Join and GroupJoin operations in C Sharp (C#) - Mini Project: Build & Apply

Join and GroupJoin operations

📖 Scenario: You work in a company that manages orders and customers. You have two lists: one with customers and one with orders. You want to connect orders to their customers to see who bought what.

🎯 Goal: Build a C# program that uses Join and GroupJoin operations to link customers with their orders and display the results.

📋 What You'll Learn

Create a list of customers with exact IDs and names

Create a list of orders with exact OrderIDs and CustomerIDs

Use Join to find matching orders and customers

Use GroupJoin to group orders by customer

Print the joined and grouped results exactly as specified

💡 Why This Matters

🌍 Real World

Joining and grouping data is common in business apps to connect related information like customers and their orders.

💼 Career

Understanding Join and GroupJoin helps you work with databases, reports, and data analysis in many programming jobs.

1

Create the customers and orders lists

Create a list called customers with these exact entries: new Customer { CustomerID = 1, Name = "Alice" }, new Customer { CustomerID = 2, Name = "Bob" }, new Customer { CustomerID = 3, Name = "Charlie" }. Also create a list called orders with these exact entries: new Order { OrderID = 101, CustomerID = 1 }, new Order { OrderID = 102, CustomerID = 2 }, new Order { OrderID = 103, CustomerID = 1 }, new Order { OrderID = 104, CustomerID = 3 }.

C Sharp (C#)

using System;
using System.Collections.Generic;

record Customer(int CustomerID, string Name);
record Order(int OrderID, int CustomerID);

class Program
{
    static void Main()
    {
        // Create customers and orders lists
        // Your code here
    }
}

Need a hint?

Use var customers = new List { ... }; and var orders = new List { ... }; with the exact entries.

2

Create a join query to match orders with customers

Create a variable called orderCustomerJoin that uses Join on orders and customers to match orders to customers by CustomerID. Select a new object with OrderID and CustomerName from the matching pairs.

C Sharp (C#)

using System;
using System.Collections.Generic;

record Customer(int CustomerID, string Name);
record Order(int OrderID, int CustomerID);

class Program
{
    static void Main()
    {
        var customers = new List
        {
            new Customer(1, "Alice"),
            new Customer(2, "Bob"),
            new Customer(3, "Charlie")
        };

        var orders = new List
        {
            new Order(101, 1),
            new Order(102, 2),
            new Order(103, 1),
            new Order(104, 3)
        };

        // Create orderCustomerJoin using Join
        // Your code here
    }
}

Need a hint?

Use orders.Join(customers, order => order.CustomerID, customer => customer.CustomerID, (order, customer) => new { order.OrderID, CustomerName = customer.Name }).

3

Create a group join query to group orders by customer

Create a variable called customerOrdersGroup that uses GroupJoin on customers and orders to group orders by each customer using CustomerID. Select a new object with CustomerName and Orders (the grouped orders).

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Linq;

record Customer(int CustomerID, string Name);
record Order(int OrderID, int CustomerID);

class Program
{
    static void Main()
    {
        var customers = new List
        {
            new Customer(1, "Alice"),
            new Customer(2, "Bob"),
            new Customer(3, "Charlie")
        };

        var orders = new List
        {
            new Order(101, 1),
            new Order(102, 2),
            new Order(103, 1),
            new Order(104, 3)
        };

        var orderCustomerJoin = orders.Join(
            customers,
            order => order.CustomerID,
            customer => customer.CustomerID,
            (order, customer) => new { order.OrderID, CustomerName = customer.Name }
        );

        // Create customerOrdersGroup using GroupJoin
        // Your code here
    }
}

Need a hint?

Use customers.GroupJoin(orders, customer => customer.CustomerID, order => order.CustomerID, (customer, ordersGroup) => new { CustomerName = customer.Name, Orders = ordersGroup }).

4

Print the join and group join results

Print the results of orderCustomerJoin by showing each OrderID and CustomerName on separate lines in the format: Order {OrderID} belongs to {CustomerName}. Then print the results of customerOrdersGroup by showing each CustomerName and their orders on separate lines in the format: {CustomerName} has orders: {comma separated OrderIDs}. If a customer has no orders, print {CustomerName} has orders: None.

C Sharp (C#)

using System;
using System.Collections.Generic;
using System.Linq;

record Customer(int CustomerID, string Name);
record Order(int OrderID, int CustomerID);

class Program
{
    static void Main()
    {
        var customers = new List
        {
            new Customer(1, "Alice"),
            new Customer(2, "Bob"),
            new Customer(3, "Charlie")
        };

        var orders = new List
        {
            new Order(101, 1),
            new Order(102, 2),
            new Order(103, 1),
            new Order(104, 3)
        };

        var orderCustomerJoin = orders.Join(
            customers,
            order => order.CustomerID,
            customer => customer.CustomerID,
            (order, customer) => new { order.OrderID, CustomerName = customer.Name }
        );

        var customerOrdersGroup = customers.GroupJoin(
            orders,
            customer => customer.CustomerID,
            order => order.CustomerID,
            (customer, ordersGroup) => new { CustomerName = customer.Name, Orders = ordersGroup }
        );

        // Print the join and group join results
        // Your code here
    }
}

Need a hint?

Use foreach loops to print each joined item and each grouped item. Use string.Join to list order IDs or print "None" if no orders.