Send Message in C

Question

I m creating an application that uses a main project that is connected to several different DLLs  From one DLL window I need to be able to open a window in another but the DLL s can t reference each other    It was suggested to me to use the sendmessage function in the first DLL and have a listener in the main program that directs that message to the appropriate DLL to open it s window    However I m not familiar at all with the sendmessage function and am having a lot of diffculty piecing things together from information I m finding online    If someone could please show me the correct way  if there is any  to use the sendmessage function and maybe how a listener captures that message that would be amazing  Here is some of the code I ve got so far I m not sure if I m heading in the right direction        DllImport  user32 dll        public static extern int FindWindow string lpClassName  String lpWindowName        DllImport  user32 dll        public static extern int SendMessage IntPtr hWnd  int wMsg  IntPtr wParam  IntPtr lParam        public void button1 Click object sender  EventArgs e                int WindowToFind   FindWindow null   Form1

User · Answer

public static extern int FindWindow string lpClassName  String lpWindowName     In order to find the window  you need the class name of the window   Here are some examples   C    const string lpClassName    Winamp v1 x   IntPtr hwnd   FindWindow lpClassName  null     Example from a program that I made  written in VB   hParent   FindWindow  TfrmMain   vbNullString    In order to get the class name of a window  you ll need something called Win Spy  Once you have the handle of the window  you can send messages to it using the SendMessage IntPtr hWnd  int wMsg  IntPtr wParam  IntPtr lParam  function   hWnd  here  is the result of the FindWindow function   In the above examples  this will be hwnd and hParent   It tells the SendMessage function which window to send the message to   The second parameter  wMsg  is a constant that signifies the TYPE of message that you are sending   The message might be a keystroke  e g  send  the enter key  or  the space bar  to a window   but it might also be a command to close the window  WM CLOSE   a command to alter the window  hide it  show it  minimize it  alter its title  etc    a request for information within the window  getting the title  getting text within a text box  etc    and so on   Some common examples include the following   Public Const WM CHAR    amp H102 Public Const WM SETTEXT    amp HC Public Const WM KEYDOWN    amp H100 Public Const WM KEYUP    amp H101 Public Const WM LBUTTONDOWN    amp H201 Public Const WM LBUTTONUP    amp H202 Public Const WM CLOSE    amp H10 Public Const WM COMMAND    amp H111 Public Const WM CLEAR    amp H303 Public Const WM DESTROY    amp H2 Public Const WM GETTEXT    amp HD Public Const WM GETTEXTLENGTH    amp HE Public Const WM LBUTTONDBLCLK    amp H203   These can be found with an API viewer  or a simple text editor  such as notepad  by opening  Microsoft Visual Studio Directory  Common Tools WINAPI winapi32 txt   The next two parameters are certain details  if they are necessary   In terms of pressing certain keys  they will specify exactly which specific key is to be pressed   C  example  setting the text of windowHandle with WM SETTEXT   x   SendMessage windowHandle  WM SETTEXT  new IntPtr 0   m strURL     More examples from a program that I made  written in VB  setting a program s icon  ICON BIG is a constant which can be found in winapi32 txt    Call SendMessage hParent  WM SETICON  ICON BIG  ByVal hIcon    Another example from VB  pressing the space key  VK SPACE is a constant which can be found in winapi32 txt    Call SendMessage button   WM KEYDOWN  VK SPACE  0  Call SendMessage button   WM KEYUP  VK SPACE  0    VB sending a button click  a left button down  and then up    Call SendMessage button   WM LBUTTONDOWN  0  0 amp   Call SendMessage button   WM LBUTTONUP  0  0 amp     No idea how to set up the listener within a  DLL  but these examples should help in understanding how to send the message

User · Answer

It doesn t sound like a good idea to use send message  I think you should try to work around the problem that the DLLs can t reference each other

User · Answer

Building on Mark Byers s answer   The 3rd project could be a WCF project  hosted as a Windows Service  If all programs listened to that service  one application could call the service  The service passes the message on to all listening clients and they can perform an action if suitable   Good WCF videos here - http   msdn microsoft com en-us netframework dd728059

User · Answer

Some other options   Common Assembly  Create another assembly that has some common interfaces that can be implemented by the assemblies     Reflection  This has all sorts of warnings and drawbacks  but you could use reflection to instantiate   communicate with the forms   This is both slow and runtime dynamic  no static checking of this code at compile time

User · Answer

You are almost there   note change in the return value of FindWindow declaration   I d recommend using RegisterWindowMessage in this case so you don t have to worry about the ins and outs of WM USER    DllImport  user32 dll        public static extern IntPtr FindWindow string lpClassName  String lpWindowName        DllImport  user32 dll        public static extern int SendMessage IntPtr hWnd  int wMsg  IntPtr wParam  IntPtr lParam        DllImport  user32 dll   SetLastError true  CharSet CharSet Auto   static extern uint RegisterWindowMessage string lpString    public void button1 Click object sender  EventArgs e                       this would likely go in a constructor because you only need to call it          once per process to get the id - multiple calls in the same instance          of a windows session return the same value for a given string      uint id   RegisterWindowMessage  MyUniqueMessageIdentifier         IntPtr WindowToFind   FindWindow null   Form1             Debug Assert WindowToFind    IntPtr Zero        SendMessage WindowToFind  id  IntPtr Zero  IntPtr Zero       And then in your Form1 class   class Form1   Form        DllImport  user32 dll   SetLastError true  CharSet CharSet Auto       static extern uint RegisterWindowMessage string lpString        private uint  messageId   RegisterWindowMessage  MyUniqueMessageIdentifier         protected override void WndProc ref Message m               if  m Msg     messageId                         do stuff                  base WndProc ref m             Bear in mind I haven t compiled any of the above so some tweaking may be necessary  Also bear in mind that other answers warning you away from SendMessage are spot on  It s not the preferred way of inter module communication nowadays and genrally speaking overriding the WndProc and using SendMessage PostMessage implies a good understanding of how the Win32 message infrastructure works   But if you want need to go this route I think the above will get you going in the right direction

User · Answer

You don t need to send messages   Add an event to the one form and an event handler to the other  Then you can use a third project which references the other two to attach the event handler to the event  The two DLLs don t need to reference each other for this to work

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