When clause in catch in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to catch only ArgumentNullException exceptions.

C Sharp (C#)

try {
    string s = null;
    Console.WriteLine(s.Length);
} catch (ArgumentNullException [1]) {
    Console.WriteLine("Null argument caught.");
}

Drag options to blanks, or click blank then click option'

Awhen (false)

Bwhen (true)

Cwhen (ex != null)

Dwhen (ex is ArgumentNullException)

Attempts:

3 left

💡 Hint

Common Mistakes

Using when (false) which never catches exceptions.

The when clause with true means the catch block will always execute for ArgumentNullException.

2fill in blank

medium

Complete the code to catch exceptions only when the message contains 'timeout'.

C Sharp (C#)

try {
    // some code
} catch (Exception ex [1] ex.Message.Contains("timeout")) {
    Console.WriteLine("Timeout error caught.");
}

Drag options to blanks, or click blank then click option'

Awhen

Bif

Ccatch

Dfilter

Attempts:

3 left

💡 Hint

Common Mistakes

Using if instead of when in catch.

The when keyword is used to filter exceptions based on a condition.

3fill in blank

hard

Fix the error in the catch block to properly use a when clause.

C Sharp (C#)

try {
    // code
} catch (Exception ex [1] ex.Message == null) {
    Console.WriteLine("Message is null.");
}

Drag options to blanks, or click blank then click option'

Aif

Bwhere

Cwhen

Dcatch

Attempts:

3 left

💡 Hint

Common Mistakes

Using if instead of when.

The correct keyword for filtering exceptions is when, not if or others.

4fill in blank

hard

Fill both blanks to catch InvalidOperationException only when the error code is 404.

C Sharp (C#)

try {
    // code
} catch (InvalidOperationException ex [1] ex.HResult [2] 404) {
    Console.WriteLine("Not found error caught.");
}

Drag options to blanks, or click blank then click option'

Awhen

B==

C!=

Dif

Attempts:

3 left

💡 Hint

Common Mistakes

Using if instead of when.

Using != instead of ==.

Use when to filter exceptions and == to compare the error code.

5fill in blank

hard

Fill all three blanks to catch exceptions when the message contains 'disk' and the error code is 1234.

C Sharp (C#)

try {
    // code
} catch (Exception ex [1] ex.Message.Contains("disk") && ex.HResult [2] [3]) {
    Console.WriteLine("Disk error caught.");
}

Drag options to blanks, or click blank then click option'

Awhen

B==

C1234

D!=

Attempts:

3 left

💡 Hint

Common Mistakes

Using if instead of when.

Using wrong comparison operators.

The when clause filters exceptions with a condition combining message and error code checks.

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