To debug the initialization code of a service application, the debugger must be attached when the service is started. This is accomplished by creating a registry key:
HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows NT\CurrentVersion\Image File Execution Options\ProgramName
The ProgramName is the image file for the service application you are debugging. Do not specify a path. For example, the ProgramName might look like MyService.exe.
Under this key create a string data value called Debugger. The value of this string should be set to the full path of the debugger that will be used. For example,
In addition to setting this registry key, the service application must be marked as "interactive". This allows your service to interact with the desktop, and allows the debugger window to appear on your desktop.
This again requires modifying a registry key: you must bitwise-or the type entry for your service with 0x100 (this is the value for SERVICE_INTERACTIVE_PROCESS according to Winnt.h). The exact location and name of this registry entry varies. For example:
Finally, you need to adjust the service application timeout. Otherwise, the service application will kill the debugger within 20 seconds after starting. Adjusting the timeout involves setting an entry in the following registry key:
Under this key, create a DWORD data value called ServicesPipeTimeout. Set this entry to the amount of time in milliseconds that you want the service to wait before timing out. For example, 60,000 is one minute, while 86,400,000 is 24 hours.
Now, when the service is started, the debugger will also start. When the debugger starts, it will stop at the initial process breakpoint, before the service has begun running. This allows you to set breakpoints or otherwise configure your debugging session to let you monitor the startup of your service. Another option is to place calls to the DebugBreak function in your service from the point at which you would like to break into the debugger. (For more information, see DebugBreak in the Platform SDK documentation.)
If your service is running with other services in a Service Host Process, you may need to isolate the service into its own Service Host Process.