In this exercise, you build a small billing calculator for a store. You organize your tax and total calculations into their own file under a Billing namespace, then bring that namespace into your main program with a using directive. You practice defining methods with parameters and return values, giving a parameter a default value, overriding that default with a named argument, and calling methods through the class name they belong to.
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/08-create-a-billing-calculator subfolder.
Select the Billing.cs file. It contains an empty shell for a Calculator class inside a Billing namespace — you’ll add methods to it in the next section:
namespace Billing
{
public static class Calculator
{
// Add the CalculateTax and CalculateTotal methods here
}
}
Now 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:
// Bring in the Billing namespace
// Set up the order
// Calculate tax and total
// Print the receipt
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.
Grouping related methods under a namespace keeps a growing project organized, and makes it clear at a glance where a method comes from.
In Billing.cs, replace the // Add the CalculateTax and CalculateTotal methods here comment with the following methods:
public static double CalculateTax(double subtotal, double taxRate = 0.08)
{
return subtotal * taxRate;
}
public static double CalculateTotal(double subtotal, double tax)
{
return subtotal + tax;
}
taxRate = 0.08 gives the parameter a default value. If a caller doesn’t supply a tax rate, 8% is used automatically.double as their return type and use return to hand a usable value back to whatever code calls them.Save Billing.cs.
Billing.cs and Program.cs are two separate files in the same project, so you use a using directive to bring the Billing namespace into scope before you can call Calculator’s methods by name.
In Program.cs, beneath the // Bring in the Billing namespace comment, add the following line:
using Billing;
Beneath the // Set up the order comment, add the following line:
double subtotal = 49.99;
Beneath the // Calculate tax and total comment, add the following lines:
double tax = Calculator.CalculateTax(subtotal);
double total = Calculator.CalculateTotal(subtotal, tax);
Calculator.CalculateTax(subtotal) calls the method through its class name, Calculator. Since no tax rate is supplied, the default value of 0.08 is used.tax value is then passed straight into CalculateTotal, which returns the final amount due.Beneath the // Print the receipt comment, add the following lines:
Console.WriteLine($"Subtotal: {subtotal:C}");
Console.WriteLine($"Tax: {tax:C}");
Console.WriteLine($"Total: {total:C}");
The :C format specifier displays each number as currency.
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/08-create-a-billing-calculator
dotnet run
You should see something like:
Subtotal: $49.99
Tax: $4.00
Total: $53.99
Tip: If you have the C# Dev Kit extension installed, you can also select the Run button that appears above your code instead of using the terminal. You only need to
cdinto the project folder once per terminal session — after that,dotnet runalone will work for the rest of this exercise.
Not every order uses the standard tax rate. You’ll add a second order that passes a different rate using a named argument, which makes the call easy to read even though it skips over the default.
Beneath your existing code (but still above the // Print the receipt output, or after it — either works), add a second order:
double outOfStateSubtotal = 80.00;
double outOfStateTax = Calculator.CalculateTax(outOfStateSubtotal, taxRate: 0.05);
double outOfStateTotal = Calculator.CalculateTotal(outOfStateSubtotal, outOfStateTax);
Console.WriteLine($"\nOut-of-state subtotal: {outOfStateSubtotal:C}");
Console.WriteLine($"Out-of-state tax: {outOfStateTax:C}");
Console.WriteLine($"Out-of-state total: {outOfStateTotal:C}");
taxRate: 0.05 names the parameter explicitly, overriding its 0.08 default with 5% for this order.taxRate: 0.05 immediately knows what that 0.05 represents, without needing to check the method’s parameter order.Run the program again with dotnet run. You should now see both receipts printed, each using a different tax rate.
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 compiler error do I get if I try to use a class from a namespace without a using directive for it? How do I read that error message to figure out the fix?”
After the AI responds: Try commenting out your using Billing; line at the top of Program.cs and running dotnet run again. Read the compiler error that appears in your terminal, then use what the AI told you to explain, in your own words, what the error is telling you. When you’re done, uncomment the using Billing; line so your program runs again.
AI Prompt:
“In C#, how can I create a shorter alias for a class from another namespace using a using directive?”
After the AI responds: Take a look at the examples the AI gives you, then try adding a using alias for Calculator at the top of Program.cs, and update your method calls to use the shorter name instead.