System.Diagnostics.Process process = new System.Diagnostics.Process();
process = System.Diagnostics.Process.Start("mspaint.exe", path);
int processId = process.Id;
AutomationElement element = FindWindowByProcessId(processId);
System.Windows.Forms.SendKeys.SendWait("^s"); //发送 Ctrl + s 键
System.Windows.Forms.SendKeys.SendWait("%{F4}"); // 发送 Alt + F4 键

public static AutomationElement FindWindowByProcessId(int processId)
{
  AutomationElement targetWindow = null;
  int count = 0;
  try
  {
    Process p = Process.GetProcessById(processId);
    targetWindow = AutomationElement.FromHandle(p.MainWindowHandle);
    return targetWindow;
  }
  catch (Exception ex)
  {
    count++;
    StringBuilder sb = new StringBuilder();
    string message = sb.AppendLine(string.Format("Target　window　is　not　existing.try　#{0}", count)).ToString();
    if (count > 5)
    {
      throw new InvalidProgramException(message, ex);
    }
    else
    {
      return FindWindowByProcessId(processId);
    }
  }
}

SendKeys.SendWait("{F5}");     //发送F5按键
SendKeys.SendWait("^s");    //发送 Ctrl + s 键
SendKeys.SendWait("%{F4}");   // 发送 Alt + F4 键
//按键 代码 
BACKSPACE {BACKSPACE}, {BS}, 或 {BKSP} 
BREAK {BREAK} 
CAPS LOCK {CAPSLOCK} 
DEL or DELETE {DELETE} 或 {DEL} 
DOWN ARROW {DOWN} 
END {END} 
ENTER {ENTER}或 ~ 
ESC {ESC} 
HELP {HELP} 
HOME {HOME} 
INS or INSERT {INSERT} 或 {INS} 
LEFT ARROW {LEFT} 
NUM LOCK {NUMLOCK} 
PAGE DOWN {PGDN} 
PAGE UP {PGUP} 
PRINT SCREEN {PRTSC} 
RIGHT ARROW {RIGHT} 
SendKeys.SendWait("+{TAB}");
SendKeys.SendWait("%f");//alt+f
SendKeys.SendWait("{Tab}");
SendKeys.SendWait("{Enter}")
//多次按键的代码
//为了指定重复键，使用 {key number} 的形式。必须在 key 与 number 之间放置一个空格。//例如，{LEFT 42} 意指 42 次按下 LEFT ARROW 键；{h 10} 则是指 10 次按下 H 键。

using System;
using System.Text;
using System.Diagnostics;
using System.Threading;
using System.Windows.Automation;
using System.Runtime.InteropServices;
using System.Windows;
using System.Windows.Forms;
namespace UIATest
{
  class Program
  {
    static void Main(string[] args)
    {
      Process process = Process.Start(@"E:\WorkBook\ATP\WpfApp\bin\Debug\WpfApp.exe");
      int processId = process.Id;
      AutomationElement element = FindElementById(processId, "textBox1");
      SendKeys sendkeys = new SendKeys();
      sendkeys.Sendkeys(element, "Sending keys to input data");
      Console.WriteLine(sendkeys.ToString());
      sendkeys.Sendkeys(element, sendkeys.ContextMenu);
      Console.WriteLine(sendkeys.ToString());
      Console.WriteLine("Test finised."); 
    }
    /// <summary>
    /// Get the automation elemention of current form.
    /// </summary>
    /// <param name="processId">Process Id</param>
    /// <returns>Target element</returns>
    public static AutomationElement FindWindowByProcessId(int processId)
    {
      AutomationElement targetWindow = null;
      int count = 0;
      try
      {
        Process p = Process.GetProcessById(processId);
        targetWindow = AutomationElement.FromHandle(p.MainWindowHandle);
        return targetWindow;
      }
      catch (Exception ex)
      {
        count++;
        StringBuilder sb = new StringBuilder();
        string message = sb.AppendLine(string.Format("Target window is not existing.try #{0}", count)).ToString();
        if (count > 5)
        {
          throw new InvalidProgramException(message, ex);
        }
        else
        {
          return FindWindowByProcessId(processId);
        }
      }
    }
    /// <summary>
    /// Get the automation element by automation Id.
    /// </summary>
    /// <param name="windowName">Window name</param>
    /// <param name="automationId">Control automation Id</param>
    /// <returns>Automatin element searched by automation Id</returns>
    public static AutomationElement FindElementById(int processId, string automationId)
    {
      AutomationElement aeForm = FindWindowByProcessId(processId);
      AutomationElement tarFindElement = aeForm.FindFirst(TreeScope.Descendants,
      new PropertyCondition(AutomationElement.AutomationIdProperty, automationId));
      return tarFindElement;
    }
  }
}