Exception handling in async code in C Sharp (C#) - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to catch exceptions in an async method.

C Sharp (C#)

try
{
    await SomeAsyncMethod();
}
catch ([1] ex)
{
    Console.WriteLine(ex.Message);
}

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

Aint

BException

Cstring

DTask

Attempts:

3 left

💡 Hint

Common Mistakes

Using a non-exception type in the catch block.

Not using await before the async method.

You catch exceptions by specifying Exception in the catch block.

2fill in blank

medium

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

C Sharp (C#)

try
{
    await SomeAsyncMethod();
}
catch (Exception ex)
{
    [1];
}

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

Athrow new Exception()

Bthrow ex

Creturn

Dthrow

Attempts:

3 left

💡 Hint

Common Mistakes

Using throw ex; which loses the original stack trace.

Using return instead of throwing.

Use throw; to rethrow the exception preserving the original stack trace.

3fill in blank

hard

Fix the error in the async method signature to enable exception handling.

C Sharp (C#)

public async [1] SomeAsyncMethod()
{
    throw new Exception("Error");
}

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

ATask

Bvoid

Cint

Dstring

Attempts:

3 left

💡 Hint

Common Mistakes

Using void return type which prevents awaiting and catching exceptions.

Returning non-task types from async methods.

Async methods that do not return a value should return Task to allow awaiting and exception handling.

4fill in blank

hard

Fill both blanks to create a dictionary comprehension that filters exceptions by message length.

C Sharp (C#)

var errors = new Dictionary<string, string>();
foreach (var ex in exceptions)
{
    if (ex.Message.Length [1] 10)
    {
        errors.Add(ex.Message, ex[2]);
    }
}

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

A>

BMessage

C<

DInnerException

Attempts:

3 left

💡 Hint

Common Mistakes

Using wrong comparison operator.

Adding the wrong property as dictionary value.

The code filters exceptions with message length greater than 10 and adds them to the dictionary using the message as key.

5fill in blank

hard

Fill all three blanks to create a LINQ query that selects exception messages with inner exceptions.

C Sharp (C#)

var messages = exceptions.Where(ex => ex.[1] != null)
    .Select(ex => ex.[2])
    .Where(msg => msg.[3]("error", StringComparison.OrdinalIgnoreCase))
    .ToList();

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

AInnerException

BMessage

CContains

DStackTrace

Attempts:

3 left

💡 Hint

Common Mistakes

Using StackTrace instead of Message.

Checking wrong property for null.

This LINQ query filters exceptions that have an inner exception, selects their messages, and filters messages containing the word "error".

Practice

1. What is the main purpose of using try and catch blocks around await calls in C# async code?

easy

A. To catch exceptions thrown during asynchronous operations and prevent program crashes.

B. To make the async method run faster.

C. To convert async methods into synchronous methods.

D. To automatically retry the async operation if it fails.

Solution

1. Step 1: Understand async exception behavior

   Exceptions in async methods are thrown when awaited, so they must be caught around await.

2. Step 2: Purpose of try-catch

   Using try and catch around await lets you handle errors gracefully and avoid crashes.

3. Final Answer:

   To catch exceptions thrown during asynchronous operations and prevent program crashes. -> Option A

4. Quick Check:

   try-catch around await = catch exceptions [OK]

Hint: Always put await inside try to catch errors [OK]

Common Mistakes:

• Assuming async methods never throw exceptions
• Catching exceptions outside async calls
• Using try-catch without await inside

2. Which of the following is the correct syntax to catch exceptions from an async method DoWorkAsync()?

easy

A. await try { DoWorkAsync(); } catch (Exception ex) { Console.WriteLine(ex.Message); }

B. try { await DoWorkAsync(); } catch (Exception ex) { Console.WriteLine(ex.Message); }

C. try { DoWorkAsync(); } catch (Exception ex) { Console.WriteLine(ex.Message); }

D. DoWorkAsync().catch(ex => Console.WriteLine(ex.Message));

Solution

1. Step 1: Identify correct try-catch with await

   The await keyword must be inside the try block to catch exceptions properly.

2. Step 2: Check syntax correctness

   try { await DoWorkAsync(); } catch (Exception ex) { Console.WriteLine(ex.Message); } correctly places await DoWorkAsync() inside try and catches exceptions.

3. Final Answer:

   try { await DoWorkAsync(); } catch (Exception ex) { Console.WriteLine(ex.Message); } -> Option B

4. Quick Check:

   await inside try = correct syntax [OK]

Hint: Put await inside try, not outside [OK]

Common Mistakes:

• Placing await outside try block
• Missing await keyword
• Using incorrect catch syntax

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

async Task TestAsync()
{
    try
    {
        await Task.Run(() => throw new InvalidOperationException("Error occurred"));
    }
    catch (InvalidOperationException ex)
    {
        Console.WriteLine("Caught: " + ex.Message);
    }
}

await TestAsync();

medium

A. Unhandled exception crash

B. No output, program runs silently

C. Caught: Error occurred

D. Compilation error

Solution

1. Step 1: Analyze async exception throwing

   The Task.Run throws an InvalidOperationException inside the task.

2. Step 2: Exception caught in catch block

   The await inside try catches the exception, printing the message.

3. Final Answer:

   Caught: Error occurred -> Option C

4. Quick Check:

   Exception caught and message printed [OK]

Hint: Exceptions in awaited tasks are caught by surrounding try [OK]

Common Mistakes:

• Expecting no output
• Thinking exception crashes program
• Missing await causes unhandled exceptions

4. Identify the error in this async exception handling code:

async Task RunAsync()
{
    try
    {
        Task.Delay(1000);
    }
    catch (Exception ex)
    {
        Console.WriteLine("Error: " + ex.Message);
    }
}

await RunAsync();

medium

A. Missing await before Task.Delay, so exceptions won't be caught.

B. Try-catch cannot be used with async methods.

C. Task.Delay does not throw exceptions, so catch is useless.

D. Catch block syntax is incorrect.

Solution

1. Step 1: Check await usage

   The Task.Delay(1000) is called without await, so exceptions won't be observed here.

2. Step 2: Effect on exception handling

   Without await, the try-catch does not catch exceptions from the async call.

3. Final Answer:

   Missing await before Task.Delay, so exceptions won't be caught. -> Option A

4. Quick Check:

   Await missing = exceptions not caught [OK]

Hint: Always await async calls inside try to catch exceptions [OK]

Common Mistakes:

• Forgetting await before async calls
• Assuming try-catch works without await
• Believing Task.Delay throws exceptions here

5. You have two async methods:

async Task MethodA()
{
    await Task.Delay(500);
    throw new Exception("Error in A");
}

async Task MethodB()
{
    try
    {
        await MethodA();
    }
    catch (Exception ex)
    {
        Console.WriteLine("Caught in B: " + ex.Message);
        throw;
    }
}

try
{
    await MethodB();
}
catch (Exception ex)
{
    Console.WriteLine("Caught in main: " + ex.Message);
}

What will be the output when this code runs?

hard

A. No output, exception swallowed

B. Caught in B: Error in A

C. Caught in main: Error in A

D. Caught in B: Error in A
Caught in main: Error in A

Solution

1. Step 1: Exception thrown in MethodA

   MethodA throws an exception after delay.

2. Step 2: Exception caught and rethrown in MethodB

   MethodB catches the exception, prints message, then rethrows it.

3. Step 3: Exception caught again in main

   Main try-catch catches the rethrown exception and prints its message.

4. Final Answer:

   Caught in B: Error in A Caught in main: Error in A -> Option D

5. Quick Check:

   Exception caught twice due to rethrow [OK]

Hint: Rethrow exceptions to propagate them up the call stack [OK]

Common Mistakes:

• Assuming exception is swallowed after first catch
• Missing that rethrow sends exception to caller
• Expecting only one catch message printed