C# Signature:

/// <summary>The GetForegroundWindow function returns a handle to the foreground window.</summary>
[DllImport("user32.dll")]
private static extern IntPtr GetForegroundWindow();

VB.NET Signature:

''' <summary>The GetForegroundWindow function returns a handle to the foreground window.</summary>
<DllImport("user32.dll", SetLastError:=True)> _
Private Shared Function GetForegroundWindow() As IntPtr
End Function

''' <summary>
    ''' Retrieves a handle to the foreground window (the window with which the user is currently working). The system assigns a slightly higher priority to the thread that creates the foreground window than it does to other threads.
    ''' </summary>
    ''' <returns>The return value is a handle to the foreground window. The foreground window can be NULL in certain circumstances, such as when a window is losing activation. </returns>
    ''' <remarks>http://msdn.microsoft.com/en-us/library/ms633505%28v=vs.85%29.aspx</remarks>
    Private Declare Function GetForegroundWindow Lib "user32.dll" () As IntPtr

User-Defined Types:

None.

Notes:

None.

Tips & Tricks:

Please add some!

Sample Code:

public ApplicationState AppState
{
    get
    {
        Process[] processCollection = Process.GetProcessesByName(ProcessName);
        if(processCollection != null && processCollection.Length  >= 1 &&
            processCollection[0] != null)
        {
            IntPtr activeWindowHandle = Win32.GetForegroundWindow();
            //Optional int ProcessID;
            //Optional Win32.GetWindowThreadProcessId(GetForegroundWindow(),out ProcessID)
            foreach(Process wordProcess in processCollection)
            {
                //Optional if( ProcessID == wordProcess.Id ) return ApplicationState.Focused;
                if(wordProcess.MainWindowHandle == activeWindowHandle)
                {
                    return ApplicationState.Focused;
                }
            }
            return ApplicationState.Running;
        }
        return ApplicationState.NotRunning;
    }
}

Alternative Managed API:

The ManagedWindowsApi project (http://mwinapi.sourceforge.net) provides a static property ManagedWinapi.SystemWindow.ForegroundWindow.