How to convert List<CustomType> to Dictionary<int, List<CustomType>>

Question

Lets say that we have this custom type:

public class Holiday
{
    public Guid Id { get; } = Guid.NewGuid();

    public string holidayName { get; set; };
    public DateTime fromDate { get; set; };
    public DateTime toDate { get; set; };
    public int year { get; set; };
}

I need to convert list of Holidays (List<Holiday>) to an dictionary (Dictionary<int, List<Holiday>>). The keys are distinct years and the values are Lists of Holidays those belong to year.

I have tried to do that by looking at this answer/Question but without success.

Answer 1

You can do this using the GroupBy method in LINQ, which

Groups the elements of a sequence according to a specified key selector function

In your case, the key would be year so the GroupBy syntax would look as follows:

List<Holiday> holidays = new List<Holiday>
{
    new Holiday
    {
        year = 1999,
        holidayName = "Easter"
    },
    new Holiday
    {
        year = 1999,
        holidayName = "Christmas"
    },
    new Holiday
    {
        year = 2000,
        holidayName = "Christmas"
    }
};

Dictionary<int, List<Holiday>> holidaysByYear = holidays
    .GroupBy(h => h.year)
    .ToDictionary(h => h.Key, h => h.ToList());

foreach (KeyValuePair<int, List<Holiday>> holidaysInYear in holidaysByYear)
{
    Console.WriteLine($"Holidays in {holidaysInYear.Key}");
    foreach (Holiday holiday in holidaysInYear.Value)
    {
        Console.WriteLine(holiday.holidayName);
    }
}

Which produces output as: