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A user defined type, or UDT, is a VB technique for defining a data type that exactly meets the needs of your program. A UDT can contain two or more individual data items that can be of different types, such as String and Integer. A UDT can even contain other UDTs and arrays. You can also create arrays of a UDT.

To define a UDT, use the Type...End Type statement, which must appear in the declarations section of a code module. Within the statement, you define the individual items, or members, that the UDT will contain.

Here's an example of a UDT you might create for keeping track of employee data:

Public Type Employee
    FirstNameAs String
    LastName As String
    DateOfHire As Date
    Salary As Currency
    EmployeeNumber As Integer
End Type

Once you define the type, you can create instances of it just like any other data type, like so:

Dim OneEmployee As Employee
Dim AllEmployees(500) As Employee

To access the members of a UDT, you use the Name.Member notation. Here are some examples:

OneEmployee.FirstName = "Jack"
OneEmployee.LastName = "Sprat"
AllEmployees(1).FirstName = "Maria"
AllEmployees(1).LastName = "Sanchez"

Nesting UDTs can be useful in some situations. Here's an example:

Public Type Person
    FirstNameAs String
    LastName As String
End Type
Public Type Employee
    Name As Person
    DateOfHire As Date
    Salary As Currency
    EmployeeNumber As Integer
End Type

When UDTs are nested, you use the same Name.Member notation but with an extra level, like this:

Dim OneEmployeeAs Employee
OneEmployee.Name.FirstName = "Jack"

You can also use UDTs for arguments to functions and procedures and as the return type for functions. They can also be helpful when storing data on disk because VB's Random file type is specifically designed to work with UDTs.

All in all, UDTs are a tool that every VB programmer should know about.

Peter Aitken has been programming with Visual Basic since Version 1.0. He has written numerous books and magazine articles on Visual Basic and other computer and programming topics.