Read and write to files

• 2 minutes

Reading data from files and writing data to files are core concepts in .NET.

Tailwind Traders needs to write the total of all the individual store sales files to a new file, then load the file into the company's sales system.

Here, you learn how to use the File class to read and write to files.

Read data from files

Files are read through the ReadAllText method on the File class.

File.ReadAllText($"stores{Path.DirectorySeparatorChar}201{Path.DirectorySeparatorChar}sales.json");

The return object from ReadAllText is a string.

{
  "total": 22385.32
}

Parse data in files

This data in its string format doesn't do you much good. It's still just characters, but now in a format that you can read. You want the ability to parse this data into a format that you can use programmatically.

There are many ways to parse JSON files with .NET, including a community library known as Json.NET.

You can add the Json.NET package to your project by using NuGet:

dotnet add package Newtonsoft.Json

Then, add using Newtonsoft.Json to the top of your class file:

using Newtonsoft.Json; 

And use the JsonConvert.DeserializeObject method:

var salesJson = File.ReadAllText($"stores{Path.DirectorySeparatorChar}201{Path.DirectorySeparatorChar}sales.json");
var salesData = JsonConvert.DeserializeObject<SalesTotal>(salesJson);

Console.WriteLine(salesData.Total);

class SalesTotal
{
  public double Total { get; set; }
}

Write data to files

You learned how to write files in the previous exercise; it's just that you wrote an empty one. To write data to a file, use the same WriteAllText method, but pass in the data that you want to write.

var data = JsonConvert.DeserializeObject<SalesTotal>(salesJson);

File.WriteAllText($"salesTotalDir{Path.DirectorySeparatorChar}totals.txt", data.Total.ToString());

// totals.txt
// 22385.32

Append data to files

In the preceding example, the file is overwritten every time you write to it. Sometimes, you don't want that. You want to append data to the file instead of replacing it entirely. You can append data with the File.AppendAllText method. By default, File.AppendAllText creates the file if it doesn't already exist.

var data = JsonConvert.DeserializeObject<SalesTotal>(salesJson);

File.AppendAllText($"salesTotalDir{Path.DirectorySeparatorChar}totals.txt", $"{data.Total}{Environment.NewLine}");

// totals.txt
// 22385.32
// 22385.32

In the next exercise, you'll finish the sales-total project for Tailwind Traders by reading all the sales files and writing the grand total to a .txt file. The company's commerce system can then process the file.