Advanced MFC Programming

Chapter 16. Context Sensitive Help
hconv
hszTopic
hszAppName
hData
lData1
lData2
Not used
Handle of the topic name, must be obtained by calling ::DdeCreateStringHandle()
Handle of service name, must be obtained by calling ::DdeCreateStringHandle()
Not used
Can be neglected
Can be neglected
In the sample of the previous section, a service name “Server” is registered on the server side. To let
the connection be set up between the server and the client, we also need to prepare a topic name that will be
used by both sides. In the sample 15.2\DDE\Server, string “Topic” is used as the topic name, whose string
handle is obtained in function CMainFrame::Hszize() and freed in function CMainFrame::UnHszize().
The client should also obtain a string handle for the topic name and use it to make connection. The
server will receive the handles of service name and topic name together with message XTYP_CONNECT. Upon
receiving this message, the server needs to check if the service name and topic name requested by the client
are supported by itself.
To compare two strings by their handles, we can call function ::DdeCmpStringHandles(…), which will
compare two DDE strings. When calling this function, we need to pass the string handles to its two
parameters. The function will return -1, 0 or 1 indicating if the first string is less than, equal to, or greater
than the second string. The following is the format of this function:
int ::DdeCmpStringHandles(HSZ hsz1, HSZ hsz2);
Now it is clear what the server should do after receiving XTYP_CONNECT message: comparing
hszAppName with its registered service name, and comparing hszTopic with its supported topic names by
calling function ::DdeCmpStringHandles(…). If the comparisons are successful, the callback function
should return TRUE, this indicates the connection request is accepted. Otherwise it should return FALSE,
in which case the connection request is rejected. The following code fragment shows how this is
implemented in sample 15.2\DDE\Server:
……
……
HDDEDATA CALLBACK CMainFrame::DdeCallback
(
UINT wType,
UINT wFormat,
HCONV hConv,
HSZ hszTopic,
HSZ hszAppName,
HDDEDATA hData,
DWORD lData1,
DWORD lData2
)
{
switch(wType)
{
}
case XTYP_CONNECT:
{
if
(
!::DdeCmpStringHandles(hszTopic, m_hszTopicName) &&
!::DdeCmpStringHandles(hszAppName, m_hszServiceName)
)
{
Printf("Connect!\r\n");
return (HDDEDATA)TRUE;
}
else
{
Printf("Connection refused, do not support this topic!\r\n");
return (HDDEDATA)FALSE;
}
}
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