In this exercise, you build a small event logger. Your program appends a new entry to a log file every time it “runs,” without erasing what’s already there, then reads every entry back one line at a time to display the full history and count how many events have been logged. You practice using File.AppendAllText() to add to a file, and a StreamReader with .ReadLine() in a loop that keeps going until it reaches the end of the file.
This exercise takes approximately 25 minutes.
You’ll write and run your code in Visual Studio Code. The starter code for this exercise lives in a GitHub repository — you’ll clone that repo now if you haven’t already.
Tip: If you’ve already cloned the repo during another exercise, skip to the next section.
Open a new Visual Studio Code window (File > New Window).
On the Welcome page, select Clone Git Repository… (or open the Command Palette with Ctrl+Shift+P and run Git: Clone).
Paste the following URL and press Enter:
https://github.com/MicrosoftLearning/mslearn-csharp-programming.git
When the file selection dialog appears, create a new folder in a convenient location to hold the repo (for example, mslearn-csharp), select it, and click Select as Repository Destination.
After the clone completes, select Open to open the folder in VS Code.
In the VS Code file explorer, navigate to the Labfiles/09-create-a-log-file-viewer subfolder.
Select the Program.cs file. You’ll see a set of guiding comments that act as an outline for your program — each one marks where a specific piece of code belongs:
// Log a new event
// Read back every logged event
// Count the logged events
Remember, comments are ignored by C# when the program runs. They’re just there to help you organize your code. In the steps that follow, you’ll add code beneath each comment.
If VS Code shows a prompt about restoring project dependencies, select Restore.
Every time the program runs, it should add one new entry to log.txt without erasing the entries from earlier runs.
Beneath the // Log a new event comment, add the following lines:
string logFile = "log.txt";
string timestamp = DateTime.Now.ToString("HH:mm:ss");
File.AppendAllText(logFile, $"[{timestamp}] Application started\n");
Console.WriteLine("Event logged.\n");
File.AppendAllText() adds to the end of log.txt, keeping every entry from previous runs intact. If the file doesn’t exist yet, C# creates it.DateTime.Now.ToString("HH:mm:ss") gives each entry its own timestamp, so you can tell runs apart.Save the file, then run your program. Open a new terminal (Terminal > New Terminal), navigate to the project folder, and run the program:
cd Labfiles/09-create-a-log-file-viewer
dotnet run
You should see:
Event logged.
Run the program two or three more times with dotnet run. Each run adds a new line to log.txt — open log.txt in the VS Code file explorer to see the entries building up, each with its own timestamp.
Tip: If you have the C# Dev Kit extension installed, you can also select the Run button that appears above your code instead of using the terminal. You only need to
cdinto the project folder once per terminal session — after that,dotnet runalone will work for the rest of this exercise.
Now you’ll read log.txt one line at a time using a StreamReader. Its .ReadLine() method returns the next line each time you call it, and returns null once there are no more lines left — that’s your signal to stop.
Beneath the // Read back every logged event comment, add the following lines:
Console.WriteLine("Event history:");
using StreamReader reader = new StreamReader(logFile);
string line;
while ((line = reader.ReadLine()) != null)
{
Console.WriteLine(line);
}
using StreamReader reader = ... opens the file and guarantees it’s closed automatically at the end of this block, even if something goes wrong while reading.(line = reader.ReadLine()) != null does two things every time through the loop: it reads the next line into line, and checks whether that line was actually there. When .ReadLine() runs out of lines, it returns null, and the loop stops.Run the program again with dotnet run. You should now see every entry logged so far, printed one per line:
Event logged.
Event history:
[09:12:04] Application started
[09:12:41] Application started
[09:13:07] Application started
Finally, you’ll keep a running count of how many entries were read, so you know at a glance how many times the program has run in total.
Beneath the // Count the logged events comment, add the following lines:
string[] allLines = File.ReadAllLines(logFile);
Console.WriteLine($"\nTotal events logged: {allLines.Length}");
File.ReadAllLines() reads the whole file again, this time as a string[] where each element is one line — a quick way to get a count without writing your own loop.allLines.Length reports how many lines (and therefore how many logged events) the file contains.Your complete program should now look like this:
// Log a new event
string logFile = "log.txt";
string timestamp = DateTime.Now.ToString("HH:mm:ss");
File.AppendAllText(logFile, $"[{timestamp}] Application started\n");
Console.WriteLine("Event logged.\n");
// Read back every logged event
Console.WriteLine("Event history:");
using StreamReader reader = new StreamReader(logFile);
string line;
while ((line = reader.ReadLine()) != null)
{
Console.WriteLine(line);
}
// Count the logged events
string[] allLines = File.ReadAllLines(logFile);
Console.WriteLine($"\nTotal events logged: {allLines.Length}");
Run the program a few more times with dotnet run and watch the total climb by one each time.
Using an AI assistant (like Copilot) is a great way to explore a programming language at your own pace. Try each prompt below, read the response carefully, and test the code examples it gives you in VS Code.
AI Prompt:
“In C#, what exception is thrown if I try to read a file with StreamReader or File.ReadAllLines and the file doesn’t exist yet? How can I check safely as a beginner?”
After the AI responds: Delete log.txt from your project folder, then run the program once. Since File.AppendAllText() creates the file automatically, this particular program won’t crash — but take a look at the examples the AI gives you, then try adding a check with File.Exists(logFile) before the reading section, so your program could handle a truly missing file gracefully in a program that doesn’t create one automatically.
AI Prompt:
“In C#, how can I let a user choose between appending to a file and overwriting it, based on their input?”
After the AI responds: Take a look at the examples the AI gives you, then try adding a prompt at the start of your program asking the user whether they want to clear the log, and use File.WriteAllText() to erase it if they say yes.