Dynamic Data Exchange and XlTable Format

Microsoft Windows provides several methods for transferring data between applications. One way to transfer data is to use Windows dynamic data exchange (DDE). DDE is a message protocol for data exchange between Windows programs. It allows software developers to design applications that share data and thereby provide the user with a more integrated Windows environment.

For complete information about DDE, see the documentation for the Microsoft Windows Software Development Kit (SDK). This article describes the DDE formats that Microsoft Excel supports, and it provides detailed information about the high-performance XlTable DDE format.

DDE Formats

Microsoft Excel supports several DDE formats. The formats are listed in the following table in the order of precedence defined by Microsoft Excel, from highest precedence (XlTable) to lowest precedence (CF_METAFILEPICT). Clipboard formats that begin with CF_ are formats that are already defined in Windows.h. Clipboard formats without CF_ must be registered before use. For more information about registering formats, see Registering Clipboard Formats.

Registering Clipboard Formats

Whenever an application uses a private Clipboard format — such as XlTable, Biff5, Biff4, Biff3, Biff, Wk1, Csv, or Rich Text Format — it must register the format before using it. Microsoft Excel registers these private Clipboard formats, and your DDE application must also register any of these formats that you want to use to exchange data.

For example, to register XlTable, use the following Windows API function call.

wCBformat = RegisterClipboardFormat((LPSTR)"XlTable");

If the function call is successful, the return value is equal to the format value for the XlTable format. This format value (type WORD) is between 0xC000 and 0xFFFF, and it's equal to the format value that Windows returned to Microsoft Excel when it registered XlTable. If Windows cannot register XlTable, the function returns 0 (zero).

Fast Table Format

The fast table format, XlTable, is designed to maximize the DDE transfer speed of Microsoft Excel. XlTable consists of a sequence of data blocks that represent a rectangular selection of cells (a table). Each data block has three parts:

WORD tdt            /* the table data type */
WORD cb            /* the size (count of bytes) of the data */
BYTE data[cb]    /* the data */

The first data block is always of type tdtTable, which specifies the number of rows and the number of columns in the table. The data blocks that follow tdtTable represent all the cells in the table. Microsoft Excel renders the reference of the cells in the table (for example, R1C1:R2C4) as the item part of the DDE message.

The cells are always rendered row-wise. In other words, all the cells in the first row of the table appear first, then all the cells in the second row, and so on. To minimize overhead, adjacent cells of the same type (tdt) are represented together in one data block, even if the cells are in different rows. In other words, one tdtFloat can contain several numbers, one tdtString can contain several strings, one tdtBool can contain several Boolean values, and so on. For examples, see the following sections, "XlTable Example 1" and "XlTable Example 2."

The data block types are described in the following table.

XlTable Example 1

The following selection of three cells . . .

. . . produces the XlTable rendering shown in the following table.

Notice that the table contains three cells, but the XlTable rendering contains only one tdtString data block.

XlTable Example 2

The XlTable format uses the tdtBlank data block to represent blank cells in a table. A sequence of several blank cells may be represented by a single tdtBlank data block.

For example, the following table . . .

. . . produces the following XlTable rendering.

Biff5, Biff4, Biff3, and Biff Formats

The Biff5, Biff4, Biff3, and Biff Clipboard formats contain a variable number of records. The records are identical to the corresponding records in the BIFF file. For more information about the BIFF5 records, see Microsoft Excel File Format. The Biff4, Biff3, and Biff formats are available for backward compatibility with existing applications. The Biff Clipboard format corresponds to the BIFF2 file format.

If you implement one of the BIFF Clipboard formats, your code should be prepared to receive all BIFF records except file-specific records such as the following:

WRITEPROT
FILEPASS
TEMPLATE
WRITEACCESS
FILESHARING
CODEPAGE
PUB
SUB
EDG
INDEX

The requisite BIFF records that your code must provide when it writes to the Clipboard are as follows:

BOF
DIMENSIONS
Cell record or records (BLANK, BOOLERR, and so on)
EOF