[ruby-on-rails] Begin, Rescue and Ensure in Ruby?

I've recently started programming in Ruby, and I am looking at exception handling.

I was wondering if ensure was the Ruby equivalent of finally in C#? Should I have:

file = File.open("myFile.txt", "w")

begin
  file << "#{content} \n"
rescue
  #handle the error here
ensure
  file.close unless file.nil?
end

or should I do this?

#store the file
file = File.open("myFile.txt", "w")

begin
  file << "#{content} \n"
  file.close
rescue
  #handle the error here
ensure
  file.close unless file.nil?
end

Does ensure get called no matter what, even if an exception isn't raised?

Yes, ensure ENSURES it is run every time, so you don't need the file.close in the begin block.

By the way, a good way to test is to do:

begin
  # Raise an error here
  raise "Error!!"
rescue
  #handle the error here
ensure
  p "=========inside ensure block"
end

You can test to see if "=========inside ensure block" will be printed out when there is an exception. Then you can comment out the statement that raises the error and see if the ensure statement is executed by seeing if anything gets printed out.

Yes, ensure like finally guarantees that the block will be executed. This is very useful for making sure that critical resources are protected e.g. closing a file handle on error, or releasing a mutex.

Yes, ensure is called in any circumstances. For more information see "Exceptions, Catch, and Throw" of the Programming Ruby book and search for "ensure".

If you want to ensure a file is closed you should use the block form of File.open:

File.open("myFile.txt", "w") do |file|
  begin
    file << "#{content} \n"
  rescue
  #handle the error here
  end
end

This is why we need ensure:

def hoge
  begin
    raise
  rescue  
    raise # raise again
  ensure  
    puts 'ensure' # will be executed
  end  
  puts 'end of func' # never be executed
end  

FYI, even if an exception is re-raised in the rescue section, the ensure block will be executed before the code execution continues to the next exception handler. For instance:

begin
  raise "Error!!"
rescue
  puts "test1"
  raise # Reraise exception
ensure
  puts "Ensure block"
end