//声明一个线程

private static System.Threading.Mutex mutex;

/// <summary>

/// 应用程序的主入口点。

/// </summary>

[STAThread]

static void Main()

{

    Application.EnableVisualStyles();

    Application.SetCompatibleTextRenderingDefault(false);

    //开启线程

    mutex = new System.Threading.Mutex(true, "OnlyRun");

    if(mutex.WaitOne(0, false))

    {

        FormLogin fl = new FormLogin();

        fl.ShowDialog();

        if(fl.DialogResult == DialogResult.OK)

        {

            Application.Run(new FormMain());

        }

        else

        {

            return;

        }

    }

    else

    {

        MessageBox.Show("程序已经在运行！", "提示", MessageBoxButtons.OK, MessageBoxIcon.Information);

        Application.Exit();

    }

}

if(textBox1.Text == "123")

{ //验证用户名密码成功

    this.DialogResult = DialogResult.OK; //返回一个登录成功的对话框状态

    this.Close(); //关闭登录窗口

}

FormLogin fl = new FormLogin();

fl.ShowDialog();

if(fl.DialogResult == DialogResult.OK)

{

    Application.Run(new FormMain());

}

else

{

    return;

}

private void button1_Click(object sender, EventArgs e)

{

    if(textBox1.Text == "aaa")

    { //验证用户名密码成功

        this.DialogResult = DialogResult.OK; //返回一个登录成功的对话框状态

        this.Close(); //关闭登录窗口

    }

}