theme: Next,1

[fit] Reading and Writing Files

[fit] Persistance

So far we have seen how to input, manage, and output data in our programs.

However, this data is only stored in the computer's memory and thus when we stop our programs all of this information is lost.

[fit] Many choices

• Files
• Remote API
• Database

[fit] What is a "file"?

A sequence of characters (really more accurately bytes)

Similar to how a string is also just a sequence of characters.

[fit] Like a string but BIGGER

Our strings have typically been fairly small.

Files can be quite large.

Files should be able to store more than one thing.

This requires structure.

[fit] Information needs structure

Often called the file format.

[.column]

Many to choose from

• CSV
• JSON
• XML
• XLS
• INI
• YAML
• TOML
• PropertyList

[.column]

We will start with CSV

[fit] CSV - Comma Separated Value

Familiar if you have used a spreadsheet app like Excel, or Numbers.

The structure of a simple CSV file looks similar to this:

"Elon Musk",42,120000
"Grace Hopper",100,240000

[fit] First Row (aka Header)

The CSV file also allows us to have a first row (aka header) that describes the data for any human and computer reader.

"Name","Department","Salary"
"Elon Musk",42,120000
"Grace Hopper",100,240000

[fit] Number Tracker

Console app to keep track of a list of numbers

dotnet new sdg-console -o NumberTracker

cd NumberTracker
sdg warp NumberTracker

[fit] Using 3rd Party Libraries

We don't have to write the code for processing CSV

But it also doesn't come "out of the box" from C# or .NET

[fit] Enter packages to the rescue

[fit] Nuget

Packages for C# and .NET are cataloged at nuget.org.

There is a LOT there.

Not all of it quality.

Not all of it appropriate to our needs.

Have a discerning eye.

[fit] CSV library we are going to use

[fit] CsvHelper

[fit] Adding to our project

Run this from the same place you'd run dotnet run

dotnet add package CsvHelper

[fit] Adding code to save the list of numbers

But first...

Tell the code where to write the data.

StreamWriter

StreamWriter accepts data and sends it to a destination.

Process data in small pieces, a flow, like water running in a stream.

//               A new stream of information to WRITE
//                |
//                |               Name of file to write to
//                |               |
//                |               |
//                v               v
var fileWriter = new StreamWriter("numbers.csv");

[fit] Using the stream

Now that we have a way to send information to a file, we need some code that knows how to write in the CSV format.

Enter CsvWriter

//                  A new stream of CSV data
//                  |
//                  |         The stream to write to
//                  |         |
//                  |         |           Rules about formatting
//                  |         |           |
//                  |         |           |
//                  v         v           v
var csvWriter = new CsvWriter(fileWriter, CultureInfo.InvariantCulture);

[fit] Write out the numbers!

//
//  Where to write
//  |
//  |                     What data to write
//  |                     |
//  v                     v
   csvWriter.WriteRecords(numbers);
//
//  Tell the object that writes to the file
//  |
//  |         That we are done and close
//  |         |
//  v         v
   fileWriter.Close();

This is how the information flows through this code:

numbers
   |
   |
   ---> csvWriter.WriteRecords
                |
                |
                ---> fileWriter
                         |
                         |
                         ---> `numbers.csv`

If the user entered a sequence of numbers: 1, 42, 99, 3, and 17 our numbers.csv would look like this:

1
42
99
3
17

[fit] Load 'em up

Now let's read this information from the file at the beginning of the code.

Just as we have a StreamWriter we also have a StreamReader we can use to load data.

var fileReader = new StreamReader("numbers.csv");

And as we have a CsvWriter we also have a CsvReader we can use to read the CSV data.

// Create a configuration that indicates this CSV file has no header
var config = new CsvConfiguration(CultureInfo.InvariantCulture)
{
   // Tell the reader not to interpret the first
   // row as a "header" since it is just the
   // first number.
   HasHeaderRecord = false,
};
var csvReader = new CsvReader(fileReader, config);

Finally, instead of WriteRecords we have a way to ReadRecords.

var numbers = csvReader.GetRecords<int>().ToList();
fileReader.Close();

NOTE: You must also add using System.Linq to be able to use ToList

[fit] Handling the case where there is no file

Algorithm:

• Detect if there is no file.
• If there is a file, use a StreamReader pointing to the file
• If there isn't a file, use a StringReader with an empty string

// Generic reader that can be a StreamReader *OR* a StringReader
TextReader reader;
// If the file exists
if (File.Exists("numbers.csv"))
{
  // Assign a StreamReader to read from the file
  reader = new StreamReader("numbers.csv");
}
else
{
  // Assign a StringReader to read from an empty string
  reader = new StringReader("");
}

Alternate algorithm:

• Create an empty list of numbers.
• If there is a file, use StreamReader + CsvReader to replace the contents of the list

Could we make the same update to SuncoastHumanResources?

• Add the loading and unloading code
• Except instead of <int> we will process <Employee>
• We also need to include the first row of header information

Code!