Why advanced LINQ matters in C Sharp (C#) - Challenge Your Understanding

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this LINQ query with grouping?

Consider the following C# code using LINQ to group numbers by their remainder when divided by 3. What will be printed?

C Sharp (C#)

var numbers = new[] {1, 2, 3, 4, 5, 6};
var groups = numbers.GroupBy(n => n % 3).OrderBy(g => g.Key);
foreach (var group in groups)
{
    Console.Write($"{group.Key}: ");
    foreach (var num in group)
        Console.Write(num + " ");
    Console.WriteLine();
}

A

0: 3 6 
1: 1 4 
2: 2 5 

B

0: 3 6 
1: 1 2 
2: 4 5 

C

0: 6 3 
1: 1 4 
2: 2 5 

D

0: 3 6 
1: 4 1 
2: 5 2 

💡 Hint

Remember that GroupBy preserves the order of elements within each group.

The GroupBy groups numbers by their remainder when divided by 3. Numbers 3 and 6 have remainder 0, 1 and 4 have remainder 1, and 2 and 5 have remainder 2. The order inside each group is preserved from the original array.

🧠 Conceptual

intermediate

Why use deferred execution in LINQ?

Which of the following best explains the advantage of deferred execution in LINQ?

A It forces all data to be loaded from the database at once to avoid multiple calls.

B It allows queries to be executed immediately and results stored in memory.

C It converts LINQ queries into SQL statements automatically.

D It delays query execution until the results are actually needed, improving performance and resource use.

💡 Hint

Think about when the data is actually fetched or processed.

Deferred execution means the query is not run until you iterate over the results. This can save time and memory by avoiding unnecessary work.

❓ Predict Output

advanced

What is the output of this LINQ query with SelectMany?

Given the following code, what will be printed?

C Sharp (C#)

var data = new[] { new[] {1, 2}, new[] {3, 4}, new[] {5} };
var result = data.SelectMany(arr => arr.Where(x => x % 2 == 0));
foreach (var num in result)
{
    Console.Write(num + " ");
}

A 2 4 5

B 1 2 3 4 5

C 2 4

D 1 3 5

💡 Hint

SelectMany flattens collections and Where filters elements.

SelectMany flattens the arrays into one sequence, but only elements that are even (x % 2 == 0) are selected. So only 2 and 4 are printed.

🔧 Debug

advanced

What error does this LINQ code produce?

Examine the following code snippet. What error will it cause when run?

C Sharp (C#)

var list = new List<int> {1, 2, 3};
var query = list.Select(x => x / (x - 2));
foreach (var val in query)
{
    Console.Write(val + " ");
}

A NullReferenceException

B DivideByZeroException

C InvalidOperationException

D No error, outputs: 1 2 3

💡 Hint

Check what happens when x equals 2.

When x is 2, the expression x - 2 is zero, causing a division by zero error during query execution.

🚀 Application

expert

How many items are in the resulting dictionary?

Consider this LINQ query that creates a dictionary from a list of strings. How many items will the dictionary contain?

C Sharp (C#)

var words = new List<string> {"apple", "banana", "apricot", "blueberry", "avocado"};
var dict = words.GroupBy(w => w[0])
                .ToDictionary(g => g.Key, g => g.Count());
Console.WriteLine(dict.Count);

A 3

B 4

C 5

D 2

💡 Hint

Look at the first letters of each word and count unique letters.

The words start with 'a' (apple, apricot, avocado) and 'b' (banana, blueberry). So there are 2 unique keys: 'a' and 'b'. The dictionary will have 2 items.