I'm hoping that its a stupid mistake I've made.
Unfortunately (or fortunately?) you are making several mistakes.
First is you need to understand that Query Notifications will invalidate one query. So you will only be notified at most once and you have to re-subscribe again (re-submit the query) if you want to receive further notifications.
Next you need to understand that you will be notified for any reason, not only for changes. In your callback you must check the reason you're notified, which are passed in via the
SqlNotificationEventArgs
.Next you need to understand asynchronous programming basic principles: if you subscribe for an event make sure you subscribe before the event can happen first time. Case in point: the
On_SqlBitChanged
can fire as soon as you submit the query. This should happen in theSqlWatcher.SqlWatcher
constructor, but you subscribe to thesqlWatcher.NewMessage
after the constructor runs.On_SqlBitChanged
can be invoked between the constructor finishes before you hook up theNewMessage
event callback in which case the notification is silently ignored.If you want to use a service make sure you start it before you use it. You are using SqlDependency in
SqlWatcher.SqlWatcher
but you start it after that when you callSqlWatcher.Start()
.Finally, if you want to be notified of changes on a query you have to submit the query. You are constructing the
SqlCommand
object, set up the notification and then... discard the object. Unless you actually submit the query, you did not yet subscribed to anything.
Suggestions for fix:
- Make
Start
andStop
statics, callStart
in application start up. - Make sure you subscribe to
NewMessage
before you submit the query - Actually submit the query (call
SqlComamnd.ExecuteQuery()
) - Inspect the
Info
,Type
andSource
in theOn_SqlBitChanged
callback, if your submission contains an error this is the only way to learn (the SqlComamnd.ExecuteQuery() will succeed even if the notification request is invalid) - You must re-subscribe once you're notified of a change, execute the query again.
One more thing: don't invoke UI code in background callbacks. You cannot call MessageBox.Show("Message Received");
from a callback, you must route through the form main thread via Form.Invoke
. YEs, I know that strictly speaking MessageBox.Show
does work on a non-UI thread but you will soon move away from alert boxes to actually form interaction and then things will break.