Reading text files in C Sharp (C#) - Step-by-Step Execution

Concept Flow - Reading text files

Start

↓

Open file

↓

Read line or all text

↓

Process or display text

↓

Close file

↓

End

The program opens a text file, reads its content line by line or all at once, processes or shows the text, then closes the file.

Execution Sample

C Sharp (C#)

using System;
using System.IO;

class Program {
  static void Main() {
    string text = File.ReadAllText("example.txt");
    Console.WriteLine(text);
  }
}

This code reads all text from 'example.txt' and prints it to the console.

Execution Table

Step	Action	Evaluation	Result
1	Start program	N/A	Program begins
2	Call File.ReadAllText("example.txt")	File exists and readable	Reads entire file content into 'text'
3	Store content in variable 'text'	Content stored	Variable 'text' holds file text
4	Call Console.WriteLine(text)	Print text to console	Text appears on screen
5	Program ends	N/A	Program stops running

💡 Program ends after printing the file content.

Variable Tracker

Variable	Start	After Step 2	After Step 3	Final
text	null	"(file content)"	"(file content)"	"(file content)"

Key Moments - 2 Insights

Why do we use File.ReadAllText instead of reading line by line?

File.ReadAllText reads the whole file at once, which is simpler for small files. Reading line by line is better for large files or when processing each line separately. See execution_table step 2 for reading all at once.

What happens if the file does not exist?

The program will throw an exception and stop unless we handle it. This example assumes the file exists. Handling errors requires extra code not shown here.

Visual Quiz - 3 Questions

Look at the execution table, what is stored in 'text' after step 3?

A The entire content of the file

B Only the first line of the file

C An empty string

D The file path string

💡 Hint

Check the 'Result' column in step 3 of the execution_table.

At which step does the program print the file content to the screen?

A Step 2

B Step 4

C Step 3

D Step 5

💡 Hint

Look for Console.WriteLine in the 'Action' column of the execution_table.

If the file was very large, what might be a better approach than File.ReadAllText?

A Ignore the file size

B Use File.ReadAllText anyway

C Read the file line by line using StreamReader

D Delete the file before reading

💡 Hint

Refer to key_moments about reading line by line for large files.

Concept Snapshot

Reading text files in C#:
- Use File.ReadAllText("filename") to read whole file as string.
- Use StreamReader to read line by line for large files.
- Always ensure file exists to avoid errors.
- Print content with Console.WriteLine.
- Close files automatically with using or rely on File methods.

Full Transcript

This example shows how a C# program reads a text file named 'example.txt'. The program starts, then calls File.ReadAllText to read the entire file content into a string variable called 'text'. Next, it prints the content to the console using Console.WriteLine. Finally, the program ends. The variable 'text' holds the full file content after reading. Beginners often wonder why we read the whole file at once; this is simple for small files but not efficient for large files. If the file does not exist, the program will crash unless error handling is added. For large files, reading line by line with StreamReader is better. This trace helps visualize each step and variable change during execution.