Visual Basic Tutorial - 46 - Optional ByVal Video Lecture | Mastering Visual Basic - Web Development

hello YouTube sam here from youtube.com
slash on live gamer for the new Boston
and in this tutorial we're going to be
taking a look at default parameters now
basically what this means is let's say
you're creating a sub or a function and
you want the user or the developer to be
able to pass a value to that function
but you don't want to force them to do
that so you can give them the option to
give you let's say a title for example
so if they want to give you a title then
it will go ahead and let them pass that
to the function but if they don't give
you anything for that spot it's not
going to return it's not going to give
them an error it's going to set the
default value for it so let's go ahead
and make a little demo application for
this so we'll type we'll go ahead and
select new project we will name it
optional make sure you have selected
windows forms application and then click
OK
so now what we're going to do is we're
going to come over here to our form 1
we'll resize it some and we'll open up
the toolbox now what we're going to do
is we're going to add a text box and
another text box now the first text box
is going to be our title and the second
text box is going to be our message now
I'll just add a button called
show message
so we'll resize that a bit so it will
fit in and we're going to go come over
here to our text box one and we'll
rename that to text title and we'll
rename this one to text message so now
let's close out of our tool box and
we'll double click on show message to
open up the event handler for that and
once we're in here I'll zoom in so now
what we're going to do is we are going
to create a sub so we're going to start
off with the keyword private and then
we'll use sub and we'll call it show
message and now what we're going to do
is we're going to force the caller of
this sub to enter in a message so we'll
do by about by Val message as string and
then we're going to have them enter in a
title now they can enter in a title if
they want but they don't have to and the
way we do this is by using the keyword
optional so we'll type optional by Val
title as string so they can call show
message with only one argument and it
will still work fine so we'll go ahead
and press ENTER and then we'll type end
sub
now what we need to do is set a default
value for our title now you cannot use
option or unless you have a default
value set so what we're going to do is
we're going to add an equal sign here so
optional by Val title a string is going
to be equal to default title and so now
we'll just do message box dot show
message and title now this isn't very
practical but it's to teach you the
concept of optionals and using default
parameters so now let's go up here to
our button one click and we're going to
call show message but there's a couple
things we need to do first we will
declare a variable so dim message as
string it's going to be equal to text
text message dot text now we're going to
create a title variable so dim title as
string and it's going to be equal to
nothing so now what we're going to do is
we're going to create a simple if
statement to see if the user has typed
anything in the title so what we do this
is we're going to do if text title dot
text length is greater than zero then go
ahead and do this now what this does is
it gets the length of the text inside of
that text box and you can see that says
text length as integer gets the length
of the text in the control so if they
actually enter something in the text
title then what we're going to do is
title is equal to text title dot text
now what we're going to do is we're
going to show message so show a message
and then we'll do message and title now
what if they didn't type anything into
the text title to handle this we're just
going to add an else
so we'll just type else right here
so if text title dot text length is
greater is equal to 0 or less than 0
which it cannot be less than 0 it's
either 0 or something greater than that
so if it is equal to 0 then we'll just
go ahead and show message and then we'll
pass message to it and no title and it
will show it will set a title to default
title so let's go ahead and run this and
here's our form right here if we go
ahead and set something that's a title
we'll type hello and then hello YouTube
if we click show message you can see the
title is set to hello and it shows hello
YouTube now if we take out hello and
leave it as nothing you can see that
when we click show message it says
default title right there so let's go
ahead and close out of that so go ahead
and make some subs or functions if you'd
like and play around with optional
bivalves or use you can use optional
bivalves with by vowels and with by revs
go ahead mix and match those get a lot
of practice with that and then once
you're done with that go ahead and move
on to the next tutorial and thanks for
watching guys