Csharp/CSharp Tutorial/LINQ/GroupBy

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GroupBy partitions a list of numbers by their remainder when divided by 5.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
public class MainClass {
    public static void Main() {
        int[] numbers = { 5, 4, 1, 3, 9 };
        var numberGroups =
            from n in numbers
            group n by n % 5 into g
            select new { Remainder = g.Key, Numbers = g };
        foreach (var g in numberGroups) {
            Console.WriteLine("Numbers with a remainder of {0} when divided by 5:", g.Remainder);
            foreach (var n in g.Numbers) {
                Console.WriteLine(n);
            }
        }
    }
}

Use Group By

using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;
using System.Linq;
using System.Reflection;
public class MainClass{
   public static void Main(){
            var query = from m in typeof(double).GetMethods()
                        select m.Name;
            var q = from m in typeof(double).GetMethods()
                    group m by m.Name into gb
                    select new {Name = gb.Key, Overloads = gb.Count()};
   }
}

uses group by to partition a list of each customer"s orders, first by year, and then by month.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
public class MainClass {
    public static void Main() {
        List<Customer> customers = GetCustomerList();
        var customerOrderGroups =
            from c in customers
            select
                new {
                    c.rupanyName,
                    YearGroups =
                        from o in c.Orders
                        group o by o.OrderDate.Year into yg
                        select
                            new {
                                Year = yg.Key,
                                MonthGroups =
                                    from o in yg
                                    group o by o.OrderDate.Month into mg
                                    select new { Month = mg.Key, Orders = mg }
                            }
                };
        Console.Write(customerOrderGroups);
    }
    static List<Customer> GetCustomerList() {
        List<Product> empTree = new List<Product>();
        empTree.Add(new Product { ProductName = "A", Category = "O", UnitPrice = 12, UnitsInStock = 5, Total = 36, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "B", Category = "O", UnitPrice = 2, UnitsInStock = 4, Total = 35, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "C", Category = "O", UnitPrice = 112, UnitsInStock = 3, Total = 34, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "D", Category = "O", UnitPrice = 112, UnitsInStock = 0, Total = 33, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "E", Category = "O", UnitPrice = 1112, UnitsInStock = 2, Total = 32, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "F", Category = "O", UnitPrice = 11112, UnitsInStock = 0, Total = 31, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        List<Customer> l = new List<Customer>();
        l.Add(new Customer { CompanyName = "A", Region = "R1", UnitsInStock = 1, Orders = empTree, CustomerId = 0 });
        l.Add(new Customer { CompanyName = "B", Region = "R2", UnitsInStock = 2, Orders = empTree, CustomerId = 1 });
        l.Add(new Customer { CompanyName = "C", Region = "R3", UnitsInStock = 3, Orders = empTree, CustomerId = 2 });
        l.Add(new Customer { CompanyName = "D", Region = "R4", UnitsInStock = 4, Orders = empTree, CustomerId = 3 });
        l.Add(new Customer { CompanyName = "E", Region = "R5", UnitsInStock = 5, Orders = empTree, CustomerId = 4 });
        return l;
    }
}
class Customer : IComparable<Customer> {
    public string CompanyName { get; set; }
    public string Region { get; set; }
    public List<Product> Orders { get; set; }
    public int UnitsInStock { get; set; }
    public int CustomerId { get; set; }
    public override string ToString() {
        return String.Format("Id: {0}, Name: {1}, Region: {3}", this.CustomerId, this.rupanyName, this.Region);
    }
    int IComparable<Customer>.rupareTo(Customer other) {
        if (other == null)
            return 1;
        if (this.CustomerId > other.CustomerId)
            return 1;
        if (this.CustomerId < other.CustomerId)
            return -1;
        return 0;
    }
}
class Product : IComparable<Product> {
    public string ProductName { get; set; }
    public string Category { get; set; }
    public int UnitPrice { get; set; }
    public int UnitsInStock { get; set; }
    public int Total { get; set; }
    public DateTime OrderDate { get; set; }
    public int Id { get; set; }
    public override string ToString() {
        return String.Format("Id: {0}, Name: {1} , Category: {3}", this.Id, this.ProductName, this.Category);
    }
    int IComparable<Product>.rupareTo(Product other) {
        if (other == null)
            return 1;
        if (this.Id > other.Id)
            return 1;
        if (this.Id < other.Id)
            return -1;
        return 0;
    }
}

uses group by to partition a list of products by category.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
public class MainClass {
    public static void Main() {
        List<Product> products = GetProductList();
        var orderGroups =
            from p in products
            group p by p.Category into g
            select new { Category = g.Key, Products = g };
        foreach (var s in orderGroups) {
            Console.Write(s.Category);
        }
    }
    static List<Product> GetProductList() {
        List<Product> empTree = new List<Product>();
        empTree.Add(new Product { ProductName = "A", Category = "O", UnitPrice = 12, UnitsInStock = 5, Total = 36, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "B", Category = "O", UnitPrice = 2, UnitsInStock = 4, Total = 35, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "C", Category = "O", UnitPrice = 112, UnitsInStock = 3, Total = 34, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "D", Category = "O", UnitPrice = 112, UnitsInStock = 0, Total = 33, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "E", Category = "O", UnitPrice = 1112, UnitsInStock = 2, Total = 32, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        empTree.Add(new Product { ProductName = "F", Category = "O", UnitPrice = 11112, UnitsInStock = 0, Total = 31, OrderDate = new DateTime(2005, 1, 1), Id = 1 });
        return empTree;
    }
}
class Product : IComparable<Product> {
    public string ProductName { get; set; }
    public string Category { get; set; }
    public int UnitPrice { get; set; }
    public int UnitsInStock { get; set; }
    public int Total { get; set; }
    public DateTime OrderDate { get; set; }
    public int Id { get; set; }
    public override string ToString() {
        return String.Format("Id: {0}, Name: {1} , Category: {3}", this.Id, this.ProductName, this.Category);
    }
    int IComparable<Product>.rupareTo(Product other) {
        if (other == null)
            return 1;
        if (this.Id > other.Id)
            return 1;
        if (this.Id < other.Id)
            return -1;
        return 0;
    }
}

uses GroupBy to partition trimmed elements of an array using a custom comparer

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
public class MainClass {
    public static void Main() {
        string[] anagrams = { "from ", " salt", " earn ", " last ", " near ", " form " };
        var orderGroups = anagrams.GroupBy(
                    w => w.Trim(),
                    a => a.ToUpper(),
                    new AnagramEqualityComparer()
                    );
        foreach (var v in orderGroups) {
            Console.WriteLine(v);
        }
    }
    public class AnagramEqualityComparer : IEqualityComparer<string> {
        public bool Equals(string x, string y) {
            return getCanonicalString(x) == getCanonicalString(y);
        }
        public int GetHashCode(string obj) {
            return getCanonicalString(obj).GetHashCode();
        }
        private string getCanonicalString(string word) {
            char[] wordChars = word.ToCharArray();
            Array.Sort<char>(wordChars);
            return new string(wordChars);
        }
    }
}
Источник — «http://nfex.ru/index.php?title=Csharp/CSharp_Tutorial/LINQ/GroupBy&oldid=6105»