Back in the days of VB 6 and earlier, writing a multithreaded VB application
was hard. MS even told ambitious VB developers not to do it, the hacks required
were not going to be supported, endorsed, blessed, etc. No soup for you.
With the release of VB.NET, we VB developers finally get to play. I'll take
you through the basics to get you started. Once you have made your first
multithreaded application, you will officially be 'dangerous.'
I've done my best to keep my explanations straightforward and accessible, but
be forewarned that multi-threading has a few hairy issues to grapple with, it
will probably take a couple reads through this article and some tinkering in
VB.NET before the topics I touch on here make thorough sense. Besides that,
we're mearly scratching the surface of many of the concepts and techniques that
VB.NET supports. When you are ready for more, MSDN
is a good place to start.
What is a Thread?
Threads are like employees at a company. In your typical company, each
employee can be working at the same time. The company is the process which
contains all the threads working in it, and which organizes them towards a
common purpose. When you write an application and run it, you are starting a
process with a single thread. So, your application can do one thing at a time
(it's a company with one employee). Your variables and objects are like the
copier and fax in an office...some things are global and shared by everyone in a
company while others are private, residing in the office of one person or
A modern operating system allows you to run a number of programs at the same
time, each of which has one or more threads. If you have one processor in your
machine, these processes (aka applications) and their threads aren't necessarily
running at the exact same time. Typically the OS let's the first process run on
the CPU for a fraction of a second, then it let's the next one run for a
fraction of a second, and so on, creating the illusion than lots of things are
happening at the same time. When the OS let's a process run, that process then
gets to decide which thread it wants to give time to first, then it gives time
to the next, etc, much as the OS does with processes.
However, with certain recent processors, and with multi-processor machines
running a multiprocessor aware operating system like Win2k or WinXP it is
possible for two processes and possibly two or more of their threads to be
running at the exact same time.
Why Use Multithreading?
Multithreading is useful for a number of reasons. Your application may need
to respond to external events. When the external event happens, it creates a new
thread to handle it appropriately without preempting freezing the rest of your
application. If you have a timer control in your VB.NET app, guess what? You
have a multithreaded application!
I've used threads in my applications to handle tasks in the background
without freezing up my application's interface while things are processing. When
the task is done, it updates a variable in my application so the main thread
will be able to tell that it's done.
You may be writing a service that need to keep track of several different
things at the same time. Threads let you juggle a lot of tasks and events
One Fax At a Time
Having two threads working at exact same time is great! But like the
employees that try to stuff paper into the shared fax machine at the exact same
time, threads can cause problems when they use shared resources.
Let's say you have a global variable called "
helps your program track the number of hits to your website. And let's say you
are running two threads that do the same thing, each tracking a different part
of the website but recording hits to the same global variable:
intWebHits += 1
Each time either thread gets a hit, it prints out the total number of hits so
far and then it increments
intWebHits. What happens if you get 2
web hits at the same time? On a machine that can run 2 threads at the same time,
you could end up running each line of the above code with each thread at the
exact same time. So if the total for
intWebHits were "129" before
the simultaneous hits, both threads would run that first line of code at the
same time, both incrementing the total and both printing "131" OR both printing
"130" depending on factors outside your control or ability to reliably predict.
What you wanted was 130 then 131; instead you got 130 and 130 or 131 and 131. If
you have a machine that can only run one thread at a time, you could still be in
trouble. What if you run line 1 on the first thread, then that thread is paused
and the second thread is given the CPU to run on for its share of time? It will
run its copy of line 1. Then it will run line 2. Then the first thread gets its
turn again. It run's its copy of line 2. What is your output? 131 and 131.
Again, not what you wanted. This behavior may be fairly harmless with a
webcounter, but you can probably imagine writing programs where being off by 1
here and there is not acceptable.
The wise people that make operating systems and compilers recognized this
problem and they have provided us with a way of dealing with it: resource locks.
A resource lock allows a thread to claim control of a variable and to be
guaranteed that no other thread will have a lock or be able to do anything to
that locked resource at the same time as the thread that has placed a lock on
it. Any other thread that needs access to that variable just has to wait its
turn. It even works with multiprocessor system.
intWebHits += 1
Now when those threads run at the same time, they will each try to get a lock
on the object "
objMyLock" but only one will get it, the other one
will wait until the lock is off "
objMyLock" before it will run.
objMyLock can by any object you care to use for a lock.
But these locks have problem of their own. Consider the following:
intWebHits += 1
intWebHits += 1
Notice that we have two slightly different routines. The first routine tries
to get a lock on
objMyLockA first and then it tries for a lock on
objMyLockB. The second routine tries for a lock on B, then on A. If
these two routines were to run at the same time, the first routine would get a
objMyLockA at the same time that the second routine would
get a lock on
objMyLockB. The first routine needs a lock on
objMyLockB in order to continue. The second routine needs a lock on
A. They both have something the other needs to continue, but neither will give
up the lock it holds until it gets the lock it needs to complete. So, these
threads would essentially stop working. This is what's known as a race
condition. You don't want one of these...so be careful not to have interlocking
locks like that in your code. These examples are simple, but if you have locks
all over you application and routine calling other routines from within a lock
protected area of code, you can end up with this problem without it being
something as easy to identify on a read through as it was here.
And one more thing. Some objects and structures built in to .NET are
multithread safe (they don't require a lock to function, though you may still
need one depending on how you use it) and some are not. Integers are multithread
safe, so you can read and write them with two different threads at the same
time, but you could have issues like those we talked about earlier...it may not
give you an error, but it probably isn't going to behave in the way you were
hoping for either. Collections, as an example, are not multithread safe. If you
have two threads try to read a collection at the same time, you will most likely
get an error. The .NET documentation usually tells you if something is thread
If it's an option, run routines in your thread that don't access shared
variables at all. You won't need locks and you won't have to worry about
avoiding or debugging race conditions! The issues related to shared variables
and the order in which threads are running are know collectively as "Thread
Cut To The Chase
Now that you know to avoid using shared variable without locks, and to avoid
race conditions whenever you use locks, you are ready for the code!
Dim thrMyThread As New System.Threading.Thread( _
Yah, that's it. These two lines create
a new thread running the
IncrementWebCountA routine. You can
set the priority of the thread and do other cool things with it, but that's the
basic usage right there. One point to note is that the starting routine must
require no parameters in order to be used to start a thread.
Whenever you run a program in VB, you specify a "startup object" like form1.
That's where your "main" thread begins...the one that starts your application,
sets up the forms, sets up the global variables, etc. It's your first employee.
Will you add more? :)