When clause in catch in C Sharp (C#) - Mini Project: Build & Apply

Handling Exceptions with When Clause in Catch

📖 Scenario: Imagine you are writing a program that processes user input numbers. Sometimes, the input might cause errors like division by zero or invalid format. You want to catch these errors but handle them differently based on the error details.

🎯 Goal: You will create a program that uses try-catch blocks with when clauses to catch exceptions only when certain conditions are met.

📋 What You'll Learn

Create a variable called numbers with the values 10, 0, and 5.

Create a variable called index and set it to 1.

Use a try block to divide 100 by the number at numbers[index].

Use a catch block with a when clause to catch DivideByZeroException only when the exception message contains the word zero.

Use another catch block with a when clause to catch IndexOutOfRangeException only when index is greater than 2.

Print the appropriate messages inside each catch block.

Print "Division result: {result}" if no exception occurs.

💡 Why This Matters

🌍 Real World

In real programs, you often need to handle errors carefully depending on the exact problem. Using <code>when</code> clauses helps you write cleaner and more precise error handling.

💼 Career

Understanding advanced exception handling is important for writing robust software that can handle unexpected situations gracefully, a key skill for software developers.

1

Create the initial data variables

Create a list of integers called numbers with the values 10, 0, and 5.

C Sharp (C#)

# Your code here

Hint

Use an array of integers with the exact values given.

2

Add the index variable

Create an integer variable called index and set it to 1.

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        int[] numbers = {10, 0, 5};
        // Your code here
    }
}

Hint

Declare index as an integer and assign it the value 1.

3

Add try block with division

Add a try block that divides 100 by numbers[index] and stores the result in an integer variable called result.

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        int[] numbers = {10, 0, 5};
        int index = 1;
        // Your code here
    }
}

Hint

Use a try block and perform the division inside it.

4

Add catch blocks with when clauses and output

Add a catch block for DivideByZeroException with a when clause that checks if ex.Message.Contains("zero"). Inside it, print "Cannot divide by zero!". Add another catch block for IndexOutOfRangeException with a when clause that checks if index > 2. Inside it, print "Index is out of range!". Finally, if no exception occurs, print "Division result: {result}".

C Sharp (C#)

using System;

class Program
{
    static void Main()
    {
        int[] numbers = {10, 0, 5};
        int index = 1;
        try
        {
            int result = 100 / numbers[index];
            // Your code here
        }
        catch (DivideByZeroException ex) when (ex.Message.Contains("zero"))
        {
            // Your code here
        }
        catch (IndexOutOfRangeException) when (index > 2)
        {
            // Your code here
        }
    }
}

Hint

Use catch (DivideByZeroException ex) when (ex.Message.Contains("zero")) and print the message. Use catch (IndexOutOfRangeException) when (index > 2) and print the other message. Print the result if no exception.

Practice

1.

What does the when clause do in a catch block in C#?

easy

A. It adds a condition to run the catch block only if the condition is true.

B. It defines a new exception type to catch.

C. It skips the catch block entirely.

D. It makes the catch block run before the try block.

Solution

1. Step 1: Understand the purpose of the when clause

   The when clause adds a condition to a catch block.

2. Step 2: Effect of the when clause in exception handling

   The catch block runs only if the condition after when is true, otherwise it skips.

3. Final Answer:

   It adds a condition to run the catch block only if the condition is true. -> Option A

4. Quick Check:

   when clause = conditional catch [OK]

Hint: when clause filters catch by condition [OK]

Common Mistakes:

• Thinking when defines a new exception type
• Assuming catch runs always regardless of when
• Confusing when with finally block

2.

Which of the following is the correct syntax to use a when clause in a catch block?

try {
    // code
} catch (Exception ex) _____ {
    // handle
}

easy

A. while (ex.Message.Contains("error"))

B. if (ex.Message.Contains("error"))

C. where (ex.Message.Contains("error"))

D. when (ex.Message.Contains("error"))

Solution

1. Step 1: Recall the correct keyword for condition in catch

   The correct keyword to add a condition in catch is when.

2. Step 2: Match the syntax with the options

   Only when (ex.Message.Contains("error")) uses when correctly with the condition.

3. Final Answer:

   when (ex.Message.Contains("error")) -> Option D

4. Quick Check:

   Use 'when' keyword for catch condition [OK]

Hint: Use 'when' keyword, not if/where/while in catch [OK]

Common Mistakes:

• Using if instead of when in catch
• Confusing when with where or while
• Missing parentheses after when

3.

What will be the output of this code?

try {
    throw new InvalidOperationException("Invalid operation");
} catch (InvalidOperationException ex) when (ex.Message.Contains("Invalid")) {
    Console.WriteLine("Caught invalid operation");
} catch (Exception) {
    Console.WriteLine("Caught general exception");
}

medium

A. Caught general exception

B. Caught invalid operation

C. No output

D. Runtime error

Solution

1. Step 1: Identify the thrown exception and matching catch

   The code throws InvalidOperationException with message containing "Invalid".

2. Step 2: Check the when condition in the first catch

   The first catch has a when clause checking if message contains "Invalid" which is true, so it runs.

3. Final Answer:

   Caught invalid operation -> Option B

4. Quick Check:

   when condition true runs first catch [OK]

Hint: Check exception type and when condition carefully [OK]

Common Mistakes:

• Ignoring the when condition and picking second catch
• Assuming no output if when is used
• Confusing exception types

4.

Find the error in this code snippet:

try {
    // some code
} catch (Exception ex) when ex.Message == "Error" {
    Console.WriteLine("Error caught");
}

medium

A. catch block must not have a condition

B. Cannot use when with Exception type

C. Missing parentheses around the when condition

D. Console.WriteLine is not allowed in catch

Solution

1. Step 1: Check syntax of when clause

   The condition after when must be enclosed in parentheses.

2. Step 2: Identify the missing parentheses in the code

   The code uses when ex.Message == "Error" without parentheses, which is invalid syntax.

3. Final Answer:

   Missing parentheses around the when condition -> Option C

4. Quick Check:

   when condition needs parentheses [OK]

Hint: Always put parentheses after when [OK]

Common Mistakes:

• Omitting parentheses around when condition
• Thinking when can't be used with Exception
• Believing catch can't have code inside

5.

Consider this code:

try {
    throw new ArgumentNullException("param");
} catch (ArgumentNullException ex) when (ex.ParamName == "param") {
    Console.WriteLine("Parameter error");
} catch (ArgumentNullException ex) {
    Console.WriteLine("Other argument null error");
}

What will be printed and why?

hard

A. Parameter error, because the when condition matches the ParamName.

B. Other argument null error, because when clause is ignored.

C. No output, because exception is not caught.

D. Runtime error due to duplicate catch blocks.

Solution

1. Step 1: Identify the thrown exception and its property

   The code throws ArgumentNullException with ParamName set to "param".

2. Step 2: Check the when clause condition in the first catch

   The first catch has a when clause checking if ex.ParamName == "param", which is true, so this catch runs.

3. Step 3: Understand catch block selection

   The second catch is ignored because the first matching catch with true when condition handles the exception.

4. Final Answer:

   Parameter error, because the when condition matches the ParamName. -> Option A

5. Quick Check:

   when true catches first matching block [OK]

Hint: when filters catch; first true condition wins [OK]

Common Mistakes:

• Ignoring when condition and picking second catch
• Thinking duplicate catch causes error
• Assuming exception is uncaught