本人新手刚学C#，怎么获取textbox的值从而进行判断，如果错了怎么弹出提示窗口，求详解

补充一下，是webForm不是windowsForm

string tx = textbox.text;
  判断用
if(**** != textbox.text)
{
    MessageBox.Show();
}
else
{   
   
}

好像不行啊

#region "非空检查"
            if (txtLoginId.Value.Trim().ToString() == "")
            {
                Response.Write("<script language:javascript>alert('登录名不能为空');</script>");
                return;
            }
            if (txtLoginPwd.Value.Trim().ToString() == "")
            {
                Response.Write("<script language:javascript>alert('登录密码不能为空');</script>");
                return;
            }
            #endregion
            string UserName = LoginManager.getLabelControlName(txtLoginId.Value.Trim().ToString(), txtLoginPwd.Value.Trim().ToString());  //调用查询方法
            if (UserName != "")
            {
                Session["Name"] = UserName;
                Response.Redirect("J28-Enclouse.aspx");
            }
            else
            {
                Response.Write("<script language:javascript>alert('用户名或密码错误');</script>");
            }

Thank you