Array bounds checking behavior in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Array bounds checking behavior

Start: Access array element

↓

Check index >= 0 and < array.Length?

No→Throw IndexOutOfRangeException

Yes↓

Return element at index

↓

End

When accessing an array element, C# checks if the index is within valid bounds. If not, it throws an exception; otherwise, it returns the element.

Execution Sample

C Sharp (C#)

int[] arr = {10, 20, 30};
int x = arr[1];
int y = arr[3];

This code accesses a valid index 1 and an invalid index 3 in the array, demonstrating bounds checking.

Execution Table

Step	Action	Index	Condition (0 <= index < Length)	Result	Output/Exception
1	Access arr[1]	1	True (0 <= 1 < 3)	Return element	20
2	Access arr[3]	3	False (3 >= 3)	Throw exception	IndexOutOfRangeException

💡 Execution stops at step 2 due to IndexOutOfRangeException caused by invalid index 3.

Variable Tracker

Variable	Start	After Step 1	After Step 2
arr	{10, 20, 30}	{10, 20, 30}	{10, 20, 30}
x	undefined	20	20
y	undefined	undefined	exception thrown, no value

Key Moments - 3 Insights

Why does accessing arr[3] cause an exception?

Because the array length is 3, valid indices are 0,1,2. Step 2 in execution_table shows index 3 is out of bounds, so C# throws IndexOutOfRangeException.

Does C# allow negative indices like arr[-1]?

No, negative indices fail the bounds check (index >= 0). The concept_flow shows any index outside 0 to Length-1 throws an exception.

What happens if you try to use the value of y after the exception?

The exception interrupts execution, so y is never assigned a value. variable_tracker shows y remains undefined after step 2.

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of x after step 1?

A 30

B 10

C 20

D undefined

💡 Hint

Check the 'Output/Exception' column at step 1 in execution_table.

At which step does the program throw an exception?

A Step 1

B Step 2

C No exception thrown

D Before step 1

💡 Hint

Look at the 'Result' and 'Output/Exception' columns in execution_table.

If the array length was 5, what would happen at step 2?

A Return element at index 3

B Throw IndexOutOfRangeException

C Return element at index 5

D Compile error

💡 Hint

Refer to the condition '0 <= index < Length' in execution_table and imagine Length=5.

Concept Snapshot

C# arrays check index bounds on every access.
If index < 0 or >= Length, an IndexOutOfRangeException is thrown.
Valid indices return the element at that position.
This prevents accessing invalid memory and bugs.
Always ensure your index is within 0 to Length-1.

Full Transcript

In C#, when you access an array element, the runtime checks if the index is within the valid range from zero up to one less than the array's length. If the index is valid, the element at that position is returned. If the index is outside this range, such as negative or too large, the runtime throws an IndexOutOfRangeException to prevent errors. For example, accessing arr[1] in an array of length 3 returns the second element, but accessing arr[3] throws an exception because the highest valid index is 2. This bounds checking helps keep programs safe and predictable.

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