GroupBy operation in C Sharp (C#) - Mini Project: Build & Apply

GroupBy operation

📖 Scenario: You work in a small store. You have a list of sales records. Each record has a Product name and a Quantity sold.You want to group these sales by product to see how many units of each product were sold in total.

🎯 Goal: Build a program that groups sales by product name and sums the quantities for each product.

📋 What You'll Learn

Create a list of sales records with exact products and quantities

Create a variable to hold the grouped sales result

Use LINQ GroupBy to group sales by product

Sum the quantities for each product group

Print the product name and total quantity sold for each group

💡 Why This Matters

🌍 Real World

Grouping sales data by product helps stores understand which products sell the most and manage inventory better.

💼 Career

Data grouping and aggregation are common tasks in software development, especially in reporting, analytics, and business intelligence roles.

1

Create the sales data list

Create a list called sales of Sale objects with these exact entries: new Sale("Apple", 10), new Sale("Banana", 5), new Sale("Apple", 7), new Sale("Orange", 3), new Sale("Banana", 2). Also define the Sale class with public properties Product (string) and Quantity (int).

C Sharp (C#)

// Define Sale class and create sales list
// Your code here

Hint

Define the Sale class first. Then create a List<Sale> called sales with the exact entries.

2

Create a variable for grouped sales

Add a variable called groupedSales of type var to hold the grouped sales result. Initialize it to null for now.

C Sharp (C#)

using System;
using System.Collections.Generic;

public class Sale
{
    public string Product { get; set; }
    public int Quantity { get; set; }

    public Sale(string product, int quantity)
    {
        Product = product;
        Quantity = quantity;
    }
}

class Program
{
    static void Main()
    {
        List sales = new List
        {
            new Sale("Apple", 10),
            new Sale("Banana", 5),
            new Sale("Apple", 7),
            new Sale("Orange", 3),
            new Sale("Banana", 2)
        };
        var groupedSales = null;
        // Your code here
    }
}

Hint

Use sales.GroupBy(sale => sale.Product) to group sales by product.

3

Sum quantities for each product group

Change the groupedSales variable to hold the result of grouping sales by Product and summing the Quantity for each group. Use LINQ GroupBy and Select to create an anonymous type with Product and TotalQuantity.

C Sharp (C#)

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

public class Sale
{
    public string Product { get; set; }
    public int Quantity { get; set; }

    public Sale(string product, int quantity)
    {
        Product = product;
        Quantity = quantity;
    }
}

class Program
{
    static void Main()
    {
        List sales = new List
        {
            new Sale("Apple", 10),
            new Sale("Banana", 5),
            new Sale("Apple", 7),
            new Sale("Orange", 3),
            new Sale("Banana", 2)
        };
        var groupedSales = sales.GroupBy(sale => sale.Product);
        // Use GroupBy and Select to sum quantities
        // Your code here
    }
}

Hint

Use Select after GroupBy to create a new object with Product and the sum of Quantity.

4

Print the grouped sales totals

Use a foreach loop to print each product and its total quantity from groupedSales. Use Console.WriteLine with the format: "Product: {product}, Total Quantity: {total}".

C Sharp (C#)

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

public class Sale
{
    public string Product { get; set; }
    public int Quantity { get; set; }

    public Sale(string product, int quantity)
    {
        Product = product;
        Quantity = quantity;
    }
}

class Program
{
    static void Main()
    {
        List sales = new List
        {
            new Sale("Apple", 10),
            new Sale("Banana", 5),
            new Sale("Apple", 7),
            new Sale("Orange", 3),
            new Sale("Banana", 2)
        };
        var groupedSales = sales.GroupBy(sale => sale.Product)
            .Select(group => new
            {
                Product = group.Key,
                TotalQuantity = group.Sum(sale => sale.Quantity)
            });
        foreach (var group in groupedSales)
        {
            // Print product and total quantity
            // Your code here
        }
    }
}

Hint

Use a foreach loop to print each group's product and total quantity using Console.WriteLine and string interpolation.

Practice

1. What does the GroupBy method do in C#?

easy

A. It sorts elements in ascending order.

B. It groups elements of a collection based on a key selector.

C. It filters elements based on a condition.

D. It removes duplicate elements from a collection.

Solution

1. Step 1: Understand the purpose of GroupBy

   The GroupBy method organizes elements by a key, creating groups of items sharing the same key.

2. Step 2: Compare with other operations

   Sorting arranges order, filtering selects items, and removing duplicates eliminates repeats, which are different from grouping.

3. Final Answer:

   It groups elements of a collection based on a key selector. -> Option B

4. Quick Check:

   GroupBy = grouping by key [OK]

Hint: GroupBy always creates groups by a key, not sorting or filtering [OK]

Common Mistakes:

• Confusing GroupBy with sorting methods
• Thinking GroupBy filters items
• Assuming GroupBy removes duplicates

2. Which of the following is the correct syntax to group a list of strings by their first character using LINQ in C#?

easy

A. var groups = list.GroupBy(s => s[0]);

B. var groups = list.GroupBy(s => s.Length);

C. var groups = list.GroupBy(s => s.ToUpper());

D. var groups = list.GroupBy(s => s.Substring(1));

Solution

1. Step 1: Identify grouping key for first character

   Grouping by the first character means using s => s[0] as the key selector.

2. Step 2: Check other options

   Grouping by length, uppercase string, or substring starting at index 1 does not group by first character.

3. Final Answer:

   var groups = list.GroupBy(s => s[0]); -> Option A

4. Quick Check:

   First char key = s[0] [OK]

Hint: Use s => s[0] to group by first character [OK]

Common Mistakes:

• Using substring starting at 1 instead of 0
• Grouping by string length instead of character
• Using ToUpper() changes case but not grouping key

3. What is the output of the following C# code?

var numbers = new[] {1, 2, 3, 4, 5, 6};
var groups = numbers.GroupBy(n => n % 2 == 0 ? "Even" : "Odd");
foreach (var group in groups) {
    Console.WriteLine($"{group.Key}: {string.Join(",", group)}");
}

medium

A. 1: 1
2: 2
3: 3
4: 4
5: 5
6: 6

B. Even: 2,4,6
Odd: 1,3,5

C. Odd: 1,3,5
Even: 2,4,6

D. Even: 1,3,5
Odd: 2,4,6

Solution

1. Step 1: Understand grouping key logic

   Numbers are grouped by whether they are even or odd using the key "Even" or "Odd".

2. Step 2: Determine group contents and order

   Group "Odd" contains 1,3,5; group "Even" contains 2,4,6. The foreach prints groups in order they appear, which is "Odd" then "Even".

3. Final Answer:

   Odd: 1,3,5 Even: 2,4,6 -> Option C

4. Quick Check:

   Group keys "Odd" then "Even" with correct items [OK]

Hint: GroupBy preserves order of first occurrence of keys [OK]

Common Mistakes:

• Assuming groups print in alphabetical order
• Mixing up even and odd groups
• Expecting separate groups for each number

4. Identify the error in this C# code that tries to group words by their length:

var words = new List<string> {"apple", "bat", "car", "dog"};
var groups = words.GroupBy(word => word.Length);
foreach (var group in groups)
    Console.WriteLine(group.Key + ": " + group.ToString());

medium

A. Using group.ToString() instead of joining group elements.

B. GroupBy cannot be used on List<string>.

C. Missing semicolon after GroupBy statement.

D. word.Length is not a valid key selector.

Solution

1. Step 1: Check GroupBy usage

   GroupBy on List<string> with word.Length is valid syntax and logic.

2. Step 2: Analyze output statement

   Using group.ToString() prints the type name, not the grouped items. We should join group elements to display them.

3. Final Answer:

   Using group.ToString() instead of joining group elements. -> Option A

4. Quick Check:

   Print grouped items by joining, not ToString() [OK]

Hint: Join group elements to print, not group.ToString() [OK]

Common Mistakes:

• Thinking GroupBy can't be used on lists
• Forgetting to join group elements for display
• Misunderstanding word.Length as key selector

5. Given a list of employees with properties Name and Department, how would you use GroupBy to create a dictionary where keys are departments and values are lists of employee names?

hard

A. var dict = employees.GroupBy(e => e.Name).ToDictionary(g => g.Key, g => g.Select(e => e.Department).ToList());

B. var dict = employees.ToDictionary(e => e.Department, e => e.Name);

C. var dict = employees.GroupBy(e => e.Department).ToList();

D. var dict = employees.GroupBy(e => e.Department).ToDictionary(g => g.Key, g => g.Select(e => e.Name).ToList());

Solution

1. Step 1: Group employees by Department

   Use GroupBy with key selector e => e.Department to group employees by their department.

2. Step 2: Convert groups to dictionary with employee names list

   Use ToDictionary with key as group.Key (department) and value as list of employee names using g.Select(e => e.Name).ToList().

3. Final Answer:

   var dict = employees.GroupBy(e => e.Department).ToDictionary(g => g.Key, g => g.Select(e => e.Name).ToList()); -> Option D

4. Quick Check:

   GroupBy + ToDictionary with Select names = correct [OK]

Hint: GroupBy then ToDictionary with Select for values list [OK]

Common Mistakes:

• Using ToDictionary directly without grouping
• Grouping by Name instead of Department
• Not converting grouped items to list of names