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.