Collection initialization syntax in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to initialize a list of integers with values 1, 2, and 3.

C Sharp (C#)

var numbers = new List<int> [1] 1, 2, 3 [2];

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

A{

B[

C}

D)

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses () instead of curly braces {} for collection initialization.

Using square brackets [] which are for arrays, not collections.

In C#, collection initializers use curly braces { } to list the elements inside the new collection.

2fill in blank

medium

Complete the code to initialize a dictionary with keys "apple" and "banana" and values 1 and 2.

C Sharp (C#)

var fruitCounts = new Dictionary<string, int> [1] {"apple", 1}, {"banana", 2} [2];

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

A{

B[

C}

D)

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses () instead of curly braces {} for dictionary initialization.

Using square brackets [] which are not valid for dictionary initializers.

Dictionaries use curly braces { } with key-value pairs inside for initialization.

3fill in blank

hard

Fix the error in the code to correctly initialize a list of strings with "red", "green", and "blue".

C Sharp (C#)

var colors = new List<string> [1] "red", "green", "blue" [2];

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

A[

B{

C}

D)

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets [] which are for arrays, not collections.

Using parentheses () which are for method calls, not initializers.

Collection initializers require curly braces { } around the elements after the new keyword.

4fill in blank

hard

Fill both blanks to initialize a dictionary with keys 1 and 2 and values "one" and "two".

C Sharp (C#)

var numbers = new Dictionary<int, string> [1] {1, "one"}, {2, "two"} [2];

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

A{

B[

C}

D)

Attempts:

3 left

💡 Hint

Common Mistakes

Using square brackets [] or parentheses () instead of curly braces.

Forgetting to close the initializer with the correct brace.

Dictionary initializers use curly braces { } to enclose the key-value pairs.

5fill in blank

hard

Fill the two blanks to initialize a list of tuples with (1, "one"), (2, "two"), and (3, "three").

C Sharp (C#)

var list = new List<(int, string)> [1] (1, "one"), (2, "two"), (3, "three") [2];

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

A{

B}

C;

D)

Attempts:

3 left

💡 Hint

Common Mistakes

Using parentheses instead of curly braces for the initializer.

Collection initializers use curly braces { } around the elements.

Practice

1. What does collection initialization syntax in C# allow you to do?
var list = new List<int> { 1, 2, 3 };

easy

A. Only create an empty collection

B. Create and fill a collection in one step

C. Fill a collection after creating it separately

D. Create a collection without specifying type

Solution

1. Step 1: Understand collection initialization

   Collection initialization lets you create a collection and add items to it in one line.

2. Step 2: Analyze the example code

   The code new List<int> { 1, 2, 3 } creates a list and fills it with 1, 2, and 3 immediately.

3. Final Answer:

   Create and fill a collection in one step -> Option B

4. Quick Check:

   Collection initialization = create + fill [OK]

Hint: Remember: initialization means create and fill together [OK]

Common Mistakes:

• Thinking it only creates empty collections
• Confusing initialization with adding items later
• Assuming type is optional in initialization

2. Which of the following is the correct syntax to initialize a dictionary with keys and values in C#?

easy

A. var dict = new Dictionary { 1: "One", 2: "Two" };

B. var dict = new Dictionary (1: "One", 2: "Two");

C. var dict = new Dictionary { (1, "One"), (2, "Two") };

D. var dict = new Dictionary { {1, "One"}, {2, "Two"} };

Solution

1. Step 1: Recall dictionary initialization syntax

   Dictionaries use curly braces with key-value pairs inside another set of braces: { {key, value}, ... }.

2. Step 2: Check each option

   var dict = new Dictionary { {1, "One"}, {2, "Two"} }; uses correct syntax with nested braces for each pair. Others use invalid syntax for C# dictionaries.

3. Final Answer:

   var dict = new Dictionary { {1, "One"}, {2, "Two"} }; -> Option D

4. Quick Check:

   Dictionary init = nested braces for pairs [OK]

Hint: Use double braces for dictionary key-value pairs [OK]

Common Mistakes:

• Using colon instead of comma between key and value
• Trying to use parentheses instead of braces
• Confusing dictionary syntax with object initializers

3. What will be the output of the following C# code?

var set = new HashSet<int> { 1, 2, 2, 3 };
Console.WriteLine(set.Count);

medium

A. 3

B. Runtime error

C. 2

D. 4

Solution

1. Step 1: Understand HashSet behavior

   A HashSet stores unique elements only, so duplicates are ignored.

2. Step 2: Analyze the initialization

   The set is initialized with {1, 2, 2, 3}, but the duplicate '2' is ignored, so the set contains 3 unique elements.

3. Final Answer:

   3 -> Option A

4. Quick Check:

   HashSet ignores duplicates = count 3 [OK]

Hint: HashSet counts unique items only [OK]

Common Mistakes:

• Counting duplicates as separate elements
• Expecting a runtime error due to duplicates
• Confusing HashSet with List behavior

4. Identify the error in this collection initialization:

var list = new List<int> { 1, 2, "3" };

medium

A. Using parentheses instead of braces

B. Missing semicolon at the end

C. Mixing int and string types in List<int>

D. List cannot be initialized with values

Solution

1. Step 1: Check the List type and values

   The list is declared as List<int>, so all elements must be integers.

2. Step 2: Identify the incorrect value

   The value "3" is a string, not an int, causing a type mismatch error.

3. Final Answer:

   Mixing int and string types in List<int> -> Option C

4. Quick Check:

   List<int> must contain only ints [OK]

Hint: All items must match the collection's declared type [OK]

Common Mistakes:

• Assuming implicit conversion from string to int
• Ignoring type mismatch errors
• Thinking semicolon or braces are the problem

5. You want to initialize a dictionary where keys are strings and values are lists of integers. Which is the correct way to do this in C#?

hard

A. var dict = new Dictionary { { "a", new List<int> {1, 2} }, { "b", new List<int> {3, 4} } };

B. var dict = new Dictionary { { "a", {1, 2} }, { "b", {3, 4} } };

C. var dict = new Dictionary { ( "a", [1, 2] ), ( "b", [3, 4] ) };

D. var dict = new Dictionary { "a": [1, 2], "b": [3, 4] };

Solution

1. Step 1: Understand nested collection initialization

   Each dictionary value is a List<int>, so you must create new List<int> instances inside the dictionary initializer.

2. Step 2: Check syntax correctness

   var dict = new Dictionary { { "a", new List<int> {1, 2} }, { "b", new List<int> {3, 4} } }; correctly uses nested initializers: dictionary with key-value pairs, where values are new List<int> with their own initializers.

3. Final Answer:

   var dict = new Dictionary { { "a", new List<int> {1, 2} }, { "b", new List<int> {3, 4} } }; -> Option A

4. Quick Check:

   Nested collections need explicit new List<int> [OK]

Hint: Use 'new' for nested collections inside dictionary [OK]

Common Mistakes:

• Omitting 'new List<int>' for nested lists
• Using parentheses or brackets incorrectly
• Trying to use colon syntax inside C# initializers