Using AppConn COM from Visual Basic

To use AppConn in a Visual Basic project, select References from the Project menu and select AppConn from the list of available references. If AppConn does not appear, use the Browse button to select the appconn.dll from the Windows\System32 folder.

Once AppConn is included in the project, all of the AppConn objects will appear in the Visual Basic IntelliSense. If AppConnLib is selected when using the IntelliSense, a list of the AppConn objects will appear. Objects may be declared with or without the AppConnLib prefix. The following declarations are the same:

  Dim Session As AppConnTable
  Dim Session As AppConnLib.AppConnTable

To create an AppConn object, use the new operator as follows:

  Set Session = new AppConnTable

Visual Basic IntelliSense provides a list of properties and methods available for the object. IntelliSense also provides a list of the parameters for methods. The IntelliSense also supplies the parameter type. Optional parameters are enclosed in square brackets (e.g. [varUserID] ).

IntelliSense will also show the return type for methods that have parameters. variables can be set to object properties and method return values. If the property return type is a COM object, Visual Basic requires using Set; otherwise a simple assignment statement can be used.

  Dim Rec As AppConnRecordSet
  Dim Index As Long
  Dim Connected As Boolean

  ' Requires Set because method returns a COM object
  Set Rec = Session.FetchRecords 

  ' Does not require Set because method returns a long
  Long Index = Session.GetCurrentRecordIndex  
  'Does not require Set because property is a Boolean 
  Connected = Session.IsConnected

Visual Basic requires that properties and methods that return a value use parentheses when parameters are included in the call.

  ' Does not require parentheses because all of the parameters are optional 
  ' and none are included
  Set Recs = Session. FetchRecords  

  ' Requires parentheses because a parameter is included
  Set Recs = Session. FetchRecords (1) 
  'Requires parentheses because parameters are included and a return value is used
  Set Recs = PerformTableProcedure("Pets",  "Get")
  ' Does not require parentheses because return value is not used
  PerformTableProcedure "Pets", "Delete"


You can use the Web Builder tool to create projects based on the "demo" models provided with Verastream Host Integrator and then examine the code in your projects to see how the connector works.

Examples for individual methods are included with the Visual Basic Methods Reference.

Handling Errors

Errors can be handled by declaring on On Error handler. A handler can be either Resume Next, GoTo 0, or GoTo <label>.

Resume Next will ignore the error and continue executing the next statement:

  On Error Resume Next

GoTo 0 will use the default error handler:

  On Error GoTo 0

GoTo <label> will cause the program to jump to the label and execute the next statement.

  On Error GoTo HandleError
     Session.ConnectToModel "localhost", "CCSDemo"
  Exit Sub

     MsgBox Err.Description, vbOKOnly + vbCritical, "Error"

Using Variable Type Parameters

Visual Basic allows variable type parameters. For example, the Item method of the AppConnStringMap can take either a number (the index of an element) or a string (the key of an element). The method will determine the type of the parameter and return the correct element for that type.

  Dim Attributes As AppConnStringMap
  Dim Attribute as String

  Set Attributes = new AppConnStringMap 'Create the string map
  Attributes.Add("LastName", "Culver") 'Populate the string map
  Attribute = Attributes.Item(1) 'Get the first element (VisualBasic indexes start at 1)
  Attribute = Attributes.Item("LastName") 'Get the element named "LastName"

Using Default Methods

Default methods can be defined for COM objects. When the method name is omitted Visual Basic will use the default method. For the AppConnRecordSet, AppConnRecord, AppConnModelRecord, AppConnStringList, and AppConnStringMap the Item method is defined as the default method. The calls to the Item method can be written as follows:

Attribute = Attributes(1)
Attribute = Attributes("LastName")

When the parameter is a string, the default method can be invoked using the exclamation point (!) and the parameter string without quotes.

Attribute = Attributes!LastName

The AppConnRecordSet, AppConnRecord, AppConnModelRecord, AppConnStringList, and AppConnStringMap objects are defined as COM collections, which means that VisualBasic can iterate over the collection using the For Each…Next construct. In the following example a For Each statement is imbedded inside a For Each statement to iterate over each record in a record set and each field in the record:

  Dim Rec
  Dim Field
  Dim Line As String
  For Each Rec In Recs
     Line = "Fields:"
     For Each Field In Rec
        Line = Line + " " + Field