C#点击按钮实现登录功能

建好登录窗体
当对窗口进行拉伸/最大化操作时，布局不会自适应会很难看
FormBorderStyle设为FixedSingle禁止拉伸
MaximizeBox设为False禁止最大化

private void btnLogin_Click(object sender, EventArgs e)
{
  //获取用户输入信息
  string uName = textUserName.Text.Trim();
  string uPwd = textUserPwd.Text.Trim();
  //判断是否为空
  if (string.IsNullOrEmpty(uName))
  {
    MessageBox.Show("请输入账号！", "登录提示", MessageBoxButtons.OK, MessageBoxIcon.Error);
    textUserName.Focus();
    return;
  }
  //与数据库通信
  string connString = "server=.;database=student;uid=yaya;pwd=521444";
  SqlConnection conn = new SqlConnection(connString);
  string sql = "select count(1) from UserInfo where Username='" + uName + "' and UserPwd='" + uPwd + "'";
  SqlCommand cmd = new SqlCommand(sql, conn);
  conn.Open();
  object o = cmd.ExecuteScalar();
  conn.Close();

  //o == null 返回为null
  //o == DBNull.Value 数据库里的值为null
  // (int)o == 0 数据库不存在/不一致
  if (o == null || o == DBNull.Value || (int)o == 0){
    MessageBox.Show("登录账号或密码错误，请重新登陆", "登录提示",
    MessageBoxButtons.OK, MessageBoxIcon.Error);
  }else{
    MessageBox.Show("登录成功", "登录提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
  }
}

参数化sql

        private void btnLogin_Click(object sender, EventArgs e)
        {
            string uName = textUserName.Text.Trim();
            string uPwd = textUserPwd.Text.Trim();
            if (string.IsNullOrEmpty(uName))
            {
                MessageBox.Show("请输入账号！", "登录提示", MessageBoxButtons.OK, MessageBoxIcon.Error);
                textUserName.Focus();
                return;
            }
            string connString = "server=.;database=student;uid=yaya;pwd=521444";
            SqlConnection conn = new SqlConnection(connString);
             //写查询语句 拼接式容易被sql注入 推荐参数化sql
            //string sql = "select count(1) from UserInfo where Username='" + uName + "' and UserPwd='" + uPwd + "'";
            string sql = "select count(1) from UserInfo where Username=@Username and UserPwd=@UserPwd";
            SqlParameter paraUName = new SqlParameter("@Username", uName);
            SqlParameter paraUPwd = new SqlParameter("@UserPwd", uPwd);

            SqlCommand cmd = new SqlCommand(sql, conn);
            cmd.Parameters.Clear();
            cmd.Parameters.Add(paraUName);
            cmd.Parameters.Add(paraUPwd);

            conn.Open();
            object o = cmd.ExecuteScalar();
            conn.Close();
            if (o == null || o == DBNull.Value || (int)o == 0){
                MessageBox.Show("登录账号或密码错误，请重新登陆", "登录提示",
                    MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
            else
            {
                MessageBox.Show("登录成功", "登录提示", MessageBoxButtons.OK, MessageBoxIcon.Information);
            }

        }