Type checking patterns in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of type pattern matching with 'is' and 'when'

What is the output of the following C# code snippet?

C Sharp (C#)

object obj = 42;
string result = obj is int i && i > 40 ? "Greater than 40" : "40 or less";
Console.WriteLine(result);

A 40 or less

B Greater than 40

C Compilation error

D Runtime exception

💡 Hint

Check the type of obj and the condition in the 'when' clause.

The variable obj is an int with value 42, which matches the pattern 'int i'. The 'when' clause checks if i > 40, which is true, so the result is 'Greater than 40'.

❓ Predict Output

intermediate

Output of switch expression with type patterns

What will be printed by this C# code?

C Sharp (C#)

object val = 3.14;
string output = val switch
{
    int i => $"Integer: {i}",
    double d => $"Double: {d}",
    _ => "Unknown type"
};
Console.WriteLine(output);

A Double: 3.14

B Unknown type

C Integer: 3

D Compilation error

💡 Hint

Look at the type of val and the matching patterns in the switch expression.

val is a double with value 3.14, so the pattern 'double d' matches and the output is 'Double: 3.14'.

🔧 Debug

advanced

Identify the runtime error in type pattern matching

This code throws an exception at runtime. Which option correctly identifies the cause?

C Sharp (C#)

object data = null;
if (data is string s)
{
    Console.WriteLine(s.Length);
}
else
{
    Console.WriteLine("Not a string");
}

A NullReferenceException because data is null and pattern matching tries to access s.Length

B Compilation error due to missing null check

C No exception, prints 'Not a string'

D InvalidCastException because data is null

💡 Hint

Consider how pattern matching with 'is' handles null values.

The pattern 'data is string s' fails if data is null, so the else branch runs printing 'Not a string'. No exception occurs.

❓ Predict Output

advanced

Output of nested type patterns with property matching

What is the output of this C# code?

C Sharp (C#)

record Person(string Name, int Age);
object obj = new Person("Alice", 30);
string message = obj switch
{
    Person { Age: > 18 } p => $"Adult: {p.Name}",
    Person p => $"Minor: {p.Name}",
    _ => "Unknown"
};
Console.WriteLine(message);

A Adult: Alice

B Minor: Alice

C Unknown

D Compilation error

💡 Hint

Check the property pattern and the value of Age.

The object is a Person with Age 30, which matches the first pattern with Age > 18, so the output is 'Adult: Alice'.

🧠 Conceptual

expert

Which option causes a compile-time error in type pattern usage?

Given the following code snippets, which one will cause a compile-time error?

A if (obj is int i && i > 0) { Console.WriteLine(i); }

B if (obj is var v && v is string s) { Console.WriteLine(s); }

C if (obj is int or string) { Console.WriteLine("int or string"); }

D if (obj is int i or string s) { Console.WriteLine(i + s.Length); }

💡 Hint

Check the syntax rules for combining patterns with 'or' and variable declarations.

Option D tries to declare two variables in an 'or' pattern, which is not allowed and causes a compile-time error.

Practice

1. What does the C# pattern if (obj is string s) do?

easy

A. Checks if obj is null

B. Converts obj to string without checking

C. Checks if obj is a string and assigns it to s if true

D. Throws an exception if obj is not a string

Solution

1. Step 1: Understand the is pattern

   The is keyword checks if an object is of a certain type.

2. Step 2: Assign variable if type matches

   If obj is a string, it assigns the value to s for use inside the block.

3. Final Answer:

   Checks if obj is a string and assigns it to s if true -> Option C

4. Quick Check:

   is Type var checks type and assigns [OK]

Hint: Remember: is Type var checks and assigns together [OK]

Common Mistakes:

• Thinking it converts without checking
• Assuming it checks for null only
• Believing it throws exceptions automatically

2. Which of the following is the correct syntax to check if obj is an int and assign it to number?

easy

A. if (obj is int number) { }

B. if (obj as int number) { }

C. if (obj == int number) { }

D. if (obj instanceof int number) { }

Solution

1. Step 1: Identify correct C# type pattern syntax

   The correct syntax uses is Type variable to check and assign.

2. Step 2: Compare options

   if (obj is int number) { } uses is int number, which is valid. Others use invalid keywords or syntax.

3. Final Answer:

   if (obj is int number) { } -> Option A

4. Quick Check:

   is Type var syntax is correct [OK]

Hint: Use is Type var for type check and assignment [OK]

Common Mistakes:

• Using 'as' instead of 'is' for type checking
• Using '==' to compare types
• Using JavaScript or Java keywords like 'instanceof'

3. What is the output of this code?

object obj = 42;
if (obj is int n)
{
    Console.WriteLine(n + 10);
}
else
{
    Console.WriteLine("Not an int");
}

medium

A. Not an int

B. 52

C. 42

D. Compilation error

Solution

1. Step 1: Check the type of obj

   obj holds the integer 42, so obj is int n is true and assigns 42 to n.

2. Step 2: Calculate the output inside the if block

   It prints n + 10 which is 42 + 10 = 52.

3. Final Answer:

   52 -> Option B

4. Quick Check:

   Type check passes, output is 42 + 10 = 52 [OK]

Hint: If type matches, variable holds value for use [OK]

Common Mistakes:

• Printing the original obj instead of n + 10
• Choosing else output wrongly
• Thinking it causes a compile error

4. Identify the error in this code snippet:

object obj = "hello";
if (obj is int number)
{
    Console.WriteLine(number);
}

medium

A. No error, code runs and prints number

B. Runtime exception thrown

C. Syntax error in the if statement

D. The variable 'number' is not assigned because obj is not int

Solution

1. Step 1: Analyze the type check

   obj is a string, so obj is int number is false and number is not assigned.

2. Step 2: Understand the effect on code execution

   The if block is skipped, so nothing prints. No error or exception occurs.

3. Final Answer:

   The variable 'number' is not assigned because obj is not int -> Option D

4. Quick Check:

   Type check false means variable not assigned [OK]

Hint: If type check fails, variable is not assigned [OK]

Common Mistakes:

• Thinking it causes syntax error
• Expecting runtime exception
• Assuming variable is assigned anyway

5. Given a list of objects List<object> items = new() { 1, "two", 3, null, 4.5 };, which code snippet correctly sums only the integer values using type checking patterns?

hard

A. int sum = 0;
foreach (var item in items)
{
if (item is int n) sum += n;
}
Console.WriteLine(sum);

B. int sum = 0;
foreach (int n in items)
{
sum += n;
}
Console.WriteLine(sum);

C. int sum = 0;
foreach (var item in items)
{
if (item is double d) sum += (int)d;
}
Console.WriteLine(sum);

D. int sum = 0;
foreach (var item in items)
{
sum += (int)item;
}
Console.WriteLine(sum);

Solution

1. Step 1: Understand the list contents

   The list has integers, a string, null, and a double. We want to sum only integers.

2. Step 2: Check each option for correct type filtering

   int sum = 0; foreach (var item in items) { if (item is int n) sum += n; } Console.WriteLine(sum); uses if (item is int n) to add only integers. int sum = 0; foreach (int n in items) { sum += n; } Console.WriteLine(sum); tries to cast all items to int in foreach, causing error. int sum = 0; foreach (var item in items) { if (item is double d) sum += (int)d; } Console.WriteLine(sum); sums doubles cast to int, which is incorrect. int sum = 0; foreach (var item in items) { sum += (int)item; } Console.WriteLine(sum); casts all items to int without checking, causing runtime errors.

3. Final Answer:

   int sum = 0; foreach (var item in items) { if (item is int n) sum += n; } Console.WriteLine(sum); -> Option A

4. Quick Check:

   Use is int var to filter integers safely [OK]

Hint: Use if (item is int n) to sum integers safely [OK]

Common Mistakes:

• Casting without checking causing exceptions
• Summing wrong types like double or string
• Assuming foreach int works on object list