Array bounds checking behavior in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What happens in C# when you try to access an array element outside its bounds?

C# throws an IndexOutOfRangeException to prevent accessing invalid memory.

beginner

Why does C# perform array bounds checking automatically?

To protect the program from errors and security issues by ensuring you only access valid array indexes.

intermediate

How can you catch an out-of-bounds error in C#?

Use a try-catch block to catch IndexOutOfRangeException and handle it gracefully.

intermediate

Does C# check array bounds at compile time or runtime?

Array bounds are checked at runtime, not at compile time.

intermediate

What is the benefit of array bounds checking in C# compared to languages like C?

It prevents bugs and crashes by stopping invalid memory access, making programs more stable and secure.

What exception does C# throw when accessing an array out of bounds?

A ArgumentException

B NullReferenceException

C IndexOutOfRangeException

D InvalidOperationException

C# throws IndexOutOfRangeException to indicate an invalid array index access.

When does C# perform array bounds checking?

A At compile time

B At runtime

C Only in debug mode

D Never

Array bounds checking happens at runtime to ensure safety during program execution.

Which of these is a benefit of array bounds checking in C#?

A Prevents invalid memory access

B Makes code run faster

C Allows accessing any memory location

D Removes the need for arrays

Bounds checking prevents invalid memory access, improving program safety.

How can you handle an out-of-bounds error in C#?

A Using try-catch for IndexOutOfRangeException

B Ignoring the error

C Using unsafe code only

D Using a debugger

You can catch the exception with try-catch to handle errors gracefully.

What happens if you access array index -1 in C#?

A Returns null

B Returns the last element

C Compiles but crashes later

D Throws IndexOutOfRangeException

Negative indexes are invalid and cause an IndexOutOfRangeException.

Explain how array bounds checking works in C# and why it is important.

Think about what happens when you try to access an invalid index.

Describe how you can handle errors caused by accessing arrays out of bounds in C#.

Consider how exceptions work in C#.

Practice

1. What happens if you try to access an array element with an index less than 0 in C#?

easy

A. It throws an IndexOutOfRangeException.

B. It returns the first element of the array.

C. It returns null.

D. It automatically adjusts the index to 0.

Solution

1. Step 1: Understand array index rules in C#

   Array indexes must be between 0 and array length - 1.

2. Step 2: Behavior on invalid index

   Accessing an index less than 0 is outside valid range and causes an exception.

3. Final Answer:

   It throws an IndexOutOfRangeException. -> Option A

4. Quick Check:

   Invalid index access = IndexOutOfRangeException [OK]

Hint: Negative indexes always cause IndexOutOfRangeException [OK]

Common Mistakes:

• Thinking negative index returns first element
• Assuming negative index returns null
• Believing index auto-corrects to 0

2. Which of the following is the correct way to declare and access the third element of an integer array named numbers in C#?

easy

A. int value = numbers[3];

B. int value = numbers[0];

C. int value = numbers[1];

D. int value = numbers[2];

Solution

1. Step 1: Recall zero-based indexing in C# arrays

   Array indexes start at 0, so the first element is numbers[0].

2. Step 2: Identify the third element index

   The third element is at index 2 (0,1,2).

3. Final Answer:

   int value = numbers[2]; -> Option D

4. Quick Check:

   Third element index = 2 [OK]

Hint: Remember array indexing starts at zero [OK]

Common Mistakes:

• Using index 3 for third element
• Confusing element position with index
• Accessing index 1 or 0 for third element

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

int[] arr = {10, 20, 30};
Console.WriteLine(arr[3]);

medium

A. 0

B. IndexOutOfRangeException

C. 30

D. 20

Solution

1. Step 1: Check array length and valid indexes

   Array has 3 elements with indexes 0, 1, 2.

2. Step 2: Accessing index 3

   Index 3 is outside the array bounds, causing an exception.

3. Final Answer:

   IndexOutOfRangeException -> Option B

4. Quick Check:

   Accessing arr[3] = IndexOutOfRangeException [OK]

Hint: Indexes must be less than array length [OK]

Common Mistakes:

• Expecting last element value instead of error
• Assuming default zero value returned
• Confusing index 3 with valid index

4. Consider this code snippet:

int[] data = new int[5];
data[5] = 100;

What is the issue and how to fix it?

medium

A. Syntax error due to array declaration.

B. No issue, code runs fine.

C. IndexOutOfRangeException because index 5 is invalid; fix by using index 4.

D. NullReferenceException because array is not initialized.

Solution

1. Step 1: Analyze array size and valid indexes

   Array size is 5, valid indexes are 0 to 4.

2. Step 2: Identify invalid index usage

   Accessing data[5] is outside bounds, causing IndexOutOfRangeException.

3. Step 3: Correct the index

   Use data[4] to access the last element safely.

4. Final Answer:

   IndexOutOfRangeException because index 5 is invalid; fix by using index 4. -> Option C

5. Quick Check:

   Index 5 invalid, max index 4 [OK]

Hint: Last index is length minus one [OK]

Common Mistakes:

• Assuming index 5 is valid for length 5
• Confusing syntax error with runtime error
• Thinking array is uninitialized

5. You have an array int[] scores = {5, 10, 15};. You want to safely print the element at index i without causing an exception. Which code snippet correctly checks bounds before accessing?

hard

A. if (i >= 0 && i < scores.Length) Console.WriteLine(scores[i]); else Console.WriteLine("Invalid index");

B. if (i > 0 && i <= scores.Length) Console.WriteLine(scores[i]); else Console.WriteLine("Invalid index");

C. if (i >= 1 && i < scores.Length) Console.WriteLine(scores[i]); else Console.WriteLine("Invalid index");

D. if (i > 0 && i < scores.Length - 1) Console.WriteLine(scores[i]); else Console.WriteLine("Invalid index");

Solution

1. Step 1: Understand valid index range for array

   Valid indexes are from 0 up to Length - 1.

2. Step 2: Check each condition

   if (i >= 0 && i < scores.Length) Console.WriteLine(scores[i]); else Console.WriteLine("Invalid index"); correctly checks i >= 0 and i < Length, ensuring safe access.

3. Step 3: Identify incorrect conditions

   Other options exclude index 0 or allow invalid upper bounds.

4. Final Answer:

   if (i >= 0 && i < scores.Length) Console.WriteLine(scores[i]); else Console.WriteLine("Invalid index"); -> Option A

5. Quick Check:

   Bounds check: 0 ≤ i < Length [OK]

Hint: Check index >= 0 and < array length [OK]

Common Mistakes:

• Using > 0 instead of >= 0
• Allowing index equal to length
• Ignoring zero index as valid