || (year % 4 == 0 && year % 100 != 0);
}

You can go a step further and make this an extension method for DateTime and DateOnly.

Like so:

public static class DateTimeExtensions
{
  public static bool IsLeapYear(this DateTime datetTime)
  {
  	return datetTime.Year % 400 == 0 || (datetTime.Year % 4 == 0 && datetTime.Year % 100 != 0);
  }
  public static bool IsLeapYear(this DateOnly dateOnly)
  {
  	return dateOnly.ToDateTime(TimeOnly.MinValue).IsLeapYear();
  }
}

.NET already has similar functionality- the static DateTime.IsLeapYear method.

Console.WriteLine(DateTime.IsLeapYear(2024));

TLDR

Rather than determining a leap year for yourself, you can use the DateTime.IsLeapYear method.

Happy hacking!