Throw and rethrow patterns in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to rethrow the caught exception preserving the original stack trace.

C Sharp (C#)

try {
    // Some code
} catch (Exception ex) {
    [1];
}

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

Athrow ex

Bthrow new Exception()

Cthrow

Dthrow ex.InnerException

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throw ex' which resets the stack trace.

Throwing a new exception loses original exception details.

Using throw alone rethrows the current exception preserving the original stack trace.

2fill in blank

medium

Complete the code to throw a new exception with a custom message inside the catch block.

C Sharp (C#)

try {
    // Some code
} catch (Exception ex) {
    throw new Exception([1], ex);
}

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

Aex.Message

Bnull

Cex

D"Error occurred"

Attempts:

3 left

💡 Hint

Common Mistakes

Passing the exception object instead of a message string.

Using null instead of a descriptive message.

The first argument is the custom message string for the new exception.

3fill in blank

hard

Fix the error in the catch block to correctly rethrow the exception preserving the stack trace.

C Sharp (C#)

try {
    // Some code
} catch (Exception ex) {
    [1];
}

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

Athrow

Bthrow new Exception(ex.Message)

Cthrow ex

Dthrow ex.InnerException

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throw ex' which resets the stack trace.

Throwing a new exception loses original stack trace.

Using throw alone rethrows the original exception with its stack trace intact.

4fill in blank

hard

Fill both blanks to create a try-catch that logs the exception message and then rethrows it preserving the stack trace.

C Sharp (C#)

try {
    // Some code
} catch (Exception ex) {
    Console.[1](ex.Message);
    [2];
}

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

AWriteLine

BWrite

Cthrow

Dthrow ex

Attempts:

3 left

💡 Hint

Common Mistakes

Using 'throw ex' which resets the stack trace.

Using Console.Write instead of WriteLine for logging.

Use Console.WriteLine to print the message and throw to rethrow preserving the stack trace.

5fill in blank

hard

Fill all three blanks to catch an exception, wrap it in a new exception with a custom message, and throw it.

C Sharp (C#)

try {
    // Some code
} catch (Exception [1]) {
    throw new Exception([2], [3]);
}

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

Aex

B"Custom error occurred"

Derror

Attempts:

3 left

💡 Hint

Common Mistakes

Using a different variable name than declared in catch.

Passing the wrong type as the message argument.

Not passing the original exception as inner exception.

The catch variable is named ex. The new exception message is a string, and the original exception ex is passed as inner exception.

Practice

1. What does the throw statement do in C# exception handling?

easy

A. It logs the error without stopping the program.

B. It ignores the error and continues execution.

C. It stops the program and signals an error.

D. It automatically fixes the error and resumes.

Solution

1. Step 1: Understand the purpose of throw

   The throw statement is used to signal that an error has occurred and to stop normal program flow.

2. Step 2: Recognize program behavior on throw

   When throw is executed, the program stops and looks for a matching catch block or terminates if none is found.

3. Final Answer:

   It stops the program and signals an error. -> Option C

4. Quick Check:

   throw stops program = C [OK]

Hint: Throw always stops execution and signals an error [OK]

Common Mistakes:

• Thinking throw continues program normally
• Confusing throw with logging
• Assuming throw fixes errors automatically

2. Which of the following is the correct syntax to rethrow an exception in a catch block without changing it?

easy

A. throw catch;

B. throw ex;

C. throw new Exception();

D. throw;

Solution

1. Step 1: Identify rethrow syntax

   To rethrow the caught exception preserving the original stack trace, use throw; without specifying the exception variable.

2. Step 2: Understand difference from throw ex;

   throw ex; resets the stack trace, which is not a pure rethrow.

3. Final Answer:

   throw; -> Option D

4. Quick Check:

   Rethrow syntax = throw; [OK]

Hint: Use plain 'throw;' to rethrow without losing stack trace [OK]

Common Mistakes:

• Using 'throw ex;' which resets stack trace
• Trying to throw a new exception instead
• Using invalid syntax like 'throw catch;'

3. What will be the output of the following C# code?

try {
  throw new Exception("Error 1");
} catch (Exception ex) {
  Console.WriteLine(ex.Message);
  throw;
}

medium

A. Error 1 printed, then program terminates with the same exception.

B. No output, program silently terminates.

C. Error 1 printed, then program continues normally.

D. Compilation error due to missing catch block.

Solution

1. Step 1: Analyze the try block

   The try block throws an exception with message "Error 1".

2. Step 2: Analyze the catch block

   The catch block prints the exception message, then rethrows the same exception using throw;.

3. Step 3: Understand program flow after rethrow

   Since the exception is rethrown and not caught again, the program terminates with the same exception after printing.

4. Final Answer:

   Error 1 printed, then program terminates with the same exception. -> Option A

5. Quick Check:

   Print then rethrow = B [OK]

Hint: Rethrow after print causes termination with printed message [OK]

Common Mistakes:

• Assuming program continues after rethrow
• Thinking no output is printed
• Confusing rethrow with new exception creation

4. Identify the error in this code snippet:

try {
  // some code
} catch (Exception ex) {
  throw ex;
}

medium

A. Try block must have a finally block.

B. Using throw ex; resets the stack trace, losing original error info.

C. Cannot catch Exception type directly.

D. Missing semicolon after throw ex.

Solution

1. Step 1: Understand throw ex; effect

   Using throw ex; throws the exception but resets the stack trace, losing original error location.

2. Step 2: Identify correct rethrow method

   To preserve stack trace, use throw; without specifying the exception variable.

3. Final Answer:

   Using throw ex; resets the stack trace, losing original error info. -> Option B

4. Quick Check:

   Throwing exception variable resets stack trace = D [OK]

Hint: Use 'throw;' not 'throw ex;' to keep original stack trace [OK]

Common Mistakes:

• Thinking 'throw ex;' preserves stack trace
• Believing finally block is mandatory
• Assuming Exception cannot be caught directly

5. You want to catch an exception, add extra info, and then rethrow it preserving the original stack trace. Which pattern is correct?

try {
  // code
} catch (Exception ex) {
  // add info
  ???
}

hard

A. throw new Exception("Extra info", ex);

B. throw ex;

C. throw;

D. throw new Exception(ex.Message);

Solution

1. Step 1: Understand wrapping exceptions

   To add extra info, create a new exception with the original as inner exception: new Exception("Extra info", ex).

2. Step 2: Preserve original stack trace

   This wrapping keeps original exception details inside the new one, preserving context.

3. Step 3: Why not other options?

   throw ex; resets stack trace, throw; rethrows original without extra info, and throw new Exception(ex.Message); loses original exception object.

4. Final Answer:

   throw new Exception("Extra info", ex); -> Option A

5. Quick Check:

   Wrap with new Exception and inner ex = A [OK]

Hint: Wrap original in new Exception to add info and preserve trace [OK]

Common Mistakes:

• Using 'throw ex;' which loses stack trace
• Using 'throw;' which loses added info
• Creating new Exception without inner exception