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Structures and Other Programming Elements (Visual Basic)

You can use structures in conjunction with arrays, objects, and procedures, as well as with each other. The interactions use the same syntax as these elements use individually.

Note Note

You cannot initialize any of the structure elements in the structure declaration. You can assign values only to elements of a variable that has been declared to be of a structure type.

A structure can contain an array as one or more of its elements. The following example illustrates this.

Public Structure systemInfo
    Public cPU As String
    Public memory As Long
    Public diskDrives() As String
    Public purchaseDate As Date
End Structure 

You access the values of an array within a structure the same way you access a property on an object. The following example illustrates this.

Dim mySystem As systemInfo
ReDim mySystem.diskDrives(3)
mySystem.diskDrives(0) = "1.44 MB"

You can also declare an array of structures. The following example illustrates this.

Dim allSystems(100) As systemInfo

You follow the same rules to access the components of this data architecture. The following example illustrates this.

ReDim allSystems(5).diskDrives(3)
allSystems(5).CPU = "386SX"
allSystems(5).diskDrives(2) = "100M SCSI"

A structure can contain an object as one or more of its elements. The following example illustrates this.

Protected Structure userInput
    Public userName As String
    Public inputForm As System.Windows.Forms.Form
    Public userFileNumber As Integer
End Structure

You should use a specific object class in such a declaration, rather than Object.

You can pass a structure as a procedure argument. The following example illustrates this.

Public currentCPUName As String = "700MHz Pentium compatible"
Public currentMemorySize As Long = 256
Public Sub fillSystem(ByRef someSystem As systemInfo)
    someSystem.cPU = currentCPUName
    someSystem.memory = currentMemorySize
    someSystem.purchaseDate = Now
End Sub

The preceding example passes the structure by reference, which allows the procedure to modify its elements so that the changes take effect in the calling code. If you want to protect a structure against such modification, pass it by value.

You can also return a structure from a Function procedure. The following example illustrates this.

Dim allSystems(100) As systemInfo
Function findByDate(ByVal searchDate As Date) As systemInfo
    Dim i As Integer
    For i = 1 To 100
        If allSystems(i).purchaseDate = searchDate Then Return allSystems(i)
    Next i
   ' Process error: system with desired purchase date not found.
End Function

Structures can contain other structures. The following example illustrates this.

Public Structure driveInfo
    Public type As String
    Public size As Long
End Structure
Public Structure systemInfo
    Public cPU As String
    Public memory As Long
    Public diskDrives() As driveInfo
    Public purchaseDate As Date
End Structure
Dim allSystems(100) As systemInfo
ReDim allSystems(1).diskDrives(3)
allSystems(1).diskDrives(0).type = "Floppy"

You can also use this technique to encapsulate a structure defined in one module within a structure defined in a different module.

Structures can contain other structures to an arbitrary depth.

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