finally, using, and validate-early

finally: always runs

try
{
    OpenConnection();
    DoWork();          // may throw
}
finally
{
    CloseConnection(); // runs on success AND on failure
}

finally executes whether the try block succeeds, throws, or returns. Put cleanup there — resource release must never depend on luck.

In modern C#, files/streams take that shape for you:

using (var reader = new StringReader(text))
{
    ... // disposed automatically, even on exception
}

Validate early, fail fast

The strongest error-handling pattern isn't a catch — it's refusing bad input before any work happens:

static int ParseAge(string input)
{
    if (input == null) throw new ArgumentException("input required");
    if (!int.TryParse(input, out int age))
        throw new FormatException("not a number");
    if (age < 0 || age > 150)
        throw new ArgumentOutOfRangeException("implausible age");
    return age;
}

Compare with catching errors after half the work is done: state is now inconsistent, cleanup is complicated, and the failure message is vague. Validate at the door.