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

alternatively you can also pass two IntPtr Variables

<DllImport("user32.dll", SetLastError := True, CharSet := CharSet.Unicode)> _
Private Function GetWindowThreadProcessId(ByVal hwnd As IntPtr, ByRef lpdwProcessId As IntPtr) As Integer
End Function  

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 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:

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