GroupJoin
Learn one-to-many joins with GroupJoin.
Hierarchy with GroupJoin
GroupJoin is the grouping cousin of Join. It pairs each outer item with the sequence of matching inner items; even outer items without a match stay in the result. This is ideal for one-to-many relations.
Example: categories.GroupJoin(products, c => c.Id, p => p.CategoryId, (c, prods) => ...). Each category arrives with its product sequence; a category with no products gets an empty sequence, not dropped.
When to use GroupJoin
GroupJoin suits listing all products of a category, all students of a class, or all orders of a customer. When you do not want unmatched outer items dropped, choose it over Join.
The result selector receives the inner sequence as IEnumerable<T>; you can call prods.Count() or iterate with foreach. It resembles a SQL left outer join.
Exercise Group categories with products
GroupJoin the categories and products lists below on Id/CategoryId. For each category print the category name and product count with Console.WriteLine.
Show example solution
" + prods.Count()); then iterate with foreach.
Example solution:
using System;
using System.Collections.Generic;
using System.Linq;
class Category
{
public int Id { get; set; }
public string Name { get; set; }
}
class Product
{
public string Name { get; set; }
public int CategoryId { get; set; }
}
class Program
{
static void Main()
{
var categories = new List<Category>
{
new Category { Id = 1, Name = "Drink" },
new Category { Id = 2, Name = "Dessert" }
};
var products = new List<Product>
{
new Product { Name = "Tea", CategoryId = 1 },
new Product { Name = "Coffee", CategoryId = 1 },
new Product { Name = "Kunefe", CategoryId = 2 }
};
var result = categories.GroupJoin(products, c => c.Id, p => p.CategoryId, (c, prods) => c.Name + ": " + prods.Count());
foreach (var r in result)
{
Console.WriteLine(r);
}
}
}Test yourself
-
Match the method to its behavior
- Join
- Only matching pairs
- GroupJoin
- Outer + inner seq
- GroupBy
- Group one sequence
- Zip
- Index-based pair
-
Complete the GroupJoin call
var result = categories.Group____(products, c => c.Id, p => p.CategoryId, (c, prods) => c.Name);Show answer
var result = categories.GroupJoin(products, c => c.Id, p => p.CategoryId, (c, prods) => c.Name); -
What does GroupJoin produce?
- Only matching pairs
- A single value
- Outer item + inner sequence
- A sorted sequence
Show answer
Outer item + inner sequence — Correct. GroupJoin pairs each outer item with its matching inner sequence.