pinvoke.net: Edit: user32.getwindowthreadprocessid

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Summary: The GetWindowThreadProcessId API

!!!!C# Signature:
 [DllImport("user32.dll", SetLastError=true)]
 static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);

// When you don't want the ProcessId, use this overload and pass IntPtr.Zero for the second parameter
 [DllImport("user32.dll")]
 static extern uint GetWindowThreadProcessId(IntPtr hWnd, IntPtr ProcessId);

!!!!VB.Net Signature:
 <DllImport("user32.dll", SetLastError:=True)> _
 Private Shared Function GetWindowThreadProcessId(ByVal hwnd As IntPtr, _
                                                  ByRef lpdwProcessId As Integer) As Integer
 End Function

''' <summary>
        ''' Retrieves the identifier of the thread that created the specified window and, optionally, the identifier of the process that created the window. 
        ''' </summary>
        ''' <param name="hwnd">A handle to the window. </param>
        ''' <param name="lpdwProcessId">A pointer to a variable that receives the process identifier. If this parameter is not NULL, GetWindowThreadProcessId copies the identifier of the process to the variable; otherwise, it does not. </param>
        ''' <returns>The return value is the identifier of the thread that created the window. </returns>
        ''' <remarks>http://msdn.microsoft.com/en-us/library/ms633522%28v=vs.85%29.aspx</remarks>
        Private Declare Auto Function GetWindowThreadProcessId Lib "user32.dll" (ByVal hwnd As IntPtr, _
                                  ByRef lpdwProcessId As Integer) As Integer

!!!!Return Value
The return value is the identifier of the thread that created the window.

!!!!Notes:
You can use Integer / Int32 when dealing with handles, but using IntPtr is much more flexible. If you need an Integer, just call IntPtr.ToInt32.

!!!!Tips & Tricks:
We can use this API to get ProcessID from Window's Title by combining this function with EnumWindows.

!!!!Sample Code (VB):
 If objAcc Is Nothing Then
  objAcc = New Application
  Dim lngPid As Integer
  Dim lngAccessHwnd As IntPtr = New IntPtr(objAcc.hWndAccessApp)
  GetWindowThreadProcessId(lngAccessHwnd, lngPid)
 End If

----
The following sample retrieves the System.Diagnostics.Process@msdn the currently "owns" the Clipboard, and outputs the process and file name.

Public Module MainModule

Public Sub Main()

Dim oProcess As Process = GetClipboardProcess()

Debug.WriteLine(oProcess.ProcessName)
            Debug.WriteLine(oProcess.MainModule.FileName)

End Sub

Public Declare Unicode Function GetClipboardOwner Lib "user32" () As IntPtr
        Public Declare Auto Function GetWindowThreadProcessId Lib "user32" (ByVal hwnd As IntPtr, ByRef lpdwProcessId As IntPtr) As IntPtr

Public Function GetClipboardProcess() As Process
            Dim oResult As Process = Nothing

Dim hOwner As IntPtr = GetClipboardOwner()

If hOwner <> IntPtr.Zero Then

Dim iProcessID As IntPtr

GetWindowThreadProcessId(hOwner, iProcessID)

oResult = Process.GetProcessById(iProcessID.ToInt32)

End If

Return oResult
        End Function

End Module

!!!!Sample Code (C#):
 IntPtr pID = GetWindowThreadProcessId(GetForegroundWindow(), IntPtr.Zero);

GetWindowThreadProcessId returns the id of the thread that created the target window. To get the process id of a window, use the first c# signature above, and:
!!!!Sample Code (C#):
 int processID = 0;
 int threadID = GetWindowThreadProcessId(hWnd, out processID);

!!!!Alternative Managed API:
Do you know one?  Please contribute it!

Documentation: GetWindowThreadProcessId@msdn on MSD