登录验证码

Question

                    string aa = this.TextBox3.Text.ToString();
                    if (aa == this.Label1.Text.ToString())
                    {
                        Response.Write("<script>alert('登陆成功！');</script>");
                        Session["adminname"] = this.TextBox1.Text.ToString();
                    }
                    else
                    {
                        Response.Write("<script>alert('验证码错误！');</script>");
                    }
登录时总是报验证码错误，是怎么回事啊求大侠帮忙！！！！！

Answer 1

   if (this.TextBox3.Text.Trim()== this.Label1.Text.Trim())
   {
   Response.Write("<script>alert('登陆成功！');</script>");
   Session["adminname"] = this.TextBox1.Text.Trim();
   }
   else
   {
   Response.Write("<script>alert('验证码错误！');</script>");
   }

Answer 2

string aa = this.TextBox3.Text.Replace(" ","");
  if (aa == this.Label1.Text.Replace(" ",""))
  {
  Response.Write("<script>alert('登陆成功！');</script>");
  Session["adminname"] = this.TextBox1.Text.ToString();
  }
  else
  {
  Response.Write("<script>alert('验证码错误！');</script>");
  }

Answer 3

replace这个用法的前提是什么啊???