// 声明部分
[StructLayout(LayoutKind.Sequential)]
internal struct MOUSEINPUT
{
    public int dx;
    public int dy;
    public int mouseData;
    public int dwFlags;
    public int time;
    public IntPtr dwExtraInfo;
}

[StructLayout(LayoutKind.Sequential)]
internal struct KEYBDINPUT
{
    public short wVk;
    public short wScan;
    public int dwFlags;
    public int time;
    public IntPtr dwExtraInfo;
}

[StructLayout(LayoutKind.Sequential)]
internal struct HARDWAREINPUT
{
    public int uMsg;
    public short wParamL;
    public short wParamH;
}

[StructLayout(LayoutKind.Explicit)]
internal struct Input
{
    [FieldOffset(0)]
    public int type;                  // Mouse: 0; Keyboard: 1; hardware: 2
    [FieldOffset(4)]
    public MOUSEINPUT mi;
    [FieldOffset(4)]
    public KEYBDINPUT ki;
    [FieldOffset(4)]
    public HARDWAREINPUT hi;
}

[DllImport("user32.dll")]
private static extern uint SendInput(uint nInputs, Input[] pInputs, int cbSize);




//使用部分
Input[] input = new Input[T_CartonID.Text.Length * 2 + 4];
int i = 0;

foreach (char nChar in MyText)
{
    input[i].type = 1;
    input[i].ki.wVk = Convert.ToInt16(nChar);

    i++;

    input[i].type = 1;
    input[i].ki.wVk = Convert.ToInt16(nChar);
    input[i].ki.dwFlags = 2;

    i++;
}

// Enter key
input[input.Length - 4].type = 1;
input[input.Length - 4].ki.wVk = 0x0D;
input[input.Length - 3].type = 1;
input[input.Length - 3].ki.wVk = 0x0D;
input[input.Length - 3].ki.dwFlags = 2;

input[input.Length - 2].type = 1;
input[input.Length - 2].ki.wVk = 0x0A;
input[input.Length - 1].type = 1;
input[input.Length - 1].ki.wVk = 0x0A;
input[input.Length - 1].ki.dwFlags = 2;

SendInput((uint)input.Length, input, Marshal.SizeOf(input[0]));

------解决方案--------------------
如果向非当前窗体发送，则需要先找到目标窗体的句柄，然后发送对应的消息即可

------解决方案--------------------
可以用API的FindWindow 和 SetForegroundWindow来强制聚焦到其他应用程序上


private void Form1_Load(object sender, EventArgs e)
{
     KeyboardHook keyboardHook = new KeyboardHook();//全局按键
      keyboardHook.KeyDown += new KeyEventHandler(keyboardHook_KeyDown);
     keyboardHook.Start();
}

void keyboardHook_KeyDown(object sender, KeyEventArgs e)
{
      if (e.KeyCode == Keys.F2)
     {
          SendKeys.Send("%A");//发送ALT+A
      }
}

------解决方案--------------------
不好意思，SendKeys.Send("%A");//发送ALT+A
A应该是小写，大写不行，写错了。