>Recently I have been getting deadlocks in my application.
>What causes a deadlock ?
>Can it be avoided ?
>How can it be resolved ?
>What causes the "File is in use by another user" to be generated ?
>I understand how a record can be in use by another user but why the file ???

Are you sure that you have a "deadlock" situation? Such a situation is only resolvable by terminating one of the participating applications. Is this how you are "solving" the problem right now?

A deadlock most frequently occurs when app#1 has item A and wants exclusive use of Item B while another application has item B and wants exclusive use of item A. Most typically 2 records of 2 different tables.
The best way to minimize these is to avoid needing 2 things at the same time, and when this is unavoidable then ensuring that different items are always required in the same sequence by all applications.

You say in a later message that the application has been running fine for a long time without problem, then this started.
So the first question is - what has changed? Code in the application itself, or another application that uses (some of) the same tables?
Some new maintenance job?
The network operating system?
It could even be a user who has started using some feature of the system recently, or possibly even mis-using a feature.

good luck