Casting with as and is operators in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of casting with 'as' operator

What is the output of this C# code snippet?

C Sharp (C#)

class Animal {}
class Dog : Animal {}

Animal a = new Animal();
Dog d = a as Dog;
Console.WriteLine(d == null ? "null" : "not null");

A null

B not null

C Compilation error

D Runtime exception

💡 Hint

The 'as' operator returns null if the cast fails instead of throwing an exception.

The variable 'a' is an Animal instance, not a Dog. Using 'as' to cast to Dog returns null because 'a' is not a Dog or derived from Dog.

❓ Predict Output

intermediate

Using 'is' operator with inheritance

What will this C# code print?

C Sharp (C#)

class Vehicle {}
class Car : Vehicle {}

Vehicle v = new Car();
Console.WriteLine(v is Car);

A Runtime exception

B True

C Compilation error

D False

💡 Hint

The 'is' operator checks if the object is compatible with the given type.

The variable 'v' refers to a Car instance, so 'v is Car' returns true.

🔧 Debug

advanced

Why does this cast cause a runtime error?

Consider this code snippet. Which option explains why it throws an exception?

C Sharp (C#)

object obj = "hello";
int num = (int)obj;

A The cast fails because 'obj' is a string, not an int, causing InvalidCastException.

B The 'as' operator returns null, so 'num' is zero.

C The code compiles but prints 'hello'.

D The 'is' operator should be used instead of cast.

💡 Hint

Direct casting throws an exception if types are incompatible.

Casting a string object to int directly causes InvalidCastException at runtime.

❓ Predict Output

advanced

Result of 'as' operator with value types

What happens when you run this C# code?

C Sharp (C#)

object o = 5;
int? n = o as int?;
Console.WriteLine(n.HasValue ? n.Value.ToString() : "null");

A Compilation error

B null

C 5

D Runtime exception

💡 Hint

Nullable types can be used with 'as' operator.

The 'as' operator can cast boxed value types to nullable types, so 'n' has value 5.

🧠 Conceptual

expert

Difference between 'is' and 'as' operators

Which statement correctly describes the difference between 'is' and 'as' operators in C#?

A 'is' throws exception on failure; 'as' returns default value.

B 'is' performs casting and returns null if it fails; 'as' checks type and returns bool.

C 'is' and 'as' are interchangeable and behave the same.

D 'is' checks type compatibility and returns a bool; 'as' attempts cast and returns null if it fails.

💡 Hint

Think about what each operator returns when the cast is not possible.

'is' returns true or false depending on type compatibility; 'as' returns the casted object or null if cast fails.

Practice

1. What does the as operator do in C#?

easy

A. It tries to cast an object to a type and returns null if it fails.

B. It checks if an object is exactly a certain type and returns true or false.

C. It converts a value type to a string representation.

D. It throws an exception if the cast is invalid.

Solution

1. Step 1: Understand the as operator behavior

   The as operator attempts to cast an object to a specified type but returns null instead of throwing an exception if the cast fails.

2. Step 2: Compare with other options

   It checks if an object is exactly a certain type and returns true or false. describes the is operator, It converts a value type to a string representation. is unrelated, and It throws an exception if the cast is invalid. is incorrect because as does not throw exceptions.

3. Final Answer:

   It tries to cast an object to a type and returns null if it fails. -> Option A

4. Quick Check:

   as returns null on failure [OK]

Hint: Remember: as returns null, no exceptions [OK]

Common Mistakes:

• Confusing as with is
• Thinking as throws exceptions
• Assuming as works with value types

2. Which of the following is the correct syntax to check if an object obj is of type string using the is operator?

easy

A. if (obj == string) { /* code */ }

B. if (obj as string) { /* code */ }

C. if (obj is (string)) { /* code */ }

D. if (obj is string) { /* code */ }

Solution

1. Step 1: Recall is operator syntax

   The correct syntax to check type is if (obj is Type), so if (obj is string) is valid.

2. Step 2: Analyze other options

   if (obj as string) { /* code */ } uses as incorrectly in an if condition, if (obj == string) { /* code */ } compares object to type which is invalid, if (obj is (string)) { /* code */ } has unnecessary parentheses.

3. Final Answer:

   if (obj is string) { /* code */ } -> Option D

4. Quick Check:

   is syntax: obj is Type [OK]

Hint: Use is like: if (obj is Type) [OK]

Common Mistakes:

• Using as in if condition directly
• Comparing object to type with ==
• Adding unnecessary parentheses in is check

3. What is the output of the following code?

object obj = "hello";
string s = obj as string;
Console.WriteLine(s != null ? s.ToUpper() : "null");

medium

A. HELLO

B. null

C. hello

D. Runtime error

Solution

1. Step 1: Analyze the as cast

   The object obj holds a string "hello". Using as string casts it successfully, so s is "hello".

2. Step 2: Evaluate the conditional output

   Since s is not null, s.ToUpper() is called, producing "HELLO".

3. Final Answer:

   HELLO -> Option A

4. Quick Check:

   as cast success means uppercase output [OK]

Hint: If as cast succeeds, use the object; else null [OK]

Common Mistakes:

• Assuming as throws exception on failure
• Forgetting to check for null after as
• Confusing output case sensitivity

4. Identify the error in this code snippet:

object obj = 123;
string s = obj as string;
Console.WriteLine(s.Length);

medium

A. The is operator is used incorrectly.

B. The as cast will fail and s will be null, causing a null reference exception.

C. You cannot use as with value types like int.

D. The code will compile but print 3.

Solution

1. Step 1: Understand as cast with incompatible types

   Since obj holds an int (123), casting it as string returns null.

2. Step 2: Analyze the usage

   Since s is null, calling s.Length causes a null reference exception.

3. Final Answer:

   The as cast will fail and s will be null, causing a null reference exception. -> Option B

4. Quick Check:

   as returns null on failure; check before use [OK]

Hint: Always check for null after as cast before use [OK]

Common Mistakes:

• Assuming is fixes null issues
• Not checking s for null before accessing members
• Thinking as works with value types

5. Given the classes:

class Animal { }
class Dog : Animal { public string Bark() => "Woof!"; }

What is the safest way to call Bark() on an Animal reference a that might be a Dog?

hard

A. if (a is Dog) { Console.WriteLine(a.Bark()); }

B. Console.WriteLine(((Dog)a).Bark());

C. Dog d = a as Dog; if (d != null) Console.WriteLine(d.Bark());

D. Console.WriteLine(a.Bark());

Solution

1. Step 1: Understand safe casting with as

   Using as casts a to Dog safely, returning null if a is not a Dog.

2. Step 2: Check for null before calling Bark()

   Checking d != null ensures Bark() is called only if a is a Dog, avoiding exceptions.

3. Step 3: Compare with other options

   if (a is Dog) { Console.WriteLine(a.Bark()); } checks type but doesn't cast, so a.Bark() is invalid because Animal has no Bark(). Console.WriteLine(((Dog)a).Bark()); casts without check, risking exceptions. Console.WriteLine(a.Bark()); is invalid because Animal has no Bark().

4. Final Answer:

   Dog d = a as Dog; if (d != null) Console.WriteLine(d.Bark()); -> Option C

5. Quick Check:

   Use as + null check for safe cast [OK]

Hint: Use as then check null before method call [OK]

Common Mistakes:

• Casting without checking type first
• Calling methods on base type without override
• Using is then casting again unnecessarily