Exception handling in async code in C Sharp (C#) - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of exception handling in async code?

Exception handling in async code helps catch and manage errors that happen during asynchronous operations, preventing the program from crashing and allowing graceful recovery.

beginner

How do you catch exceptions from an async method in C#?

You use a try-catch block around the await call to the async method. If the async method throws an exception, it will be caught in the catch block.

intermediate

What happens if you don't await an async method and it throws an exception?

If you don't await the async method, exceptions may go unobserved and can cause the program to crash later or behave unpredictably.

intermediate

Explain how to handle multiple exceptions from multiple awaited async calls.

You can use multiple try-catch blocks for each awaited call or use a single try-catch around all awaits. To handle multiple exceptions together, use Task.WhenAll and catch exceptions from the awaited task, which are wrapped in an AggregateException.

advanced

What is the role of AggregateException in async exception handling?

AggregateException groups multiple exceptions thrown by tasks running in parallel. When awaiting Task.WhenAll, exceptions are wrapped in AggregateException to handle them together.

Which keyword is used to wait for an async method to complete and catch exceptions it throws?

A await

B async

C try

D catch

The 'await' keyword waits for the async method to finish and allows exceptions to be caught in the surrounding try-catch.

What happens if you call an async method without awaiting it and it throws an exception?

A Program automatically retries the async method

B Exception is ignored and program continues safely

C Exception is caught immediately

D Exception may go unobserved and cause issues later

Without awaiting, exceptions may not be observed immediately and can cause crashes or unexpected behavior later.

How can you handle exceptions from multiple async tasks running in parallel?

A Use multiple try-catch blocks separately

B Ignore exceptions from parallel tasks

C Use Task.WhenAll and catch AggregateException

D Use async void methods

Task.WhenAll waits for all tasks and throws an AggregateException containing all exceptions, which you can catch and handle.

Which of these is NOT a good practice for exception handling in async code?

A Ignore exceptions from async methods

B Use try-catch around awaited calls

C Always await async methods

D Handle AggregateException when using Task.WhenAll

Ignoring exceptions can cause hidden bugs and crashes; always handle exceptions properly.

What type of exception wraps multiple exceptions from parallel async tasks?

A InvalidOperationException

B AggregateException

C NullReferenceException

D TimeoutException

AggregateException groups multiple exceptions thrown by parallel tasks.

Describe how you would catch and handle exceptions from an async method in C#.

Think about how you wait for the async method and where you put the try-catch.

Explain the role of AggregateException when working with multiple async tasks.

Consider what happens when many tasks fail at once.

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