Collection Item Method

Named Arguments

Syntax objectvariable .Item( index) objectvariable Use: Required

Data Type: Collection object

An object variable of type Collection.


Use: Required

Data Type: Numeric or String

If a string, index is the key; if numeric, index is the ordinal position. Description

Returns the member of the collection whose key or ordinal position corresponds to index.

Rules at a Glance

• If index is a string, it's taken to be the key, and the member of the collection with the key of index is returned.

• If index is a number, it's taken to be the index number and the member in the ordinal position index is returned.

• If index is a string and the key doesn't exist in the collection, an error (runtime error 5, "Invalid procedure call or argument") is generated.

• If index is numeric, it must be between 1 and the maximum number of items in the collection, or an error (runtime error 9, "Subscript out of range") is generated.

Programming Tips & Gotchas

• When writing wrapper classes for collections, you can make your object model more readable by making the name of the property that wraps the

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Item method the same as the name of the object being obtained from the collection. For example, if your collection class is called Employees and is a collection of Employee records, your object model reads much better with an Employee Property Get procedure, as follows:

Public Property Get Employee(vEmpCode as Variant) _ As Boolean Employee = mcolEmployees.Item(vEmpCode) End Property

• Note that in the above Property Get procedure, the parameter is passed as a variant. This is because a Collection item can be extracted by its key (a string) or by its ordinal number (integer or long). Therefore, by passing a variant, your Property Get procedure can accept a number or a string, and the Item method determines whether to access the collection by its key or its ordinal number. There is, however, one little glitch here: what happens if you have used the string representation of a number for the key? For example:

mcol.Add Item:="Somestuff" Key:=CStr(iKey)

In this case, passing a string representation of the number as a variant doesn't work; the Item method is passed a number and assumes you want the item at that ordinal position.

• Unlike the Dictionary object in VB6, there is no Exists method in the Collection object, so you can't find out in advance if a particular key exists within the Collection. However, you can create an "Exists" function by calling the Item method with a given key and returning True if an item is returned or False if an error is generated, as the following code shows:

Public Function Exists(vKey As Variant) As Boolean On Error Resume Next msValue = mcolMyCollection.Item(vKey)

If Err.Number = 0 Then Exists = True


Exists = False End If

End Function

• When I create wrapper classes for collections, I always include an Exists function like the one shown above, but with a little extra—if the Item requested by the client doesn't exist, I attempt to load it into the collection, as this code demonstrates:

Public Function Exists(sEmpCode As String) As Boolean

On Error Goto Exists_Err

'declare an object variable to hold the test object Dim oTest as Employee

'attempt to get the Employee object from

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'the Employees collection oTest = mcolEmpoyees.Item(sEmpCode) 'if the code gets here then we know all is well 'clean up and leave Set oTest = Nothing Exists = True Exit Function Exists_Err:

'was the error because the Employee object did not 'exist in the collection? If Err.Number = 5 Then

'attempt to get the Employee object If GetEmployee(sEmpCode) then

'we got the employee record and added it to 'the collection Exists = True Else

'the employee code does not exist so we 'couldn't get the record Exists = False End If Else

Exists = False End If

End Function

With this function in place, I can always ensure that the client-side code operates smoothly:

Dim oEmployees As New Employees Dim oEmployees As Employee

If oEmployees.Exists(sEmpCode) Then

Set oEmployee = oEmployees.Employee(sEmpCode)


MsgBox "This Employee Code could not be found" End if

• The Item method is the default member of the Collection object. This means that, when retrieving a member of a collection, you don't have to actually include an explicit call to the Item method. The following two statements, for example, are identical to one another:

set objMember = objCollection.Item(6) set objMember = objCollection(6)

See Also

Collection Object, Collection.Add Method, Collection.Count Property, Collection.Remove Method

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