Dictionary key-value collection in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a Dictionary in C#?

A Dictionary in C# is a collection that stores data as key-value pairs, where each key is unique and is used to access its corresponding value quickly.

beginner

How do you add an item to a Dictionary in C#?

Use the Add(key, value) method or the indexer syntax dictionary[key] = value; to add a new key-value pair.

intermediate

What happens if you try to add a duplicate key to a Dictionary?

Adding a duplicate key causes an exception because keys must be unique in a Dictionary.

beginner

How can you check if a key exists in a Dictionary?

Use the ContainsKey(key) method which returns true if the key exists, otherwise false.

intermediate

How do you retrieve a value safely from a Dictionary without risking an exception?

Use TryGetValue(key, out value) which returns true if the key exists and outputs the value; otherwise, it returns false.

What type of data structure is a Dictionary in C#?

A Key-value collection

B List of values

C Array of keys

D Stack

A Dictionary stores data as key-value pairs, making it a key-value collection.

Which method checks if a key exists in a Dictionary?

A ContainsKey()

B Exists()

C ContainsValue()

D FindKey()

ContainsKey() checks if a specific key exists in the Dictionary.

What happens if you add a key that already exists in a Dictionary using Add()?

A The Dictionary clears all items

B The value is updated

C The key is ignored

D An exception is thrown

Adding a duplicate key with Add() throws an exception because keys must be unique.

Which method safely retrieves a value from a Dictionary?

A FetchValue()

B GetValue()

C TryGetValue()

D Retrieve()

TryGetValue() safely retrieves a value and avoids exceptions if the key is missing.

How do you add a key-value pair to a Dictionary using indexer syntax?

A dictionary.Add(key, value);

B dictionary[key] = value;

C dictionary.Insert(key, value);

D dictionary.Set(key, value);

Using dictionary[key] = value; adds or updates the key-value pair.

Explain what a Dictionary key-value collection is and how it works in C#.

Think about how you use a real dictionary to find a word's meaning.

Describe how to safely retrieve a value from a Dictionary without causing an error if the key does not exist.

It's like checking if a word exists before reading its meaning.

Practice

1. What is the main purpose of a Dictionary in C#?

easy

A. To create graphical user interfaces

B. To store data in a sequential list

C. To perform mathematical calculations

D. To store data as key-value pairs for quick access

Solution

1. Step 1: Understand the Dictionary concept

   A Dictionary stores data as pairs where each key is linked to a value for fast lookup.

2. Step 2: Compare with other options

   Options B, C, and D describe lists, math, and UI, which are unrelated to Dictionary's purpose.

3. Final Answer:

   To store data as key-value pairs for quick access -> Option D

4. Quick Check:

   Dictionary = key-value pairs [OK]

Hint: Remember: Dictionary = keys + values for fast lookup [OK]

Common Mistakes:

• Confusing Dictionary with List
• Thinking Dictionary stores only values
• Assuming Dictionary is for UI or math

2. Which of the following is the correct way to add a key-value pair to a Dictionary<int, string> named dict?

easy

A. dict.Add(1, "apple");

B. dict.Insert(1, "apple");

C. dict["1"] = "apple";

D. dict.Put(1, "apple");

Solution

1. Step 1: Recall Dictionary methods

   The method to add a new key-value pair is Add(key, value).

2. Step 2: Check options

   Only dict.Add(1, "apple"); uses Add. dict[1] = "apple"; uses indexer which sets or updates but is not the method to add explicitly.

3. Final Answer:

   dict.Add(1, "apple"); -> Option A

4. Quick Check:

   Add() method adds key-value pair [OK]

Hint: Use Add() to insert new pairs, indexer to update [OK]

Common Mistakes:

• Using Insert or Put which don't exist
• Confusing Add() with indexer for adding
• Trying to add with wrong method name

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

var dict = new Dictionary<int, string>();
dict.Add(1, "one");
dict[2] = "two";
Console.WriteLine(dict[1] + ", " + dict[2]);

medium

A. two, one

B. 1, 2

C. one, two

D. Runtime error

Solution

1. Step 1: Understand dictionary additions

   Key 1 is added with value "one" using Add(). Key 2 is set to "two" using indexer.

2. Step 2: Check output of Console.WriteLine

   It prints values for keys 1 and 2 joined by comma: "one, two".

3. Final Answer:

   one, two -> Option C

4. Quick Check:

   dict[1] = one, dict[2] = two [OK]

Hint: Indexer sets or updates; Add inserts new pairs [OK]

Common Mistakes:

• Confusing keys with values in output
• Expecting keys printed instead of values
• Thinking indexer causes error if key missing

4. What is wrong with this code snippet?

var dict = new Dictionary<int, string>();
dict.Add(1, "apple");
dict.Add(1, "banana");

medium

A. It will compile and run without errors

B. It causes a runtime exception due to duplicate key

C. It overwrites the first value with the second

D. It causes a compile-time error

Solution

1. Step 1: Understand Add() behavior with duplicate keys

   Adding a key that already exists causes a runtime exception.

2. Step 2: Check code behavior

   Second Add with key 1 throws an ArgumentException at runtime.

3. Final Answer:

   It causes a runtime exception due to duplicate key -> Option B

4. Quick Check:

   Duplicate keys cause runtime error [OK]

Hint: Add() fails if key exists; use indexer to overwrite [OK]

Common Mistakes:

• Assuming Add overwrites existing key
• Expecting compile-time error instead of runtime
• Confusing Add with indexer behavior

5. Given a list of user IDs and names:

var users = new List<(int id, string name)> { (1, "Alice"), (2, "Bob"), (3, "Charlie") };

Which code correctly creates a Dictionary<int, string> mapping IDs to names using a dictionary comprehension style?

hard

A. var dict = users.ToDictionary(u => u.id, u => u.name);

B. var dict = new Dictionary<int, string>(); foreach(var u in users) dict.Add(u.name, u.id);

C. var dict = users.Select(u => new { u.id, u.name }).ToList();

D. var dict = new Dictionary<string, int>(); foreach(var u in users) dict[u.id] = u.name;

Solution

1. Step 1: Understand ToDictionary usage

   ToDictionary converts a list to a dictionary by specifying key and value selectors.

2. Step 2: Check each option

   var dict = users.ToDictionary(u => u.id, u => u.name); correctly maps id to name. var dict = new Dictionary<int, string>(); foreach(var u in users) dict.Add(u.name, u.id); swaps key and value and uses wrong types. var dict = users.Select(u => new { u.id, u.name }).ToList(); creates a list, not dictionary. var dict = new Dictionary<string, int>(); foreach(var u in users) dict[u.id] = u.name; has wrong dictionary types and indexer usage.

3. Final Answer:

   var dict = users.ToDictionary(u => u.id, u => u.name); -> Option A

4. Quick Check:

   ToDictionary creates dictionary from list [OK]

Hint: Use ToDictionary with key and value selectors [OK]

Common Mistakes:

• Swapping key and value in Add
• Using Select instead of ToDictionary
• Mismatching dictionary key-value types