The LA Fox Developer Newsletter

April2000

ADO Jumpstart for Microsoft

Visual FoxPro Developers

Part 3

John V Petersen

(Ed. Note: Reprinted with the author’s full permission. This is

the third installment of a multi-part article begun in February’s

LA Fox Developer.]

Along with GetChunk, examine the AppendChunk method. The

first time this method is called for a field, it overwrites any data

in the field. Successive calls then append the data, until pend-

ing edits are cancelled or updated. The following code illustrates

how this method works:

For x = 1 To 100

oRecordset.Fields("notes").AppendChunk(Str(x)+Chr(10) 0)+Chr(1 3))

Next x

Both the GetChunk and AppendChunk methods are ideal for

dealing with low memory scenarios.

The Attributes Property

An attribute specifies the characteristics of something. As a

person, you have many attributes, eye color, height, weight, and

so forth. In the OOP world, objects have many attributes. Most

of the time, attributes are exposed in the form of properties. A

Visual FoxPro form has several properties such as Width,

Height, and BackColor, just to name a few. The same is true for

objects in ADO. Sometimes, however, it is not convenient to

have a one-to-one correspondence between attributes and

properties. Often, you can pack large amounts of information

into a smaller space through the power of setting bits. A bit is

much like a switch. It is either on or off or 1 or 0. If you string

these bits together, you gain the ability to store multiple values

in a small space. This is how the Attributes property works.

The Connection, Parameter, Field, and Property objects all have

an Attributes property. If you have never worked with bit opera-

Lions before, working with this property can be quite challenging.

In some situations, as is the case with the GetChunk and

AppendChunk methods, you will need to refer to the Attributes

property of the Field object to determine whether those methods

are available.

Using the Field object to illustrate how the Attributes property

Norks, you can associate the following attributes with a Field

Dbject and its associated binary values:

adFldMayDefer—lndicates that the field contents are retrieved

Dnly when referenced—0x00000002

adFldupdateable—lndicates that the field can be updated—

Dx00000004

adFldUnkownupdateable—lndicates that the provider does

riot know whether the field is updateable—0x00000008

adFldFixed—lndicates that the field contains fixed length

data—Ox000000l 0

adFldlsNullable—lndicates that the field can accept a null

value dunng a write operation—0x00000020

adFldMayBeNullable—Indiates that the field may contain a

null value—0x00000040

adFldlong—lndicates that the field contains long binary data

and that the GetChunk and AppendChunk methods can be

used—0x00000080

adFldRowlD—Indicates that the field contains a row ID and

cannot be updated. This does not relate to a field that may

contain the identity value or some other auto-incrementing

value. Rather, it relates to a ROW ID that is unique across the

database. Oracle has this feature—Ox00000lOO

adFldRowVersion—Indicates whether the field indicates the

version of the row. For example, a SQL TimeStamp field may

have this attribute set—0x00000200

adFldCachedDeferred—lndjcates that once this field has

been read, future references will be read from the cache—

Ox0000l 000

Usually, more than one of these attributes are present at any

given time. Yet the Attributes property is a single value. Using

the Employees table Notes field as an example, you will see

that the Attributes property yields a value of 234. The value 234

represents the sum of the attributes for that field. For example,

nullable and long attributes have decimal values of 32 and 128

respectively. This means that the Attributes property evaluates

to 160. This works like the Windows Messagebox dialog box

with regard to specifying the icon and types of buttons that are

present.

Knowing that the Attributes property is a sum of the attributes of

a Field object does not help in determining whether a specific

attribute is present. This is where understanding bit operations

comes in handy. The first step is to convert the sum (such as

234, above) into a binary equivalent:

11101010

Working from right to left, (or from the least significant bit to the

most significant)—and beginning with zero, see that bits 1, 3, 5,

6, and 7 are set, (indicated by their values of I in those posi-

tions). Bits 0, 2, and 4 are not set. The next step is to deter-

mine whether a field is “long.”

To determine whether a field is a long field, we must first convert

the adFldLong constant, which specifies which bit if set,

indicates that the field is long. The adFldLong constant has a

hex value of 0x00000080. This translates into a decimal value of

128. The following is the binary equivalent:

10000000

(Con’t, page 4)

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