// https://teratail.com/questions/115871
using System;
using System.Diagnostics;
using System.Linq;
using System.Runtime.InteropServices;

namespace ConsoleApp1
{
    class Program
    {
        [DllImport("user32.dll")]
        static extern IntPtr PostMessage(IntPtr hWnd, uint Msg, IntPtr wParam, IntPtr lParam);

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

        // 送信するメッセージ
        const uint WM_KEYDOWN = 0x100;
        const uint WM_LBUTTONDOWN = 0x0201;
        const uint WM_LBUTTONUP = 0x0202;
        const uint None = 0;
        const uint MK_LBUTTON = 0x0001;

        /// <summary>
        /// メモ帳にaと入力する
        /// </summary>
        static void Test1()
        {
            // https://www.papy.in/bbs/vc/200609/06090004.html
            var hwnd = Process.GetProcessesByName("notepad").First().MainWindowHandle;
            hwnd = FindWindowEx(hwnd, IntPtr.Zero, "Edit", null);

            // aと入力
            PostMessage(hwnd, WM_KEYDOWN, (IntPtr)0x41, IntPtr.Zero);
        }

        /// <summary>
        /// ペイントでクリックする
        /// </summary>
        static void Test2()
        {
            // https://social.msdn.microsoft.com/Forums/ja-JP/90258227-a820-4de3-ae71-4bbf88076dd2/wpfc?forum=wpfja
            var hwnd = Process.GetProcessesByName("mspaint").First().MainWindowHandle;
            hwnd = FindWindowEx(hwnd, IntPtr.Zero, "MSPaintView", null);
            hwnd = FindWindowEx(hwnd, IntPtr.Zero, null, null);

            UInt16 x = 300;
            UInt16 y = 300;
            UInt32 pos = ((UInt32)y << 16) | (UInt32)x;

            // クリックする
            PostMessage(hwnd, (uint)WM_LBUTTONDOWN, new IntPtr((int)MK_LBUTTON), new IntPtr(pos));
            PostMessage(hwnd, (uint)WM_LBUTTONUP, new IntPtr((int)None), new IntPtr(pos));
        }

        static void Main(string[] args)
        {
            Test1();
            Test2();
        }
    }
}