Attach after a launch
Applies to: 
Visual Studio 
Visual Studio for Mac
Note
This article applies to Visual Studio 2017. If you're looking for the latest Visual Studio documentation, see Visual Studio documentation. We recommend upgrading to the latest version of Visual Studio. Download it here
After a program launches, the debug session is ready to attach the debug engine (DE) to said program.
Design decisions
Because communication is easier within a shared address space, you must choose between two design approaches: set communication between the debug session and the DE. Or, set communication between the DE and the program. Choose between the following:
If it makes more sense to set up the communication between the debug session and the DE, the debug session co-creates the DE and asks the DE to attach to the program. This design leaves the debug session and DE together in one address space, and the run-time environment and program together in another.
If it makes more sense to set up the communication between the DE and the program, the run-time environment co-creates the DE. This design leaves the SDM in one address space and the DE, run-time environment, and program together in another. This design is typical of a DE that is implemented with an interpreter to run scripted languages.
Note
How the DE attaches to the program is implementation-dependent. Communication between the DE and the program is also implementation-dependent.
Implementation
Programmatically, when the session debug manager (SDM) first receives the IDebugProgram2 object that represents the program to be launched, it calls the Attach method, passing it an IDebugEventCallback2 object, which is later used to pass debug events back to the SDM. The IDebugProgram2::Attach method then calls the OnAttach method. For more information on how the SDM receives the IDebugProgram2 interface, see Notifying the port.
If your DE needs to run in the same address space as the program you're debugging: because the DE is typically part of an interpreter that's running a script, the IDebugProgramNodeAttach2::OnAttach method returns S_FALSE. The S_FALSE return indicates that it completed the attach process.
If, however, the DE runs in the address space of the SDM: the IDebugProgramNodeAttach2::OnAttach method returns S_OK, or the IDebugProgramNodeAttach2 interface isn't implemented at all on the IDebugProgramNode2 object associated with the program you're debugging. In this case, the Attach method is eventually called to complete the attach operation.
In the latter case, you must call the GetProgramId method on the IDebugProgram2 object that was passed to the IDebugEngine2::Attach method, store the GUID in the local program object, and return this GUID when the IDebugProgram2::GetProgramId method is subsequently called on this object. The GUID is used to identify the program uniquely across the various debug components.
In the case of the IDebugProgramNodeAttach2::OnAttach method returning S_FALSE, the GUID to use for the program is passed to that method and it's the IDebugProgramNodeAttach2::OnAttach method that sets the GUID on the local program object.
The DE is now attached to the program and ready to send any startup events.