public partial class Form1 : Form
    {
protected override CreateParams CreateParams
        {
get
            {
                CreateParams cp = base.CreateParams;
                cp.ExStyle |= (int)0x08000000L;      //把窗体设置成WS_EX_NOACTIVATE样式
                 return cp;
            }
        }
public Form1()
        {
this.TopMost = true;
            InitializeComponent();
        }

private void button1_Click(object sender, EventArgs e)
        {
            SendKeys.Send("A");
        }
    }