求 文本框输入的数据如何与数据库中的数据相比较
这是我写的代码
  protected void Btn_Add_Click(object sender, EventArgs e)
    {
        
            string con = ConfigurationManager.ConnectionStrings["enventprocessConnectionString"].ConnectionString;
            SqlConnection mycon = new SqlConnection(con);
            mycon.Open();

            string sql1 = "select username from userinfo";
            SqlCommand cm = new SqlCommand(sql1, mycon);
            SqlDataReader dr=cm.ExecuteReader(CommandBehavior.CloseConnection);
          while (dr.Read())
          {

              for (int i=0;i<dr.FieldCount;i++)
              {
                 if (Username.Text.ToString()==dr[i].ToString())
                 {
                 
                 Response.Write("<script>alert('你输入的用户名已经存在');</script>");
                 
                 }
                 else
                 {
                     SqlConnection cn = binddata.con();
                     string sql = "insert into userinfo(username,password,identify) values               ('" + Username.Text + "','" + Password.Text + "','录入员')";
                     SqlCommand cmd = new SqlCommand(sql,cn );
                     cmd.ExecuteNonQuery();
                     Response.Write("<script>alert('添加用户成功')</script>");
                     Username.Text = "";
                     cn.Close();
                     
                 }
        当输入的数据在数据库中存在时,没有提示"已经存在数据"     是什么原因呢?
 急需   

如果你要查找输入的数据是否在数据库里存在的话要定义一个变量接收他的个数

string sql1 = string.format("select count(*) from userinfo where username='{0}' ","这里写文本框控件的名字.TEXT");
在做判读
如果
count>0说明存在