Join and GroupJoin operations in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to join two lists on the Id property.

C Sharp (C#)

var result = from c in customers
             join o in orders on c.Id [1] o.CustomerId
             select new { c.Name, o.OrderId };

Drag options to blanks, or click blank then click option'

A==

Bequals

CequalsTo

Dequal

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of 'equals' in join clause.

Using incorrect keywords like 'equal' or 'equalsTo'.

In C#, the join clause uses the keyword equals to match keys.

2fill in blank

medium

Complete the code to perform a GroupJoin and select the customer name with their orders.

C Sharp (C#)

var result = customers.GroupJoin(orders,
                c => c.Id,
                o => o.CustomerId,
                (c, os) => new { c.Name, Orders = [1] });

Drag options to blanks, or click blank then click option'

Ao

Borders

CorderList

Dos

Attempts:

3 left

💡 Hint

Common Mistakes

Using the original orders list instead of the grouped orders parameter.

Using a variable name not defined in the lambda parameters.

The GroupJoin result selector uses the second parameter (here named os) which represents the grouped orders for each customer.

3fill in blank

hard

Fix the error in the join clause by completing the missing keyword.

C Sharp (C#)

var query = from a in authors
            join b in books on a.Id [1] b.AuthorId
            select new { a.Name, b.Title };

Drag options to blanks, or click blank then click option'

Aequals

BequalsTo

CequalTo

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of 'equals'.

Using incorrect keywords like 'equalTo' or 'equalsTo'.

The join clause requires the keyword equals to compare keys, not '==' or other variants.

4fill in blank

hard

Fill both blanks to create a dictionary of customer names and their order counts using GroupJoin.

C Sharp (C#)

var customerOrderCounts = customers.GroupJoin(orders,
                            [1],
                            [2],
                            (c, os) => new { c.Name, Count = os.Count() })
                            .ToDictionary(x => x.Name, x => x.Count);

Drag options to blanks, or click blank then click option'

Ac => c.Id

Bo => o.CustomerId

Cc => c.Name

Do => o.Id

Attempts:

3 left

💡 Hint

Common Mistakes

Mixing up key selectors by using names instead of Ids.

Using order Id instead of CustomerId for matching.

The GroupJoin requires key selectors for customers and orders: customer Id and order CustomerId.

5fill in blank

hard

Fill all three blanks to join authors and books, then select author name, book title, and year published.

C Sharp (C#)

var joinedData = from a in authors
                 join b in books on a.Id [1] b.AuthorId
                 select new { [2], [3], b.YearPublished };

Drag options to blanks, or click blank then click option'

Aequals

Ba.Name

Cb.Title

D==

Attempts:

3 left

💡 Hint

Common Mistakes

Using '==' instead of 'equals' in join.

Selecting wrong properties or missing one.

The join uses equals. The select projects author name and book title along with year.

Practice

1.

What is the main difference between Join and GroupJoin in C# LINQ?

easy

A. Join matches one item to one item; GroupJoin matches one item to many items as a group.

B. Join is used only for numbers; GroupJoin is used only for strings.

C. Join creates groups; GroupJoin creates flat lists.

D. Join works only with arrays; GroupJoin works only with lists.

Solution

1. Step 1: Understand Join behavior

   Join pairs each item from the first list with exactly one matching item from the second list based on a key.

2. Step 2: Understand GroupJoin behavior

   GroupJoin pairs each item from the first list with a collection (group) of matching items from the second list.

3. Final Answer:

   Join matches one item to one item; GroupJoin matches one item to many items as a group. -> Option A

4. Quick Check:

   Join = one-to-one, GroupJoin = one-to-many [OK]

Hint: Remember: Join = pairs, GroupJoin = groups [OK]

Common Mistakes:

• Thinking Join creates groups
• Confusing GroupJoin with Join syntax
• Assuming Join works only with arrays

2.

Which of the following is the correct syntax to perform a Join operation in C# LINQ?

var result = from a in listA
             join b in listB on a.Id equals b.Id
             select new { a.Name, b.Value };

easy

A. var result = from a in listA join b in listB on a.Id == b.Id select new { a.Name, b.Value };

B. var result = from a in listA join b in listB where a.Id == b.Id select new { a.Name, b.Value };

C. var result = from a in listA join b in listB on a.Id equals b.Id select new { a.Name, b.Value };

D. var result = from a in listA join b in listB by a.Id equals b.Id select new { a.Name, b.Value };

Solution

1. Step 1: Check correct join syntax

   The correct syntax uses join b in listB on a.Id equals b.Id to specify the matching keys.

2. Step 2: Identify errors in other options

   var result = from a in listA join b in listB where a.Id == b.Id select new { a.Name, b.Value }; uses where instead of on ... equals, which is invalid for join. var result = from a in listA join b in listB by a.Id equals b.Id select new { a.Name, b.Value }; misplaces by. var result = from a in listA join b in listB on a.Id == b.Id select new { a.Name, b.Value }; uses == instead of equals.

3. Final Answer:

   var result = from a in listA join b in listB on a.Id equals b.Id select new { a.Name, b.Value }; -> Option C

4. Quick Check:

   Join syntax = on ... equals ... [OK]

Hint: Use 'on key1 equals key2' for Join syntax [OK]

Common Mistakes:

• Using '==' instead of 'equals' in join
• Replacing 'on' with 'where' in join
• Adding extra keywords like 'by'

3.

What is the output of the following C# code?

var students = new[] {
    new { Id = 1, Name = "Alice" },
    new { Id = 2, Name = "Bob" }
};
var scores = new[] {
    new { StudentId = 1, Score = 90 },
    new { StudentId = 1, Score = 85 },
    new { StudentId = 2, Score = 88 }
};

var query = students.GroupJoin(scores,
    s => s.Id,
    sc => sc.StudentId,
    (s, scGroup) => new {
        Student = s.Name,
        Scores = scGroup.Select(x => x.Score).ToList()
    });

foreach (var item in query) {
    Console.WriteLine($"{item.Student}: {string.Join(",", item.Scores)}");
}

medium

A. Alice: 90,85,88
Bob:

B. Alice: 90,85
Bob: 88

C. Alice: 90
Bob: 85,88

D. Alice:
Bob: 90,85,88

Solution

1. Step 1: Understand GroupJoin matching

   Each student is matched with all scores where StudentId equals Id. Alice (Id=1) matches scores 90 and 85; Bob (Id=2) matches score 88.

2. Step 2: Output formatting

   The code prints each student's name followed by their scores joined by commas. So output lines are "Alice: 90,85" and "Bob: 88".

3. Final Answer:

   Alice: 90,85 Bob: 88 -> Option B

4. Quick Check:

   GroupJoin groups scores by student [OK]

Hint: GroupJoin groups many matches per key [OK]

Common Mistakes:

• Assuming Join returns groups
• Mixing up StudentId and Id keys
• Printing scores incorrectly

4.

Identify the error in this C# LINQ GroupJoin code snippet:

var result = listA.GroupJoin(listB,
    a => a.Id,
    b => b.Id,
    (a, bGroup) => new {
        a.Name,
        bGroup
    });

medium

A. Cannot use bGroup directly without projection like ToList().

B. Missing select keyword before the new anonymous object.

C. The lambda parameters a and b must have the same name.

D. The GroupJoin method requires a third collection parameter.

Solution

1. Step 1: Check usage of bGroup in projection

   bGroup is an IEnumerable of matching items. It cannot be used directly in the anonymous type without materializing it (e.g., bGroup.ToList()) because it is lazily evaluated and multiple enumerations can lead to unexpected behavior or performance issues.

2. Step 2: Rule out other options

   No select keyword is needed--the lambda is the projection. Lambda parameters a and b can have different names. GroupJoin requires only two collections.

3. Final Answer:

   Cannot use bGroup directly without projection like ToList(). -> Option A

4. Quick Check:

   GroupJoin returns groups; project with ToList() for clarity [OK]

Hint: Always project groups to list for easier use [OK]

Common Mistakes:

• Thinking GroupJoin needs select keyword
• Assuming lambda parameter names must match
• Expecting extra collection parameter

5.

You have two lists: authors and books. Each book has an AuthorId. You want to create a list showing each author and all their book titles, but if an author has no books, show an empty list. Which LINQ operation is best?

hard

A. Use SelectMany to flatten authors and books.

B. Use Join to match authors and books.

C. Use Where to filter authors with books only.

D. Use GroupJoin to group books by author.

Solution

1. Step 1: Understand requirement for grouping

   You want each author with a collection of their books, including authors with no books (empty list).

2. Step 2: Choose correct LINQ operation

   GroupJoin creates a one-to-many relationship and includes authors with no matching books, producing empty groups as needed.

3. Final Answer:

   Use GroupJoin to group books by author. -> Option D

4. Quick Check:

   GroupJoin = one-to-many with empty groups included [OK]

Hint: GroupJoin keeps authors without books with empty groups [OK]

Common Mistakes:

• Using Join excludes authors without books
• Filtering authors removes those without books
• SelectMany flattens, losing grouping