Boolean chain

Leap
Leap in C# 
public static bool IsLeapYear(int year)
{
    return year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
}

The first boolean expression uses the remainder operator to check if the year is evenly divided by 4.

  • If the year is not evenly divisible by 4, then the chain will "short circuit" due to the next operator being a logical AND (&&), and will return false.
  • If the year is evenly divisible by 4, then the year is checked to not be evenly divisible by 100.
  • If the year is not evenly divisible by 100, then the expression is true and the chain will "short-circuit" to return true, since the next operator is a logical OR (||).
  • If the year is evenly divisible by 100, then the expression is false, and the returned value from the chain will be if the year is evenly divisible by 400.
year year % 4 == 0 year % 100 != 0 year % 400 == 0 is leap year
2020 true true not evaluated true
2019 false not evaluated not evaluated false
2000 true false true true
1900 true false false false

The chain of boolean expressions is efficient, as it proceeds from testing the most likely to least likely conditions.

Shortening

When the body of a function is a single expression, the function can be implemented as an expression-bodied member, like so

public static bool IsLeapYear(int year) =>
    year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);

or

public static bool IsLeapYear(int year) => year % 4 == 0 && (year % 100 != 0 || year % 400 == 0);
4th Sep 2024 · Found it useful?

Other Approaches to Leap in C#

Other ways our community solved this exercise
public static bool IsLeapYear(int year)
{
    return year % 100 == 0 ? year % 400 == 0 : year % 4 == 0;
}
Ternary operator

Use a ternary operator of boolean expressions. 

return (year % 4, year % 100, year % 400) switch
{
    (_, 0, 0) => true,
    (_, 0, _) => false,
    (0, _, _) => true,
    _ => false,
};
switch on a tuple

Use a switch on a tuple made from boolean expressions. 

public static bool IsLeapYear(int year)
{
    return (new DateTime(year, 2, 28)).AddDays(1.0).Day == 29;
}
DateTime addition

Use DateTime addition. 

public static bool IsLeapYear(int year)
{
    return DateTime.IsLeapYear(year);
}
Built-in method

Use the built-in method.