C# Tutorial · Advanced C# · Lesson 18 of 22

C# Exceptions

try, catch, finally, throw, custom exceptions and using.

Handling exceptions

Example 1

try
{
    int[] arr = { 1, 2, 3 };
    Console.WriteLine(arr[5]);
}
catch (IndexOutOfRangeException e)
{
    Console.WriteLine($"Oops: {e.Message}");
}
finally
{
    Console.WriteLine("finally always runs");
}

try { int.Parse("abc"); }
catch (FormatException) when (DateTime.Now.Year > 2000) { Console.WriteLine("Not a number (filtered catch)"); }

Output

Oops: Index was outside the bounds of the array.
finally always runs
Not a number (filtered catch)

Run this on your computer — see “Get Started” for setup.

Custom exceptions and using

Derive from Exception for domain errors. using disposes resources (files, connections) automatically when they go out of scope:

Example 2

try
{
    using var conn = new FakeConnection("db");
    conn.Query("SELECT 1");
    throw new InsufficientFundsException(30m);
}
catch (InsufficientFundsException e)
{
    Console.WriteLine($"{e.Message} (short by {e.ShortBy})");
}

class InsufficientFundsException(decimal shortBy) : Exception("Insufficient funds")
{
    public decimal ShortBy { get; } = shortBy;
}

class FakeConnection : IDisposable
{
    private readonly string _name;
    public FakeConnection(string name) { _name = name; Console.WriteLine($"open {_name}"); }
    public void Query(string sql) => Console.WriteLine($"query: {sql}");
    public void Dispose() => Console.WriteLine($"close {_name}");
}

Output

open db
query: SELECT 1
close db
Insufficient funds (short by 30)

Run this on your computer — see “Get Started” for setup.

Test yourself

What does a using declaration guarantee?

The namespace is imported
Dispose() is called when the variable goes out of scope
Exceptions are ignored
The object is never null

Show answer

Answer: Dispose() is called when the variable goes out of scope