Leap Year Calculator Second Attempt

FirstImpl.cs

public static bool IsLeapYear(int year)
{
    return true;
}

FirstTest.cs

[Test]
public void TestThat_IsLeapYear_ShouldReturnTrue_WhenTheYearIsDivisibleByFour()
{
    const int year = 1984;

    var isLeapYear = LeapYearCalculator.IsLeapYear(year);

    Assert.IsTrue(isLeapYear);
}

FourthImpl.cs

public static bool IsLeapYear(int year)
{
    if (year % 100 == 0)
    {
        return false;
    }

    return year % 4 == 0;
}

FourthTest.cs

[Test]
public void TestThat_IsLeapYear_ShouldReturnFalse_WhenTheYearIsDivisibleByOneHundred()
{
    const int year = 1700;

    var isLeapYear = LeapYearCalculator.IsLeapYear(year);

    Assert.IsFalse(isLeapYear);
}

Refactor.cs

public static bool IsLeapYear(int year)
{
    if (IsCentury(year))
    {
        return year % 400 == 0;
    }

    return year % 4 == 0;
}

private static bool IsCentury(int year)
{
    return year % 100 == 0;
}

SecondImpl.cs

public static bool IsLeapYear(int year)
{
    return year % 4 == 0;
}

SecondTest.cs

[Test]
public void TestThat_IsLeapYear_ShouldReturnFalse_WhenTheYearIsNotDivisibleByFour()
{
    const int year = 1985;

    var isLeapYear = LeapYearCalculator.IsLeapYear(year);

    Assert.IsFalse(isLeapYear);
}

ThirdImpl.cs

public static bool IsLeapYear(int year)
{
    if (year % 100 == 0)
    {
        return year % 400 == 0;
    }

    return year % 4 == 0;
}

ThirdTest.cs

[Test]
public void TestThat_IsLeapYear_ShouldReturnTrue_WhenTheYearIsDivisibleByFourHundred()
{
    // Whoops, this doesn't fail.

    const int year = 1600;

    var isLeapYear = LeapYearCalculator.IsLeapYear(year);

    Assert.IsTrue(isLeapYear);
}