public partial class Login : System.Web.UI.Page
{
    private static string sValidator = "";
    private readonly string sValitorImageUrl = "ValidateImage.aspx?Validator=";
    private StringBuilder LetterList = new StringBuilder();//可变字符串,用于生成验证码
    

    protected void Page_Load(object sender, EventArgs e)
    {
        lab_RegistID.Text = Session["RegistID"].ToString();
        if (!Page.IsPostBack)
        {
            sValidator = CreatValitedateString(6);
            ValidateImageLo.ImageUrl = sValitorImageUrl + sValidator;
            compValidator = sValidator;
        }
    }

    private int GetRandomint(int min, int max)//获取基础字符串的随机数位
    {
        Random random = new Random();

        return (random.Next(min, max));

    }

    private void GetLeterlist()//创建验证码的基础字符串
    {
        for (int i = 0; i < 10; i++)
        {
            LetterList.Append(i.ToString());

        }
        for (int i = 0; i < 26; i++)
        {
            LetterList.Append(((char)(i + (int)'a')).ToString());
        }
        for (int i = 0; i < 26; i++)
        {
            LetterList.Append(((char)(i + (int)'A')).ToString());
        }


    }

    private string CreatValitedateString(int n)
    {
        GetLeterlist();

        StringBuilder vs = new StringBuilder();
        for (int i = 0; i < n; i++)
        {
            int index = GetRandomint(0, LetterList.Length - 1);
            vs.Append(LetterList[index].ToString());
            LetterList.Remove(index, 1);
        }
        return (vs.ToString());

    }
    protected void btn_Login_Click(object sender, EventArgs e)
    {

        if (txt_VlidateNum.Text != sValidator)
        {
            lab_ValidateMsg.Text = "请输入正确的验证码!";
           

            return;

        }
        else
        {
            Response.Redirect("~/HomePage.aspx");
        }
        

        
    }
}

public string Text 
{   
     get { return (string)ViewState["Text"]; }   
     set { ViewState["Text"] = value; }
}

------解决方案--------------------
我用一个LinkBtton想来执行刷新验证码的
但是clik事件没发生前，RequiredFieldValidator就提示验证码不能为空了
应该怎么做？才能绕开？
=====
其实不用linkbutton来刷新 用啊

function DianJi()//刷新验证码
   { document.getElementById("yan").src = "../include/ValidateCode.aspx" + "?" + Math.random(); }

<img id="yan" alt="刷新验证码"  style=" cursor:pointer" onclick="javascript:DianJi();" src="../include/ValidateCode.aspx"  />


------解决方案--------------------
验证码的生成是在ValidateImage.aspx页面里的。在其他页面的无效