Interact with a form using handle C#

I am trying to automate a dynamically appearing dialogue box. I need to pass text to it,s text field and then press a button over this. What I have tried so far.

 [DllImport("user32.dll", EntryPoint = "FindWindow", SetLastError = true)]
        static extern IntPtr FindWindowByCaption(IntPtr ZeroOn开发者_开发技巧ly, string lpWindowName);

IntPtr handle= FindWindowByCaption(System.IntPtr.Zero, "Caption Of File");

I am getting the handle of dialogue box correctly.

List<IntPtr> childWindows=  GetChildWindows(handle);//To get the child controls in this dialogue box

Source

But when I try to cast it to control I get null.

foreach (IntPtr i in childWindows) 
{
    Control c = Control.FromHandle(i);
}

So can any body tell what is wrong.I am supposing that I shall cast handle to control and then interact with control properties(e.g: text).

I've been using code like this sucessfully for years to perform single sign on to an application that prompts the user for their username/pwd/domain. The only caution is you need to know the control structure of the dialog you're targeting, but this is easily accomplished with Spy++ and rarely changes. Of course you will need to modify this code for the control structure of your window.

        [DllImport("user32.dll")]
        static extern IntPtr FindWindowEx(IntPtr hwndParent, IntPtr hwndChildAfter,
            string lpszClass, string lpszWindow);

        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        public static extern IntPtr SendMessage(IntPtr hWnd, uint msg, int wParam, string lParam);

        [DllImport("User32.Dll")]
        public static extern IntPtr PostMessage(IntPtr hWnd, uint msg, int wParam, int lParam);

        private const uint WM_GETTEXTLENGTH = 0x000E;
        private const uint WM_SETTEXT = 0x000C;
        private const uint WM_GETTEXT = 0x000D;
        private const uint BM_CLICK = 0x00F5;
        private const uint WM_CLOSE = 0x0010;

        enum GetWindow_Cmd : uint
        {
            GW_HWNDFIRST = 0,
            GW_HWNDLAST = 1,
            GW_HWNDNEXT = 2,
            GW_HWNDPREV = 3,
            GW_OWNER = 4,
            GW_CHILD = 5,
            GW_ENABLEDPOPUP = 6
        }

var dialog FindWindow("optionalClassNameHere", "Log On"); //Get the handle of the window
 var w3 = GetWindow(dialog , (uint)GetWindow_Cmd.GW_CHILD);  //I use GetWindow to walk the window controls
 var wUid = FindWindowEx(w3, IntPtr.Zero, "Edit", "");
 var w4 = GetWindow(wUid, (uint)GetWindow_Cmd.GW_HWNDNEXT);
 var wPwd = FindWindowEx(w4 , IntPtr.Zero, "Edit", "");
 var wOK = FindWindowEx(w3, IntPtr.Zero, "Button", "OK");
 SendMessage(wUid, WM_SETTEXT, 0, _WinDomain + "\\" + Username); //Send username to username edit control
 SendMessage(wPwd, WM_SETTEXT, 0, Password); //Send password to password edit control
 PostMessage(wOK, BM_CLICK, 0, 0); //Send left click(0x00f5) to OK button

Control.FromHandle can only work for controls in your process that are implemented by Control descendents. I'd guess this window is outside your process.

You need to use Win32 API methods to modify it.