Dictionary key-value collection in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ query_result

intermediate

What is the output of this C# dictionary lookup?

Consider the following C# code snippet:

var dict = new Dictionary<string, int>() { {"apple", 3}, {"banana", 5} };
int value = dict["banana"];
Console.WriteLine(value);

What will be printed?

C Sharp (C#)

var dict = new Dictionary<string, int>() { {"apple", 3}, {"banana", 5} };
int value = dict["banana"];
Console.WriteLine(value);

A 5

B KeyNotFoundException

C 0

D 3

💡 Hint

Look at the key used to get the value from the dictionary.

The dictionary contains the key "banana" with value 5, so accessing dict["banana"] returns 5.

📝 Syntax

intermediate

Which option correctly adds a new key-value pair to a C# dictionary?

You want to add a new entry with key "orange" and value 7 to an existing dictionary dict. Which code snippet is correct?

A dict.Add("orange", 7);

B dict["orange"] = 7;

C dict.Insert("orange", 7);

D dict.Put("orange", 7);

💡 Hint

Remember the method name to add a new key-value pair in Dictionary.

The Add method is used to add a new key-value pair. Using the indexer with assignment also works but is not the method call asked here. Insert and Put are not valid Dictionary methods.

🔧 Debug

advanced

What error does this code raise?

Examine this C# code:

var dict = new Dictionary<string, int>();
int value = dict["missingKey"];
Console.WriteLine(value);

What happens when this code runs?

C Sharp (C#)

var dict = new Dictionary<string, int>();
int value = dict["missingKey"];
Console.WriteLine(value);

A Prints 0

B Throws KeyNotFoundException

C Prints null

D Compilation error

💡 Hint

Think about what happens when you access a key that does not exist in a dictionary.

Accessing a missing key with the indexer throws a KeyNotFoundException at runtime.

❓ optimization

advanced

Which approach is best to check if a key exists and get its value in C# dictionary?

You want to check if the key "pear" exists in a dictionary and get its value if it does. Which code snippet is the most efficient and safe?

A if (dict.ContainsKey("pear")) { var val = dict["pear"]; }

B var val = dict["pear"]; if (val != null) { /* use val */ }

C dict.TryGetValue("pear", out var val);

D var val = dict.GetValueOrDefault("pear");

💡 Hint

Look for a method that combines checking and retrieving in one call.

TryGetValue checks if the key exists and returns the value in one call without throwing exceptions, making it efficient and safe.

🧠 Conceptual

expert

What is the count of items after this sequence?

Given an empty dictionary dict of type Dictionary<string, int>, you run these commands:

dict["a"] = 1;
dict["b"] = 2;
dict["a"] = 3;
dict.Add("c", 4);

How many key-value pairs does dict contain now?

C Sharp (C#)

var dict = new Dictionary<string, int>();
dict["a"] = 1;
dict["b"] = 2;
dict["a"] = 3;
dict.Add("c", 4);
int count = dict.Count;

A Throws exception

B 4

C 2

D 3

💡 Hint

Remember what happens when you assign a value to an existing key.

Assigning to an existing key updates its value without adding a new pair. So keys are "a", "b", and "c" → total 3.