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Select the application window and PostMessage

I created the application (KeySimulator) which is running on the Windows VM. This app is sending an 'ENTER' key to another application (DraftSight). DraftSight is running on the same VM. Maine purpose of my app is to select DraftSight and click 'ENTER' just in case if any messages are pop up on the DraftSight.
Everything is working except my app sometimes is not recognizing the DS app.
My app is running 24/7 and I have a Task to start my app on a log-in or if my app is not running.
This is my code for that function:
[DllImport("USER32.DLL", CharSet = CharSet.Unicode)]
public static extern IntPtr FindWindow(string lpClassName, string lpWindowName);
[DllImport("USER32.DLL")]
public static extern bool SetForegroundWindow(IntPtr hWnd);
IntPtr draftSightHandle = FindWindow("Qt5QWindowIcon", null);
SetForegroundWindow(draftSightHandle);
const UInt32 WM_KEYDOWN = 0x0100;
PostMessage(draftSightHandle, WM_KEYDOWN, (int)Keys.Enter, 0);

Send Mouse Clicks to a Minimized C# Application using The Same App

There're a lot of ways to simulate a mouse click in an inactive window. I want my program to make clicks even when it's minimized and of course without capturing the main mouse.
Specifically, my C# program will contain a Web Control displaying some flash content and this is where I want the clicks to occur.
here's the techniques I tried:
1.[DllImport("user32.dll", CharSet=CharSet.Auto, CallingConvention=CallingConvention.StdCall)]
public static extern void mouse_event(uint dwFlags, uint dx, uint dy, uint cButtons, uint dwExtraInfo);
2.[DllImport("user32.dll", SetLastError = true)]
public static extern bool SendMessage(int hWnd, UInt32 Msg, IntPtr wParam, IntPtr lParam);
Both of them did not do any good in my case. Surprisingly, I was able to use the SendMessage() function to send mouse clicks in a minimized chrome displaying some flash content. but, it doesn't seem to do that in my C# program. Is there a possibility that my program not receiving the message for some reason?
Any Hints on how can I do this?

Are there any dangers of using Windows API calls in C#

I want to watermark a textbox, and found several different ways of doing it, but one that I liked uses SendMessage and an external DLL. However, I think I heard somewhere that doing it this way can cause BSOD since it isn't managed. Is this true, or is it just hear-say.
http://vidmar.net/weblog/archive/2008/11/05/watermarked-textbox-in-windows-forms-on-.net.aspx
private const uint ECM_FIRST = 0x1500;
private const uint EM_SETCUEBANNER = ECM_FIRST + 1;
[DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = false)]
static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, uint wParam, [MarshalAs(UnmanagedType.LPWStr)] string lParam);

The short answer is no. It won't cause a BSOD, although it could crash your program.
WinForms is basically built on top of Windows API calls, so when done right, custom API calls should work good as well.
One other thing to keep in mind is that if you do call the Windows API, it may create portability issues, such as when porting to Mono, as those DLLs will most likely not be available.

How to dispose the Win32 APIs in c#.net

I am using c#.net to develop a winform application.My winform application is using the below components
1)Win 32 dlls (using System.Runtime.InteropServices)
2)Timers(3 in count) (System.Timers)
3)Excel Interop
The memory of the application is not at all coming down .As timers are running continuosly so i cannot dispose the
So would like to implement dispose patterns .
Is it necessary to dispose the win32 APIs apart from Excel interop.?
If necessary can you please suggest the best way to call and dispose the win32 APIs.
Some of the Win32 APIs Used in application are listed below.
DllImport("user32.dll", SetLastError = true, CharSet = CharSet.Auto)]
static extern uint RegisterWindowMessage(string lpString);
[DllImport("wininet.dll")]
private extern static bool InternetGetConnectedState(out int netConnection, int val);
[DllImport("Oleacc.dll")]
private static extern int AccessibleObjectFromWindow(IntPtr hwnd, uint dwObjectID, byte[] riid, ref Excel.Window ptr);
[DllImport("WtsApi32.dll")]
private static extern bool WTSRegisterSessionNotification(IntPtr hWnd, [MarshalAs(UnmanagedType.U4)]int dwFlags);
[DllImport("WtsApi32.dll")]
private static extern bool WTSUnRegisterSessionNotification(IntPtr hWnd);

The hWnd parameters in your function calls are all window handles. As a general rule, whenever you have finished using a window handle in the windows API, you need to explicitly release it using the CloseHandle function

How can I put the monitor to sleep when using C#/WPF (Not WinForms!)

The trouble I'm having is with using...
[DllImport("user32")]
public static extern IntPtr SendMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);
...and then...
SendMessage(???, WM_SYSCOMMAND, (IntPtr)SC_MONITORPOWER, (IntPtr)MONITOR_OFF);
SendMessage wants the Form's Handle but I'm not using Forms so cannot get the Handle.
Is there any other way I can put the monitor to sleep OR get the Handle in WPF?

To get the Handle for a WPF Window use:
new WindowInteropHelper(YourWPFWindow).Handle
MSDN reference

Write new WindowInteropHelper(someWindow).Handle

Or use
HwndSource source = (HwndSource)HwndSource.FromVisual(this);
source.Handle...
to get a handle from a single element in the form, also runs for the whole window.