第一次用javascript写了一个不用刷新就能实现－－用户名验证的例子

<script type="text/javascript" >
        var xmlHttp;
        function createXMLHttpRequest()
        ...{
            xmlHttp = false
            if(window.XMLHttpRequest)//开始初始化XMLHttpRequest对象//Mozilla浏览器
            ...{
                xmlHttp = new XMLHttpRequest();
                if(xmlHttp.overrideMimeType) //设置Mime类别
                ...{           
                    xmlHttp.overrideMimeType("text/xml");
                }
            }
            else if(window.ActiveXObject)
            ...{
                try
                ...{
                    xmlHttp = new ActiveXObject("Msxml2.XMLHTTP");
                }
                catch(e)
                ...{
                    try
                    ...{
                        xmlHttp = new ActiveXObject("Micosoft.XMLHTTP");
                    }
                    catch(e)
                    ...{
                    }
                }
            }
            if(!xmlHttp)
            ...{
                //异常,创建实例失败
                window.alert("不能创建XMLHttpRequest对象实例");
                return false;
            }
        }

        function CheckUserNameURL(url)
        ...{
            createXMLHttpRequest();
            xmlHttp.onreadystatechange = processRequest;
            xmlHttp.open("POST",url,true);
            xmlHttp.send(null);
        }

        //处理返回信息的函数
        function processRequest()
        ...{
            if(xmlHttp.readyState == 4)//判断对象状态
            ...{
                if(xmlHttp.status == 200)//信息已经成功返回,开始处理信息
                ...{ 
                    var aa = xmlHttp.responseText;
                    var bb = aa.substr(0,1);
                    if(bb == 0)
                    ...{
                        alert("可以注册");
                    }
                    else if(bb == 1)
                    ...{
                        alert("用户已经存在，请选择其它用户名称");
                    }
                    else
                    ...{
                        alert("出现错误");
                    }
                }
                else
                ...{
                    //页面不正常
                    alert("你所请求的页面有异常");
                }
            }
        }

        function userCheck()
        ...{
            var userName = document.getElementById("userName").value;
            if(userName == "")
            ...{
                alert("用户名不能为空");
                document.getElementById("userName").focus();
                return false;
            }
            else
            ...{
                CheckUserNameURL("logon.aspx?userName=" + userName );
            }
        }
        </script>

上面这段代码是用来对用户进行判断,如果已经注册了,那么就会弹出一个对话框

说＂用户已经存在，请选择其它用户名称＂

如果不存在就提示＂可以注册＂；

下面是webForm1.aspx的部门代码，

                <TABLE id="Table1" style="WIDTH: 408px; HEIGHT: 99px" cellSpacing="1" cellPadding="1" width="408"
                    border="0">
                    <TR>
                        <TD align="right">用户名:</TD>
                        <TD><asp:textbox id="userName" runat="server"></asp:textbox>
                            <INPUT id="checkUsername" type="button" value="验证" onmousemove="userCheck();">
                        </TD>
                    <TR>
                        <TD align="right">密 码:</TD>
                        <TD><asp:textbox id="txtPassword" runat="server"></asp:textbox></TD>
                    </TR>
                    <TR>
                        <TD align="right"></TD>
                        <TD>
                            <asp:Button id="btnCheckUser" runat="server" Text="验  证"></asp:Button>
                            <DIV></DIV>
                        </TD>
                    </TR>
                </TABLE>

下面是logon.aspx.cs的代码，这段代码放在page_load里就行了

            string userName = "";
            try
            ...{
                userName = this.Request.QueryString["userName"].ToString();
                string sqlString = "SELECT COUNT(*) FROM Users WHERE userName ='" + userName + "'";
                string count = SetConnection.ExecuteScalar(sqlString );
                this.Response.Write(count);
                this.lblMessage.Text = count ;
            }
            catch
            ...{
            }

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本来是想把 javaScript 代码放到一个.js的代码里，可是不知道为什么一放到去然后在title里引用时老是出错，不知道是什么原因．

在写这个验证当中遇到的最大的困难就是，xmlHttp.responseText 返回来的字符串如何把第一个字取回来，弄了老久，今天看到可以用string.substr()这个方法，解决了上面问题．

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