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WHAT I HAVE:
Visual Basic 2019, WinForms
MY QUESTION:
True or false: Multiple application domains within the same process can use the same thread, but multiple processes can't do so. If I look within Process.GetProcesses and ProcessThread.Id for the thread identifier, returned by lParam of WM_ACTIVATEAPP, for an opposing program, the first local process with that Id is the only one, right?
See the doc :
"An application consists of one or more processes. A process, in the simplest terms, is an executing program. One or more threads run in the context of the process. "
https://learn.microsoft.com/en-us/windows/win32/procthread/processes-and-threads
That doesn't answer my question. How many processes can use the same thread? And is the value returned by WndProc / WM_ACTIVATEAPP for LParam.ToInt32, representing the identifier for the other program being swapped with the current one, the same as the value returned by ProcessThread.Id? If I find an identifier match in the ProcessThreadCollection of a local Process, do I need to check the threads of any other local processes?
All in all, is the process owned by a thread unique?! The doc is a little cryptic!
That doesn't answer my question: Is the process owning a thread unique?
That doesn't answer my question.
It does : "One or more threads run in the context of the process. "
A thread belongs to a process, not the reverse...
So the answer is, "Only 1 process owns a thread", right? Therefore, if I use the thread identifier returned by WM_ACTIVATEAPP and go through the ProcessThreadCollection of each process in Process.GetProcesses looking for a ProcessThread with a matching Id, then I've found the (only) owning process for the thread with the very first match I get, right? A simple yes or no will do here. (The answer appears to be yes, but I want to be sure.}
Windows process ids and thread ids are valid while there is an open handle to the related process or thread. Both ids can be reused by Windows. The ProcessThreadCollection is a snapshot of the threads in a process at a point in time. There is always a possibility that a thread could terminate and its id is invalidated (or reused) between the time the snapshot is taken and the thread id is used by the caller.
Instead of searching through all of these collections for every running process an alternative would be to obtain a process id from a thread id by using P/Invoke and the Windows API. Use OpenThread to obtain a handle from the thread id. Use GetProcessIdOfThread to obtain the process id from the thread's handle. This seems to me to be a quicker and more direct way to obtain the process id.
Finally, its been my observation that there is not always a valid thread id in the lParam for the WM_ACTIVATEAPP message. Many times it contains 0, which is not a valid thread id.
Can you show me VB.NET code for using OpenThread and GetProcessIdOfThread (I'm not that good at using unmanaged APIs)? Also, does your final observation mean there's no really reliable way of determining, when the current process is activated or deactivated, which process is the opposing process which was switched out for the current one or switched in from it?
So the answer is, "Only 1 process owns a thread", right?
Yes, you can check for example with Spy++ :
Example from .Net Framework 4.8 Winform project. The form contains a single ListBox.
Public Class Form1
Protected Overrides Sub WndProc(ByRef m As Message)
Dim processId As Integer = 0
If m.Msg = Win32.WM_ACTIVATEAPP Then
If m.LParam <> 0 Then
Dim handle As IntPtr = Win32.OpenThread(Win32.THREAD_QUERY_LIMITED_INFORMATION, False, m.LParam.ToInt32())
If handle <> IntPtr.Zero Then
processId = Win32.GetProcessIdOfThread(handle)
If processId = 0 Then
ListBox1.Items.Add($"Error {Marshal.GetLastWin32Error()} getting processid")
End If
Win32.CloseHandle(handle)
Else
ListBox1.Items.Add($"Error {Marshal.GetLastWin32Error()} opening thread {m.LParam}")
End If
Else
ListBox1.Items.Add("Invalid thread id, lParam was 0")
End If
If m.WParam <> 0 Then
If processId <> 0 Then
ListBox1.Items.Add($"Window Activated, other process is {processId}")
Else
ListBox1.Items.Add("Window Activated, other process is unknown")
End If
Else
If processId <> 0 Then
ListBox1.Items.Add($"Window Deactivated, other process is {processId}")
Else
ListBox1.Items.Add("Window Deactivated, other process is unknown")
End If
End If
End If
MyBase.WndProc(m)
End Sub
End Class
Public Class Win32
Public Const WM_ACTIVATEAPP As Integer = &H1C
Public Const THREAD_QUERY_LIMITED_INFORMATION As Integer = &H800
<DllImport("kernel32.dll", ExactSpelling:=True, SetLastError:=True)>
Public Shared Function GetProcessIdOfThread(handle As IntPtr) As Integer
End Function
<DllImport("kernel32.dll", ExactSpelling:=True, SetLastError:=True)>
Public Shared Function OpenThread(desiredAccess As Integer, inherit As Boolean, threadId As Integer) As IntPtr
End Function
<DllImport("kernel32.dll", ExactSpelling:=True, SetLastError:=True)>
Public Shared Function CloseHandle(handle As IntPtr) As Boolean
End Function
End Class
Sample results when switching between the exampIe winforms application, notepad, calculator and Task Manager. Note the process ids in Task manager and that when switching to Calculator the thread id was from Application Frame Host.
Have you tried the sample code that I posted at your request?
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