Best way to handle a typical precondition exception?

Question

Which of the following ways of handling this precondition is more desirable and what are the greater implications?

1:

If Not Exists(File) Then 
    ThrowException
    Exit
End If
File.Open
...work on file...

2:

If Exists(File) Then 
    File.Open 
    ....work on file...
Else
    ThrowException
    Exit
End 

Note: The File existence check is just an example of a precondition to HANDLE. Clearly, there is a good case for letting File existence checks throw their own exceptions upwards.

Answer 1

I prefer the first variant so it better documents that there are preconditions

Answer 2

Separating the pre-condition check from work is only valid if nothing can change between the two. In this case an external event could delete the file before you open it. Hence the check for file existence has little value, the open call has to check this anyway, let it produce the exception.