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Visual Basic allows the programmer to define more complex data
types using combinations of the simpler data types to construct
them.
To dig in a bit further let's consider a scenario where the
programmer must deal with records which represent people and
their accompanying attributes.
Many applications need to keep track of a person's name, their
address, phone number, and/or other data about the person. The
process of loading in all of the attributes of a single person from
a file and displaying them in the interface form can get quite
unwieldy if the programmer attempts to use single discrete variables
for each attribute. He could define several arrays to keep track of
each attribute, one for names, one for street address, one for phone
number.
Along with each of these arrays if would be necessary to also
define an array index variable to keep track of where we are in each
array. It should be obvious that it wouldn't take much of a mistake
to wind up with the wrong attributes being displayed in a record
with a particular name.
User defined types collect these attributes together in a data
structure which can be used to declare an array of people which is
much easier to keep track of, as there is only one array index
value.
To declare a user defined type you will need to add a module to
your project as they cannot be defined within a form module. To do
so select Insert, Module from the Visual Basic IDE menu. This module
has no form interface, just code. Use the View Code button to view
the code for the module. In it's General, Declarations section is
where you will define your user defined type.
Here is an example of the declaration section from a module which
defines a user defined type.
Public Type Person
BirthName As String
StreetAddress As String
City As String
State
As String
ZipCode As String
PhoneArea As Integer
PhonePrefix As Integer
PhoneSuffix As Integer
Comments As String
End Type
Public People() As Person
In this sample Person is the user defined type. Using the newly
defined type an array of People has been further defined. It now
becomes much simpler to refer to any attribute of a particular
person in code. Supposing that an array index variable named
ArrayPosition has also been defined, we can now assign values to
data with dot notation in the following manner.
People(ArrayPosition).BirthName =
"Joseph Jones"
People(ArrayPosition).StreetAddress = "123 3rd Street"
Ok, even that can get to be quite a bit of repetitious typing
while coding, so Visual Basic has kindly provided us with the With
statement which makes the process even easier. Consider the
following example:
With People(ArrayPosition)
.BirthName = "Joseph Jones"
.StreetAddress = "123 3rd Street"
.City =
"Anytown"
End With
As you can see it is now much easier to refer to any individual
attribute of a person when using a user defined type. Be sure to
include the dot before each term within the With statement.
If you are careful to sum the size of the individual elements of
the user defined type to arrive at it's total size for each record,
and then in turn sum the size of all of the records stored in the
People() array you can write the entire array of records out to file
in one operation, and then read it back in in one operation as well.
Now that you know how to define a user defined type I'm sure you
can think of many other uses for them. To see how to read and write
arrays of user defined types to and from a file see Reading and Writing UDTs to files.
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