Benefits of asynchronous file operations ( async/ await)

1. Introduction

Imagine your app is a cafe. And this cafe has only one waiter (that's our main thread of execution). When a customer (user) orders a coffee (some action), the waiter goes to the kitchen, makes the coffee, and only after that returns to the table to take the next order.

Now imagine someone ordered... borscht. Not just borscht, but a huge pot of borscht that needs two hours to cook! What will our waiter do? He'll stand by the stove for those two hours, doing nothing except waiting for the borscht to finish. All other customers will sit, wave their hands, complain, and he just won't see them. The cafe is "frozen" because the waiter is blocked.

In programming this is called a blocking operation. When you call a normal method to read or write a file (for example, FileStream.Read() or StreamReader.ReadLine()), your current thread of execution is blocked. It stops executing everything else until the I/O operation completes.

Let's look at a simple example:

// Let's create a "large" file for demonstration
string largeFilePath = "LargeOrder.txt";
using (StreamWriter sw = new StreamWriter(largeFilePath))
{
    for (int i = 0; i < 1000000; i++) // 1 million lines
        sw.WriteLine($"Stroka {i}: Kakaya-to ochen' vazhnaya informatsiya...");
} // The file is closed here so it can be read

// !!! WARNING: This is a blocking operation !!!
string content = File.ReadAllText(largeFilePath);

Run this code. You'll see that while writing and reading files the program "freezes". The console doesn't accept input, no new messages are printed, until the file is fully read. Only after that the following lines will execute.

This might not be so noticeable in tiny console apps, but imagine:

• GUI application: You click "Load file" and the program window "freezes". You can't move the window, press other buttons, menus don't respond. That's a terrible user experience.
• Web server: The server handles user requests. If one request requires reading a very large file, then all other requests will wait in line until that one thread is freed. That leads to huge latency and poor scalability.

That's why we need asynchrony!

2. Advantages of asynchronous file operations

Asynchrony doesn't make the disk faster. The disk will still work at the same speed. Asynchrony is about not waiting for a slow operation to finish, and freeing the execution thread to do other work.

Back to our cafe. Now the waiter is smart and multitasking. When a customer orders a "VERY BIG BORSHCH" (reading a large file), the waiter doesn't stand by the stove. He starts the borscht cooking, then returns to the hall and starts taking orders from other tables, clearing dishes, serving other customers. As soon as the borscht is ready, the kitchen calls him, he returns, grabs the borscht and delivers it to the customer.

Key difference: the waiter is not blocked waiting for the borscht. He productively uses that time for other tasks.

Application responsiveness (User Interface Responsiveness)

This is probably the most noticeable and important benefit for most desktop and mobile apps. If your program does something long (reads a file, downloads data from the internet, processes large amounts of data), the async approach allows you to:

• Keep the UI interactive: The user can keep pressing buttons, moving windows, viewing other info while a heavy operation runs in the background.
• Show progress indicators: You can show a nice loading animation or progress bar so the user knows the app hasn't hung and is working.

Imagine that instead of a "frozen" console while the waiter reads a file, you'd see him continuing to take other "orders" (for example, showing another message or reacting to user input), while the file read happens somewhere "in the background". That's much better!

Efficient resource usage and scalability

This is critical for server apps (for example, web services, APIs, backends) that must serve many users simultaneously.

• Don't waste threads: In the synchronous model each "long" user request blocks one thread on the server. If you have 1000 such requests, you'll need 1000 threads. Each thread consumes memory and CPU resources. The OS spends time context-switching between these threads. Asynchrony lets the same thread, while waiting for an I/O operation to finish, start processing another request. When the I/O completes, it returns to the first request.
• Free up the CPU: While data is being read from disk or transferred over the network, the processor is idle. Asynchrony lets it work on other useful computations during that time.
• Less memory: Fewer active threads means lower RAM usage by the server.

3. Simplifying code (in C# with async/await)

Writing async code used to be hard, verbose and error-prone. You had to manually manage threads, callbacks and synchronization. It was like trying to build a space station from LEGO in complete darkness.

But C# introduced the async and await keywords. It's like a magic wand that lets you write async code almost as simply and readably as regular synchronous code. You just tell the compiler: "This part might take long, wait, but don't block the whole world."

// This is an EXAMPLE of how async code might look (no deep explanations yet)
// We'll dig into this in later lectures!

public static async Task Main(string[] args) // This is how an async Main looks
{
    Console.WriteLine("Program starts reading a VERY LARGE FILE asynchronously...");

    Stopwatch stopwatch = Stopwatch.StartNew();
    
    // !!! WARNING: Asynchronous operation !!!
    await File.ReadAllTextAsync(largeFilePath); // This does not block the thread
    
    stopwatch.Stop();

    Console.WriteLine($"File read! Time spent: {stopwatch.ElapsedMilliseconds} ms.");
    // There could be other work here while the file was being read!
}

Note: asynchrony does not make the I/O operation itself faster at the disk level. If reading a 1 GB file takes 15 seconds, it'll still take 15 seconds. The difference is what your CPU and threads do during those seconds. In the synchronous case they are idle, in the asynchronous case they work on other tasks.

Let's summarize this in a small table:

In essence, asynchrony is a way to make your app more responsive and scalable, especially when interacting with external slow resources like the file system or network. It doesn't replace buffering; it complements it. Buffering speeds up the actual data transfer, while asynchrony ensures your program doesn't stand still while that transfer happens.

4. Behind the scenes

Real-world examples: video editors, games, websites

Almost all modern programs that work with large files use asynchronous approaches. Popular media players don't block the UI when loading a movie. Servers don't "freeze" when one client downloads a huge file. Even a simple backup program or cloud client does everything "in the background" so the user can continue working.

How it works (plain language)

Asynchronous file methods in .NET (for example, ReadAsync, WriteAsync) actually use OS capabilities that allow not blocking the program thread during long operations. This is possible thanks to system calls that tell the OS: "Read this file for me, and when you're done — let me know."

Visual element: how async reading works (diagram)

sequenceDiagram
    participant UserCode as Vash kod
    participant OS as Operatsionnaya sistema
    participant Disk as Disk

    UserCode->>OS: Zapros na asinhronnoe chtenie faila
    OS->>Disk: Chitaet dannye
    UserCode->>UserCode: Prodolzhaet vypolnyat' drugie zadachi
    OS->>OS: Ozhidaet zaversheniya chteniya
    Disk-->>OS: Dannye gotovy
    OS-->>UserCode: Soobshchenie o zavershenii chteniya
    UserCode->>UserCode: Obrabatyvaet dannye

5. Where async operations give the biggest win

• GUI applications (UI doesn't get blocked).
• Servers that handle many concurrent file requests.
• Scripts that process large volumes of data in automated modes (for example, backups).
• Tools that work with slow or network drives.

The more data and the slower the storage, the more noticeable the benefit of asynchrony. Even if you don't build huge apps, it's good to get used to Async methods: their support is a standard in modern C#.

Now you know why asynchronous file operations are not just a trendy thing, but an important technique for building fast and responsive apps on .NET 9. In the next lectures we'll dive into syntax, practice and typical use cases of ReadAsync, WriteAsync and their friends!

6. Asynchrony

We'll study exactly how asynchrony works in levels 55-62. Right now I just want to introduce you to the concept. If you suddenly didn't understand anything in this level — no problem. Just skip these lectures and tasks, and come back to them after learning about asynchrony.

Parallelism vs asynchrony

If you launched several Windows apps on your computer and they all do something at the same time, programmers will say that tasks are running in parallel.

If you minimized a game on your phone and switched to something else, and the minimized app is paused, that's more like asynchronous work. Asynchrony is less about simultaneous execution and more about simultaneous waiting.

Example

Suppose you decided to do house chores. You loaded the dishwasher, and while it's running you put clothes into the washing machine. While the washer is washing, you put a pie in the oven. You work alone, but do many things at once because you switch to something else instead of waiting for a particular operation to finish.

As soon as the washer finishes, it notifies you and you return to dealing with the laundry. From its point of view, you just waited for it to finish, although you were working the whole time.

You can't simultaneously load the dishwasher, put a pie in the oven, and remove laundry from the washer. But if one of them is busy, you're not obliged to just wait — you can use that time productively.

Important nuance

If you have only one task, you won't see any difference between "waiting for the task to finish" and "being able to work on other tasks while waiting". But if there are many tasks, the difference becomes noticeable.

7. Common mistakes and pitfalls

The most common misconception: if you just call an async method, everything will magically get faster. In reality, if you use it incorrectly (for example, forget to await), the code becomes "unpredictable": the result isn't ready yet but the program already uses it. In GUI apps event handlers almost always should be async, otherwise the UI will "freeze".

Another thing: asynchrony is not about speeding up the read/write itself, but about allowing your program to be efficient and responsive during slow operations.

What's next?

Now that we understand why we need asynchrony, it's time to learn how to use it. In the next lecture we'll dive into the syntax of asynchronous file reading and writing, get familiar with async versions of methods (for example, ReadAsync and WriteAsync) and start writing our first real async code! It will be interesting, stay tuned!