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.