How Do I Extract/Set Data From/To a Variant Type of a COM Object? (Magic xpa 4.x)

Many COM objects use the VT_VARIANT data type. The definition of the VT_VARIANT, is, in essence, that it can hold any data type. However, the object you are using will be expecting data of a certain format for a specific function.

Usually, Magic xpa will handle the conversions automatically. If you know that a certain VT_VARIANT parameter is expecting an array, for instance, and you set up a vector, you just pass the vector in and Magic xpa does the rest (see How Do I Send and Retrieve Array Values from COM Objects? for an example). Similarly, if the object is sending back a negative integer in a VT_VARIANT, and you accept the parameter in a numeric variable that allows negatives, Magic xpa will handle the underlying conversion correctly.

However, if you need a more precise level of control, you can use a BLOB data type to pass data back and forth, and use the Variant functions to set up exactly the type of variant you want to send. You can also use the same functions to extract data from a variant that is sent back from the object.

Here is a summary of the functions used.

• VariantCreate(): Copies data into a BLOB variant. See Creating a Variant with VariantCreate().

• VariantGet(): Copies data from a BLOB variant into a variable. See Extracting data from a Variant using VariantGet.

• VariantGetVector(): Copies data from a BLOB variant into a vector. See the Magic xpa Help files.)

• VariantAttr(): Returns the Magic xpa data type (Alpha, Numeric, Date, etc.) of the variant. See Extracting the attribute type using VariantAttr().

• VariantType(): Returns the data type (a number representing the standard variant types, such as VT_DATE, VT_l4, etc.). See Extracting the data type using VariantType().

Creating a Variant with VariantCreate()

You can use the VariantCreate() function to populate the variant BLOB. In this example, we used VariantCreate() to move a Date and a Time into the VT_DATE variant, which can actually hold a date/time stamp.

The syntax of VariantCreate() is:

VariantCreate(VT Type, Value, Time value)

where:

• VT Type: A number representing the data type. For instance, in our example, 7 represents the date type. See Variant Data Types for a list of the types.

• Value: The value you are putting into the variant. This could be any data type, a date, string, number, or BLOB.

• Time value: (optional) This allows you to move a time into a VT_Date type. In this example, we pass in a Time as the third parameter.

Extracting data from a Variant using VariantGet()

The syntax of VariantGet() is:

VariantGet(Variant Value, Attribute)

where:

• Variant Value: The BLOB that has your Variant.

• Attribute: A letter representing the type you are extracting. See Variant Data Attributes below for a list of the codes.

In this example, the variant is a VT_DATE type, which holds both a Date and a Time, so we can extract both from the same variant using VariantGet().

Variant Data Attributes

Attribute Letter	Magic xpa Data Attribute
A	Alpha
N	Numeric
L	Logical
D	Date
T	Time
B	Boolean
U	Unicode

Variant Data Types

Here is a list of the data type that are used in the Variant functions.

VarType returns	Enumeration symbol	VariantAttr returns
0	VT_EMPTY	No value specified
1	VT_NULL	SQL-style Null
2	VT_I2	Signed 2-byte integer	–32,768 to 32,767
3	VT_I4	Signed 4-byte integer	–2,147,483,648 to 2,147,483,647
4	VT_R4	Signed 4-byte real	1.1E -38 to 3.4E +38 (7 digits)
5	VT_R8	Signed 8-byte real	2.2E -308 to 1.7 E +308 (15 digits)
6	VT_CY	Currency
7	VT_DATE	Date
8	VT_BSTR	Automation string
9	VT_DISPATCH	A pointer to an object that implements IDispatch
10	VT_ERROR	SCODE
11	VT_BOOL	Boolean
12	VT_VARIANT
13	VT_UNKNOWN	A pointer to an object that implements IUnknown
14	VT_DECIMAL	Decimal
16	VT_I1	1-byte character	–128 to 127
17	VT_UI1	Unsigned 1-byte character	0 to 255
18	VT_UI2	Unsigned 2-byte integer	0 to 65,535
19	VT_UI4	Unsigned 4-byte integer	0 to 4,294,967,295
22	VT_INT	Signed machine integer
23	VT_UINT	Unsigned machine integer
36	VT_RECORD	User defined type
8192	VT_ARRAY		An array of data type
16384	VT_BYREF		A reference to data type