When clause in catch in C Sharp (C#) - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of try-catch with when clause filtering

What will be the output of this C# program when the input number is 5?

C Sharp (C#)

using System;
class Program {
    static void Main() {
        int number = 5;
        try {
            if (number < 10) throw new Exception("Small number");
            Console.WriteLine("Number is large");
        }
        catch (Exception ex) when (number > 10) {
            Console.WriteLine("Caught exception for large number");
        }
        catch (Exception ex) when (number < 10) {
            Console.WriteLine("Caught exception for small number");
        }
    }
}

A Caught exception for small number

B Caught exception for large number

C Number is large

D No output

💡 Hint

Look at the when clauses and which one matches the number 5.

The exception is thrown because number is 5 (less than 10). The first catch has when (number > 10) which is false, so it is skipped. The second catch has when (number < 10) which is true, so it catches and prints 'Caught exception for small number'.

❓ Predict Output

intermediate

Behavior of multiple catch blocks with when clauses

What will this program print when the input number is 15?

C Sharp (C#)

using System;
class Program {
    static void Main() {
        int number = 15;
        try {
            if (number > 10) throw new InvalidOperationException("Too large");
            Console.WriteLine("Number is small");
        }
        catch (InvalidOperationException ex) when (number < 10) {
            Console.WriteLine("Caught invalid operation for small number");
        }
        catch (InvalidOperationException ex) when (number > 10) {
            Console.WriteLine("Caught invalid operation for large number");
        }
    }
}

A Caught invalid operation for small number

B Unhandled exception

C Caught invalid operation for large number

D Number is small

💡 Hint

Check which when clause matches the number 15.

The exception is thrown because number is 15 (greater than 10). The first catch has when (number < 10) which is false, so it is skipped. The second catch has when (number > 10) which is true, so it catches and prints 'Caught invalid operation for large number'.

❓ Predict Output

advanced

Effect of when clause on exception filtering

What will be the output of this program?

C Sharp (C#)

using System;
class Program {
    static void Main() {
        int number = 0;
        try {
            number = 10 / 0;
        }
        catch (DivideByZeroException ex) when (number == 0) {
            Console.WriteLine("Divide by zero caught when number is zero");
        }
        catch (DivideByZeroException ex) {
            Console.WriteLine("Divide by zero caught");
        }
    }
}

A Divide by zero caught when number is zero

B Divide by zero caught

C No output

D Unhandled exception

💡 Hint

Check the value of number when the exception is thrown and the when clause condition.

The exception is thrown before number is assigned any value other than 0. The when clause checks if number == 0, which is true, so the first catch block runs and prints 'Divide by zero caught when number is zero'.

❓ Predict Output

advanced

When clause with multiple conditions

What will this program print?

C Sharp (C#)

using System;
class Program {
    static void Main() {
        int number = 7;
        try {
            if (number % 2 == 1) throw new Exception("Odd number");
        }
        catch (Exception ex) when (number > 5 && number < 10) {
            Console.WriteLine("Caught exception for odd number between 5 and 10");
        }
        catch (Exception ex) {
            Console.WriteLine("Caught exception");
        }
    }
}

A Caught exception

B Caught exception for odd number between 5 and 10

C No output

D Unhandled exception

💡 Hint

Check the when clause condition and the value of number.

The exception is thrown because number is 7 (odd). The when clause checks if number is greater than 5 and less than 10, which is true, so the first catch block runs and prints 'Caught exception for odd number between 5 and 10'.

🧠 Conceptual

expert

Understanding when clause effect on exception propagation

Consider this code snippet. Which statement is true about the behavior of the when clause in the catch block?

C Sharp (C#)

try {
    throw new Exception("Error");
}
catch (Exception ex) when (false) {
    Console.WriteLine("This will never run");
}
catch (Exception ex) {
    Console.WriteLine("Caught in general catch");
}

A The first catch block will catch the exception because the when clause is ignored.

B The exception will not be caught and will crash the program.

C The program will not compile because when clause cannot be false.

D The exception will be caught by the second catch block because the when clause in the first catch evaluates to false.

💡 Hint

Think about how the when clause filters exceptions.

The when clause acts as a filter. If it evaluates to false, the catch block is skipped and the runtime looks for another matching catch block. Here, the first catch is skipped because when(false) is false, so the second catch block catches the 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