File class static methods in C Sharp (C#) - Time & Space Complexity

Time Complexity: File class static methods

O(n)

Understanding Time Complexity

When using static methods of the File class, it is important to understand how the time taken grows as the file size or number of files increases.

We want to know how the work done by these methods changes when the input gets bigger.

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

string[] lines = File.ReadAllLines("data.txt");
foreach (string line in lines)
{
    Console.WriteLine(line);
}
    

This code reads all lines from a file into an array, then prints each line to the console.

Identify Repeating Operations

• Primary operation: Reading each line from the file and storing it in an array.
• How many times: Once for each line in the file, repeated for all lines.
• Secondary operation: Looping through the array to print each line, also once per line.

How Execution Grows With Input

As the number of lines in the file grows, the time to read and print grows roughly the same way.

Input Size (n lines)	Approx. Operations
10	About 10 reads and 10 prints
100	About 100 reads and 100 prints
1000	About 1000 reads and 1000 prints

Pattern observation: The total work grows directly with the number of lines; doubling lines doubles work.

Final Time Complexity

Time Complexity: O(n)

This means the time taken grows in a straight line with the number of lines in the file.

Common Mistake

[X] Wrong: "Reading a file with File.ReadAllLines is always fast and constant time regardless of file size."

[OK] Correct: The method reads every line, so the time depends on how many lines there are. Bigger files take longer.

Interview Connect

Understanding how file reading scales helps you explain performance in real apps, showing you know how input size affects work done.

Self-Check

"What if we used File.ReadLines instead of File.ReadAllLines? How would the time complexity change?"