Async/Await and Error Handling

Modern applications must handle I/O operations (network calls, file access, database queries) without blocking. C# provides a first-class async/await model built on the Task type. This lesson also covers structured error handling with exceptions.

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Error Handling with Exceptions

try/catch/finally

try
{
    string content = File.ReadAllText("data.txt");
    int number = int.Parse(content);
    Console.WriteLine($"Result: {number}");
}
catch (FileNotFoundException ex)
{
    Console.WriteLine($"File not found: {ex.FileName}");
}
catch (FormatException ex)
{
    Console.WriteLine($"Invalid format: {ex.Message}");
}
catch (Exception ex)
{
    // Catch-all for unexpected exceptions
    Console.WriteLine($"Unexpected error: {ex.Message}");
}
finally
{
    // Always runs — cleanup code
    Console.WriteLine("Cleanup complete");
}

Exception Hierarchy

Exception	When
Exception	Base class for all exceptions
SystemException	Base for runtime exceptions
ArgumentException	Invalid argument
ArgumentNullException	Null argument
InvalidOperationException	Operation not valid for current state
NullReferenceException	Accessing a member on null
IndexOutOfRangeException	Array index out of bounds
IOException	I/O operation failed
HttpRequestException	HTTP request failed
TaskCanceledException	Task was cancelled

Throwing Exceptions

public void SetAge(int age)
{
    if (age < 0)
        throw new ArgumentOutOfRangeException(nameof(age), "Age cannot be negative");

    Age = age;
}

Custom Exceptions

public class InsufficientFundsException : Exception
{
    public decimal Balance { get; }
    public decimal Amount { get; }

    public InsufficientFundsException(decimal balance, decimal amount)
        : base($"Cannot withdraw {amount:C}. Balance: {balance:C}")
    {
        Balance = balance;
        Amount = amount;
    }
}

Exception Filters

try
{
    await httpClient.GetAsync(url);
}
catch (HttpRequestException ex) when (ex.StatusCode == HttpStatusCode.NotFound)
{
    Console.WriteLine("Resource not found");
}
catch (HttpRequestException ex) when (ex.StatusCode == HttpStatusCode.Unauthorized)
{
    Console.WriteLine("Not authorised");
}

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The using Statement

Automatically dispose of resources when done:

// Traditional using block
using (var reader = new StreamReader("file.txt"))
{
    string content = reader.ReadToEnd();
}
// reader is disposed here

// Modern using declaration (C# 8+)
using var reader2 = new StreamReader("file.txt");
string content2 = reader2.ReadToEnd();
// reader2 is disposed when the enclosing scope ends

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Introduction to Async/Await

Why Async?

Synchronous I/O blocks the calling thread:

// BAD — blocks the thread while waiting
string content = httpClient.GetStringAsync(url).Result;

Asynchronous I/O releases the thread while waiting:

// GOOD — thread is free while waiting
string content = await httpClient.GetStringAsync(url);

The Task Type