摘自http://msdn.microsoft.com/zh-tw/library/ms171548(v=vs.110).aspx

首先，使用DllImport引入两个函数：

// Get a handle to an application window.
[DllImport("USER32.DLL", CharSet = CharSet.Unicode)]
public static extern IntPtr FindWindow(string lpClassName,
string lpWindowName);

// Activate an application window.
[DllImport("USER32.DLL")]
public static extern bool SetForegroundWindow(IntPtr hWnd);

然后首先使用FindWindow函数获取到需要按键的窗口句柄，以计算器为例。这里体现了这个方法的局限性，就是似乎不能触发全局快捷键。

// Get a handle to the Calculator application. The window class
// and window name were obtained using the Spy++ tool.
IntPtr calculatorHandle = FindWindow("CalcFrame","Calculator");

然后使用SetForegroundWindow函数将这个窗口调到最前。

SetForegroundWindow(calculatorHandle);

接下来就可以直接使用SendKeys.SendWait之类的发送按键了。

SendKeys.SendWait("111");
SendKeys.SendWait("*");
SendKeys.SendWait("11");
SendKeys.SendWait("=");

完整代码如下：

using System;
using System.Runtime.InteropServices;
using System.Drawing;
using System.Windows.Forms;
// Get a handle to an application window.
[DllImport("USER32.DLL", CharSet = CharSet.Unicode)]
public static extern IntPtr FindWindow(string lpClassName,
string lpWindowName);

// Activate an application window.
[DllImport("USER32.DLL")]
public static extern bool SetForegroundWindow(IntPtr hWnd);

// Send a series of key presses to the Calculator application.
private void button1_Click(object sender, EventArgs e)
{
// Get a handle to the Calculator application. The window class
// and window name were obtained using the Spy++ tool.
IntPtr calculatorHandle = FindWindow("CalcFrame","Calculator");

// Verify that Calculator is a running process.
if (calculatorHandle == IntPtr.Zero)
{
MessageBox.Show("Calculator is not running.");
return;
}

// Make Calculator the foreground application and send it
// a set of calculations.
SetForegroundWindow(calculatorHandle);
SendKeys.SendWait("111");
SendKeys.SendWait("*");
SendKeys.SendWait("11");
SendKeys.SendWait("=");
}

注意如果是命令行程序的话需要手动添加System.Windows.Forms引用，否则找不到SendKeys类。