Exercise - Throw and catch exceptions in a C# console application - Training

15 minutes

In this exercise, you'll develop a try code block and catch clause in the top-level statements, create and throw exceptions in the MakeChange method, and then complete the catch code block using an exception object. You complete the following tasks during this exercise:

Update top-level statements: Implement a try-catch pattern in the top-level statements. The try code block will contain the call to MakeChange.
Update MakeChange method: Create and throw exceptions for "Insufficient till" and "Underpayment" issues.
Update the catch code block to use properties of the thrown exception.
Verification test: perform verification tests for the code that you develop in this exercise.

Add a `try-catch` pattern to the top-level statements

In this task, you'll enclose the call to the MakeChange method inside a try statement and create the corresponding catch clause.

Ensure that the Program.cs file is open in the Visual Studio Code Editor.

Locate the following code lines:

// MakeChange manages the transaction and updates the till 
string transactionMessage = MakeChange(itemCost, cashTill, paymentTwenties, paymentTens, paymentFives, paymentOnes);

// Backup Calculation - each transaction adds current "itemCost" to the till
if (transactionMessage == "transaction succeeded")
{
    Console.WriteLine($"Transaction successfully completed.");
    registerCheckTillTotal += itemCost;
}
else
{
    Console.WriteLine($"Transaction unsuccessful: {transactionMessage}");
}

Take a minute to consider the purpose of this code.

Notice that MakeChange returns a string value. The return value is assigned to a variable named transactionMessage. If transactionMessage is equal to "transaction succeeded", then the cost of the purchased item is added to registerCheckTillTotal. The variable registerCheckTillTotal is used to verify the till balance calculated by the MakeChange method.

To enclose the call to the MakeChange method in a try statement code block, update your code as follows:

try
{
    // MakeChange manages the transaction and updates the till 
    string transactionMessage = MakeChange(itemCost, cashTill, paymentTwenties, paymentTens, paymentFives, paymentOnes);
}

Add the following catch clause after the try statement code block:
```
catch
{
}
```
You'll finish developing the catch clause once you've created and thrown the exceptions.

Create and throw exceptions in the `MakeChange` method

In this task, you'll update MakeChange to create and throw custom exceptions when a transaction can't be completed.

The MakeChange method includes two issues that should result in exceptions:

Underpayment issue: This issue occurs when the customer offers a payment that is less than the item cost. If the customer hasn't offered sufficient payment, MakeChange should throw an exception.
Insufficient till issue: This issue occurs when the till doesn't contain the bills required to produce the exact change. If the till can't make exact change, MakeChange should throw and exception.

Scroll down to the MakeChange method.

Locate the following code lines:

if (changeNeeded < 0)
    transactionMessage = "Not enough money provided.";

Take a minute to consider the issue that this code is addressing.

If changeNeeded is less than zero, the customer has not provided enough money to cover the purchase price of the item they're buying. The purchase price and the money provided by the customer are parameters of the MakeChange method. The method is unable to complete the transaction when the customer doesn't provide enough money. In other words, the operation fails.

There are two exception types that appear to match these conditions:
- InvalidOperationException: An InvalidOperationException exception should only be thrown when the operating conditions of a method don't support the successful completion of a particular method call. In this case the operating conditions are established by the parameters supplied to the method.
- ArgumentOutOfRangeException - An ArgumentOutOfRangeException exception should only be thrown when the value of an argument is outside the allowable range of values as defined by the invoked method. In this case the money provided must be greater than the cost of the item.
Either exception type could work, but InvalidOperationException is a slightly better match in the context of this application.

Update the code as follows:

if (changeNeeded < 0)
    throw new InvalidOperationException("InvalidOperationException: Not enough money provided to complete the transaction.");

Scroll down to locate the following code lines:

if (changeNeeded > 0)
    transactionMessage = "Can't make change. Do you have anything smaller?";

Take a minute to consider the issue that this code is addressing.

If changeNeeded is greater than zero after the while loops that prepare the change, then the till has run out of bills that can be used to make change. The method is unable to complete the transaction when the till lacks the bills required to make change. In other words, the operation fails.

The InvalidOperationException exception should be used to create the exception.

Update the code as follows:

if (changeNeeded > 0)
    throw new InvalidOperationException("InvalidOperationException: The till is unable to make the correct change.");

Complete the `catch` code block

In this task, you'll update the catch clause to catch a specific exception type.

Scroll up above the MakeChange method, and locate the following code:
```
catch
{
}    
```
To catch the exception type thrown in the MakeChange method, update the code as follows:
```
catch (InvalidOperationException e)
{
    Console.WriteLine($"Could not complete transaction: {e.Message}");
}    
```
The InvalidOperationException exception object thrown in MakeChange will be caught, but other exception types won't. Since you're not prepared to handle other exception types, it's important to let them be caught lower in the call stack. If you become aware that other exception types are expected within MakeChange, you can add additional catch clauses.
Use the File menu to save your updates.

Convert the MakeChange method from "string" to "void" and access exception properties

In this task, you'll update MakeChange to be of type void, and then use exception properties to communicate issue details to the user.

Scroll to the top of the MakeChange method.

To convert the MakeChange method from type string to type void, update the code as follows:

static void MakeChange(int cost, int[] cashTill, int twenties, int tens = 0, int fives = 0, int ones = 0)

Delete the following variable declaration:
```
string transactionMessage = "";
```
Scroll to the bottom of the MakeChange method.

Delete the following code lines:

if (transactionMessage == "")
    transactionMessage = "transaction succeeded";

return transactionMessage;

Scroll up to the top-level statements and locate the try code block.

Update the try code block as follows:

try
{
    // MakeChange manages the transaction and updates the till 
    MakeChange(itemCost, cashTill, paymentTwenties, paymentTens, paymentFives, paymentOnes);

    Console.WriteLine($"Transaction successfully completed.");
    registerCheckTillTotal += itemCost;
}

Locate and then delete the following code lines:
```
// Backup Calculation - each transaction adds current "itemCost" to the till
if (transactionMessage == "transaction succeeded")
{
    Console.WriteLine($"Transaction successfully completed.");
    registerCheckTillTotal += itemCost;
}
else
{
    Console.WriteLine($"Transaction unsuccessful: {transactionMessage}");
}
```
The try and catch code blocks are now communicating the transaction "success" and "failure" messages to the user. Since the exception's Message property describes the issue, a single Console.WriteLine() statement addresses both issues. Your code is easier to read and maintain after these updates.
Use the File menu to save your updates.

Check your work

In this task, you'll run your application and verify that your updated code works as intended.

Scroll up to find the while loop in the top-level statements.

This loop is used to iterate through the transactions.
Locate the following code a few lines before the start of the while loop.
```
int transactions = 10;
```
Update the number of transactions to 40 as follows:
```
int transactions = 40;
```
Locate the following code line inside the while loop.
```
int itemCost = valueGenerator.Next(2, 20);
```
Update the itemCost random number generator as follows:
```
int itemCost = valueGenerator.Next(2, 50);
```
This cost range is a better match for items that customers will be purchasing.
Use the File menu to save your updates.
On the Run menu, select Start Debugging.
Review the output in the TERMINAL panel.

Verify that the messages associated with the two exception types are displayed:

Your transactions report should include the following "Could not make transaction" messages:

Customer is making a $42 purchase
         Using 2 twenty dollar bills
         Using 0 ten dollar bills
         Using 0 five dollar bills
         Using 0 one dollar bills
Could not make transaction: InvalidOperationException: Not enough money provided to complete the transaction.

Customer is making a $23 purchase
         Using 2 twenty dollar bills
         Using 0 ten dollar bills
         Using 0 five dollar bills
         Using 1 one dollar bills
Cashier prepares the following change:
         A five
         A five
         A one
         A one
Could not make transaction: InvalidOperationException: The till is unable to make change for the cash provided.

Congratulations, you've debugged the cash register application to fix a code logic issue, and you've updated the application to use proper exception handling techniques.

Note

The reported output shows that the money till is no longer balanced. There are additional logic bugs in the code. A challenge project module is available if you're interested in demonstrating your Visual Studio Code debugging skills!

Continue

Exercise - Throw and catch exceptions in a C# console application

Add a try-catch pattern to the top-level statements

Create and throw exceptions in the MakeChange method

Complete the catch code block

Convert the MakeChange method from "string" to "void" and access exception properties

Check your work

Feedback

Add a `try-catch` pattern to the top-level statements

Create and throw exceptions in the `MakeChange` method

Complete the `catch` code block