SendMessageW function (winuser.h)
Sends the specified message to a window or windows. The SendMessage function calls the window procedure for the specified window and does not return until the window procedure has processed the message.
To send a message and return immediately, use the SendMessageCallback or SendNotifyMessage function. To post a message to a thread’s message queue and return immediately, use the PostMessage or PostThreadMessage function.
Syntax
Parameters
A handle to the window whose window procedure will receive the message. If this parameter is HWND_BROADCAST ((HWND)0xffff), the message is sent to all top-level windows in the system, including disabled or invisible unowned windows, overlapped windows, and pop-up windows; but the message is not sent to child windows.
Message sending is subject to UIPI. The thread of a process can send messages only to message queues of threads in processes of lesser or equal integrity level.
The message to be sent.
For lists of the system-provided messages, see System-Defined Messages.
Additional message-specific information.
Additional message-specific information.
Return value
The return value specifies the result of the message processing; it depends on the message sent.
Remarks
When a message is blocked by UIPI the last error, retrieved with GetLastError, is set to 5 (access denied).
Applications that need to communicate using HWND_BROADCAST should use the RegisterWindowMessage function to obtain a unique message for inter-application communication.
The system only does marshalling for system messages (those in the range 0 to (WM_USER-1)). To send other messages (those >= WM_USER) to another process, you must do custom marshalling.
If the specified window was created by the calling thread, the window procedure is called immediately as a subroutine. If the specified window was created by a different thread, the system switches to that thread and calls the appropriate window procedure. Messages sent between threads are processed only when the receiving thread executes message retrieval code. The sending thread is blocked until the receiving thread processes the message. However, the sending thread will process incoming nonqueued messages while waiting for its message to be processed. To prevent this, use SendMessageTimeout with SMTO_BLOCK set. For more information on nonqueued messages, see Nonqueued Messages.
An accessibility application can use SendMessage to send WM_APPCOMMAND messages to the shell to launch applications. This functionality is not guaranteed to work for other types of applications.
Examples
The winuser.h header defines SendMessage as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.
SendMessage function (winuser.h)
Sends the specified message to a window or windows. The SendMessage function calls the window procedure for the specified window and does not return until the window procedure has processed the message.
To send a message and return immediately, use the SendMessageCallback or SendNotifyMessage function. To post a message to a thread’s message queue and return immediately, use the PostMessage or PostThreadMessage function.
Syntax
Parameters
A handle to the window whose window procedure will receive the message. If this parameter is HWND_BROADCAST ((HWND)0xffff), the message is sent to all top-level windows in the system, including disabled or invisible unowned windows, overlapped windows, and pop-up windows; but the message is not sent to child windows.
Message sending is subject to UIPI. The thread of a process can send messages only to message queues of threads in processes of lesser or equal integrity level.
The message to be sent.
For lists of the system-provided messages, see System-Defined Messages.
Additional message-specific information.
Additional message-specific information.
Return value
The return value specifies the result of the message processing; it depends on the message sent.
Remarks
When a message is blocked by UIPI the last error, retrieved with GetLastError, is set to 5 (access denied).
Applications that need to communicate using HWND_BROADCAST should use the RegisterWindowMessage function to obtain a unique message for inter-application communication.
The system only does marshalling for system messages (those in the range 0 to (WM_USER-1)). To send other messages (those >= WM_USER) to another process, you must do custom marshalling.
If the specified window was created by the calling thread, the window procedure is called immediately as a subroutine. If the specified window was created by a different thread, the system switches to that thread and calls the appropriate window procedure. Messages sent between threads are processed only when the receiving thread executes message retrieval code. The sending thread is blocked until the receiving thread processes the message. However, the sending thread will process incoming nonqueued messages while waiting for its message to be processed. To prevent this, use SendMessageTimeout with SMTO_BLOCK set. For more information on nonqueued messages, see Nonqueued Messages.
An accessibility application can use SendMessage to send WM_APPCOMMAND messages to the shell to launch applications. This functionality is not guaranteed to work for other types of applications.
SendMessageA function (winuser.h)
Sends the specified message to a window or windows. The SendMessage function calls the window procedure for the specified window and does not return until the window procedure has processed the message.
To send a message and return immediately, use the SendMessageCallback or SendNotifyMessage function. To post a message to a thread’s message queue and return immediately, use the PostMessage or PostThreadMessage function.
Syntax
Parameters
A handle to the window whose window procedure will receive the message. If this parameter is HWND_BROADCAST ((HWND)0xffff), the message is sent to all top-level windows in the system, including disabled or invisible unowned windows, overlapped windows, and pop-up windows; but the message is not sent to child windows.
Message sending is subject to UIPI. The thread of a process can send messages only to message queues of threads in processes of lesser or equal integrity level.
The message to be sent.
For lists of the system-provided messages, see System-Defined Messages.
Additional message-specific information.
Additional message-specific information.
Return value
The return value specifies the result of the message processing; it depends on the message sent.
Remarks
When a message is blocked by UIPI the last error, retrieved with GetLastError, is set to 5 (access denied).
Applications that need to communicate using HWND_BROADCAST should use the RegisterWindowMessage function to obtain a unique message for inter-application communication.
The system only does marshalling for system messages (those in the range 0 to (WM_USER-1)). To send other messages (those >= WM_USER) to another process, you must do custom marshalling.
If the specified window was created by the calling thread, the window procedure is called immediately as a subroutine. If the specified window was created by a different thread, the system switches to that thread and calls the appropriate window procedure. Messages sent between threads are processed only when the receiving thread executes message retrieval code. The sending thread is blocked until the receiving thread processes the message. However, the sending thread will process incoming nonqueued messages while waiting for its message to be processed. To prevent this, use SendMessageTimeout with SMTO_BLOCK set. For more information on nonqueued messages, see Nonqueued Messages.
An accessibility application can use SendMessage to send WM_APPCOMMAND messages to the shell to launch applications. This functionality is not guaranteed to work for other types of applications.
Examples
The winuser.h header defines SendMessage as an alias which automatically selects the ANSI or Unicode version of this function based on the definition of the UNICODE preprocessor constant. Mixing usage of the encoding-neutral alias with code that not encoding-neutral can lead to mismatches that result in compilation or runtime errors. For more information, see Conventions for Function Prototypes.
how do i send a custom message to a windows service?
I have implemented a windows xp service application that starts a couple of working threads. From one of the threads i need to send custom messages back to the service. How do i do that?
4 Answers 4
One option is to use OmniThreadLibrary (read this blog post for an example).
Thanks for your help. Here is how I solved the problem:
In the Service class definition:
In the ServiceExecute method:
In ServiceStop method:
And the method for handling messages:
In MyThread.Execute method:
Its working just fine.
I agree with TOndrej that shared objects should be sufficient.
On the other hand you can use my IPC (Cromis IPC) which works just fine inside services. It is easy to use, message oriented, so you don’t need to know how named pipes work and very fast. The server part also uses a thread pool, so there is no waiting for something to be processed. You can use the fire and forget scenario.
Or if you think a little redesign is ok, you can try OmniThreadLibrary which has all the messaging already build in and is made for tasks like this.
EDIT:
Ok probably the cleanest way to go, without any redesing is to have a common object list which is protected by a critical section. The working thread is adding objects that need to be processed in the list. When and object is added the working thread signals an event. Then you have an object processing thread which is waiting with WaitForSingleObject for this event. As soon as something is added to the list the event is signaled and the processing thread just processes all the objects it finds in the list. Then it waits again. All you need to do is to protect the access to the common list.
Arcane Code
Yesterday we looked at starting and stopping your windows service from another application. It would also be helpful to be able to send other commands beyond the basic set of Start/Stop/Pause etc.
On the good side, there is an API through which it is possible to send a command to your windows service, fairly easily as a matter of fact. The down side is it’s a one way communication, through the built in mechanism it’s not possible to return any data. Even more limiting is the only data you are allowed to pass in is an integer, and it’s value must be between 128 and 255.
Even with those limitations, if all you need to do is execute a few simple commands the built in API can be more than enough to meet your needs.
To illustrate, we’ll expand the windows service we’ve been using as an example. We’ll add the ability to send a command that will force the event log to be updated immediately, rather than waiting on the timer to fire off its event.
First, we’ll add two items to the service. The first is a public enum. Strictly speaking we don’t have to use an enum, but it makes for more readability.
// Must be int between 128 and 255
public enum commands
Next we’ll add a new method to the windows service called OnCustomCommand. This is an override to the base classes method. As you can see it first calls the base method, then checks the value of the integer that was passed in, in this case against our enum. If it finds a match, it calls the WriteToLog method immediately. (The WriteToLog was discussed yesterday, so I won’t reiterate here).
protected override void OnCustomCommand( int command)
if (command == ( int ) commands .LogIt)
WriteToLog( “Arcane LogIt:” );
OK, that’s all that’s needed for the service. Let’s switch to our program, and add another button called LogIt.
Now we’ll add a little code to the click event for the log it button.
private void btnLogIt_Click( object sender, EventArgs e)
ServiceController sc = new ServiceController ( “ArcaneTimeLogging” );
Like in our other examples we create a reference to our service by creating a new ServiceController object and passing in the name of our service. Once we have an sc object we call the ExecuteCommand method. This allows us to send messages to a windows service.
Numbers 0-127 are reserved for windows, and are handled in the base class. 128 to 255 are there for your own use. In the example above I used 255 just to show that you could pass an integer value directly without using an enum.
One last small item, we don’t want the LogIt button to be enabled if our service isn’t running. We’ll add a little logic to the SetDisplay, Start and Stop methods to include setting the buttons enabled status properly. Here’s the updated routines.
private void SetDisplay( ServiceController sc)
