Welcome to this lesson on reading data from archived files using C#. In our previous discussions, we explored how to handle ZIP archives, a crucial skill in managing compressed data forms in C#. Now, we're advancing to an equally important aspect: reading the actual content from these archived files and performing operations on it. This skill has broad applications, from data analysis to software management, where data is often stored compactly to save space. By the end of this lesson, you will be able to efficiently read data from a specific file within a ZIP archive and conduct basic operations, such as arithmetic calculations.

In our last lesson, we delved into the essentials of opening a ZIP archive using the System.IO.Compression namespace in C#. We covered how to open these archives, iterate through the files they contain, and access each file's name, providing a solid foundation for archive navigation. Remember that handling archives effectively is the first step; today's focus is on accessing and extracting the content within these files efficiently.

Before reading data from a ZIP archive, it's essential to comprehend the file and folder structure within such archives. A ZIP file can contain both files and directories, mimicking a file system structure. Each entry in a ZIP archive represents either a file or a directory, and each has a specific path that is relative to the root of the archive.

Consider a ZIP archive named archive.zip with the following structure:

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1archive.zip
2└── data.txt

In this structure, data.txt is a file located directly in the root of the archive, and its content is as follows:

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11 5 3 5 2 4 3

This understanding is crucial when you need to access files within the archive, as you'll need to specify their relative paths accurately when navigating through or targeting these entries.

In C#, when accessing entries in a ZIP archive, we use properties like FullName to identify each entry's full path within the archive and Name to get the entry's name. For example, if you want to access data.txt, knowing it is in the root allows you to use its name directly.

To access the data inside data.txt, we first need to access open the ZIP archive using the ZipFile.OpenRead method. This method returns a ZipArchive object, which allows us to iterate over the entries in the archive.

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1const string zipFileName = "archive.zip";
2
3// Open the ZIP archive for reading
4using (ZipArchive archive = ZipFile.OpenRead(zipFileName))
5{
6    // Iterate over each entry in the ZIP archive
7    foreach (ZipArchiveEntry entry in archive.Entries)
8    {
9        // Check if the entry name matches "data.txt"
10        if (entry.FullName == "data.txt")
11        {
12            Console.WriteLine($"Found file: {entry.FullName}");
13        }
14    }
15}

By iterating over the archive.Entries, we examine each entry to determine if its FullName matches our target file, data.txt. This allows us to ascertain its presence within the archive and subsequently perform further operations on it if it exists.

Once we've accessed the file within the archive, the next step is reading its content. We can use a StreamReader to manage the data efficiently, ensuring that we can handle text files without exhausting memory.

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1using (ZipArchive archive = ZipFile.OpenRead(zipFileName))
2{
3    foreach (ZipArchiveEntry entry in archive.Entries)
4    {
5        if (entry.FullName == "data.txt")
6        {
7            Console.WriteLine($"Found file: {entry.FullName}");
8            // Create a StreamReader to read the text content from the opened stream
9            using (StreamReader reader = new StreamReader(entry.Open()))
10            {
11                // Read the content of the entry
12                string content = reader.ReadToEnd();
13            }
14        }
15    }
16}

The entry.Open() method is used to get access to the data inside the file within the ZIP archive by creating a stream, which acts as a conduit or channel for data to flow from the file to your program. Once the stream is open, we utilize a StreamReader, which is a handy tool designed to read text efficiently from the stream, allowing us to fetch and process the content of the file in a straightforward manner.

With the file content successfully extracted, you can process these data bits. Let's consider the scenario where data.txt contains a list of integers you want to sum:

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1using (ZipArchive archive = ZipFile.OpenRead(zipFileName))
2{
3    foreach (ZipArchiveEntry entry in archive.Entries)
4    {
5        if (entry.FullName == "data.txt")
6        {
7            Console.WriteLine($"Found file: {entry.FullName}");
8            using (StreamReader reader = new StreamReader(entry.Open()))
9            {
10                string content = reader.ReadToEnd();
11
12                // Split the content into an array of numbers using spaces as separators
13                string[] numbers = content.Split(' ');
14
15                int sum = 0;
16
17                // Iterate over the array, parsing and summing the numbers
18                foreach (string number in numbers)
19                {
20                    if (!string.IsNullOrWhiteSpace(number))
21                    {
22                        sum += int.Parse(number);
23                    }
24                }
25
26                // Display the calculated sum
27                Console.WriteLine($"Sum of numbers in data.txt: {sum}");
28                // Expected output: Sum of numbers in data.txt: 23
29            }
30        }
31    }
32}

After reading the content from the file, we break it down into individual parts using spaces as separators, creating an array of strings where each string represents a number. This is accomplished with the content.Split(' ') method. Next, we utilize a loop to iterate over each element in this array. Within the loop, we check if the string element is not null or whitespace before converting it to an integer using int.Parse(), and we continually add these integers together to calculate their total sum.

In this lesson, you learned how to access and read data from files within a ZIP archive using the System.IO.Compression namespace in C#. Starting from verifying and opening a file within an archive, we proceeded through the process of reading its content efficiently with StreamReader and finally demonstrated processing extracted data to achieve a meaningful outcome.

These skills will set the foundation for the upcoming practice exercises where you'll apply what you've learned to real-world scenarios. As you continue with the course, remember these principles, as they form the backbone of effective large data handling in virtually any software application context. Happy coding!