StreamReader and StreamWriter in C Sharp (C#) - Step-by-Step Execution

Concept Flow - StreamReader and StreamWriter

Start

↓

Open file with StreamWriter

↓

Write text to file

↓

Close StreamWriter

↓

Open file with StreamReader

↓

Read text from file

↓

Close StreamReader

↓

End

This flow shows writing text to a file using StreamWriter, then reading it back using StreamReader.

Execution Sample

C Sharp (C#)

using System.IO;

var writer = new StreamWriter("test.txt");
writer.WriteLine("Hello World");
writer.Close();

var reader = new StreamReader("test.txt");
var text = reader.ReadLine();
reader.Close();

Writes 'Hello World' to a file and then reads it back.

Execution Table

Step	Action	File State	Variable Values	Output
1	Open StreamWriter for 'test.txt'	File opened for writing (empty)	writer=open, reader=null, text=null
2	WriteLine 'Hello World'	File contains 'Hello World\n'	writer=open, reader=null, text=null
3	Close StreamWriter	File saved with 'Hello World\n'	writer=closed, reader=null, text=null
4	Open StreamReader for 'test.txt'	File opened for reading	writer=closed, reader=open, text=null
5	ReadLine from file	File unchanged	writer=closed, reader=open, text='Hello World'
6	Close StreamReader	File closed	writer=closed, reader=closed, text='Hello World'

💡 StreamReader and StreamWriter closed, text read successfully.

Variable Tracker

Variable	Start	After Step 2	After Step 5	Final
writer	null	open	closed	closed
reader	null	null	open	closed
text	null	null	'Hello World'	'Hello World'

Key Moments - 2 Insights

Why do we need to close StreamWriter before opening StreamReader?

Closing StreamWriter flushes and saves the data to the file. Without closing, StreamReader might read incomplete or no data. See execution_table steps 3 and 4.

What happens if we try to read from StreamReader before writing anything?

The file would be empty or not created, so StreamReader.ReadLine() returns null or empty. In our trace, writing happens before reading (steps 2 and 5).

Visual Quiz - 3 Questions

Look at the execution_table, what is the value of 'text' after step 5?

A 'Hello World'

B null

C Empty string

D Throws error

💡 Hint

Check the 'Variable Values' column at step 5 in execution_table.

At which step is the file content actually saved to disk?

A Step 4 - Open StreamReader

B Step 2 - WriteLine

C Step 3 - Close StreamWriter

D Step 6 - Close StreamReader

💡 Hint

Look at the 'File State' column; file is saved when StreamWriter is closed.

If we skip closing StreamWriter, what likely happens when opening StreamReader?

A StreamReader reads full text

B StreamReader reads partial or no text

C Program crashes immediately

D StreamWriter closes automatically

💡 Hint

Refer to key_moments about why closing StreamWriter is important before reading.

Concept Snapshot

StreamWriter writes text to files; StreamReader reads text from files.
Always close StreamWriter to save data before reading.
Use WriteLine to write lines; ReadLine to read lines.
Remember to close both to free resources.
Simple way to save and load text data in C#.

Full Transcript

This example shows how to write text to a file using StreamWriter and then read it back using StreamReader in C#. First, StreamWriter opens the file and writes 'Hello World' with WriteLine. Then it is closed to save the data. Next, StreamReader opens the same file and reads the line back into a variable. Finally, StreamReader is closed. Variables track the open/closed state of streams and the text read. Closing StreamWriter before reading is important to ensure data is saved. This process is a basic way to handle text files in C#.