New LINQ Features in .NET 9

.NET 9 introduces several enhancements to LINQ, providing developers with more powerful and efficient data querying capabilities. Below are the key additions:

1. CountBy

The CountBy method groups elements based on a specified key selector and counts the occurrences within each group. This simplifies the process of determining the frequency of items in a collection without the need for intermediate groupings.

var fruits = new[] { "apple", "banana", "apple", "orange", "banana", "apple" };
var fruitCounts = fruits.CountBy(fruit => fruit);
// Result: { ("apple", 3), ("banana", 2), ("orange", 1) }

2. AggregateBy

The AggregateBy method allows for aggregation over groups defined by a key selector. It streamlines operations such as summing or averaging values within grouped data, eliminating the need for separate grouping and aggregation steps.

var sales = new[]
{
    new { Region = "North", Amount = 100 },
    new { Region = "South", Amount = 150 },
    new { Region = "North", Amount = 200 },
    new { Region = "South", Amount = 50 }
};

var totalSalesByRegion = sales.AggregateBy(
    sale => sale.Region,
    0,
    (total, sale) => total + sale.Amount
);
// Result: { ("North", 300), ("South", 200) }

3. Index

The Index method pairs each element of a sequence with its zero-based index, facilitating scenarios where the position of elements within a collection is relevant.

var colors = new[] { "red", "green", "blue" };
var indexedColors = colors.Index();
// Result: { (0, "red"), (1, "green"), (2, "blue") }

4. ChunkBy

The ChunkBy method partitions a sequence into chunks based on a specified key selector. This is particularly useful when grouping items that share a common attribute.

var salesData = new List<Sale>
{
    new Sale { Date = new DateTime(2023, 1, 1), Amount = 100 },
    new Sale { Date = new DateTime(2023, 1, 2), Amount = 150 },
    new Sale { Date = new DateTime(2023, 2, 1), Amount = 200 },
    // More sales...
};

var monthlyChunks = salesData.ChunkBy(s => s.Date.Month);
foreach (var month in monthlyChunks)
{
    Console.WriteLine($"Month: {month.First().Date.Month}");
    foreach (var sale in month)
    {
        Console.WriteLine($"\tSale Amount: {sale.Amount}");
    }
}

5. MinBy and MaxBy

The MinBy and MaxBy methods retrieve the minimum or maximum element in a sequence based on a specified selector function, simplifying the process of finding elements with the smallest or largest key values.

var employees = new List<Employee>
{
    new Employee { Name = "Alice", Salary = 60000 },
    new Employee { Name = "Bob", Salary = 75000 },
    new Employee { Name = "Charlie", Salary = 50000 },
};

var highestPaid = employees.MaxBy(e => e.Salary);
var lowestPaid = employees.MinBy(e => e.Salary);
Console.WriteLine($"Highest Paid: {highestPaid.Name} - {highestPaid.Salary}");
Console.WriteLine($"Lowest Paid: {lowestPaid.Name} - {lowestPaid.Salary}");

For more detailed information on these and other new features in .NET 9, refer to the official Microsoft documentation: What's new in .NET 9.