页面上有5个FileUpload，FileUpload1到FileUpload5
想点击按钮实现多文件上传，并保存到数据库，点击时验证下FileUpload是否有文件
        protected void btnupload_Click(object sender, EventArgs e)
        {
            int j;
            for (j = 1; j <= 5;j++ )
            {
                string fileup="FileUpload"+j.ToString();
                upload((FileUpload)this.FindControl(fileup));//调用upload方法  
             }
        }
     protected void upload(FileUpload myFileUpload)  
     {  
          if (myFileUpload.HasFile)
         {
          //保存数据库
         }
     }
但是myFileUpload.HasFile总出错，提示未将对象引用设置到对象的实例。检查发现myFileUpload始终是NULL
这个该怎么解决，请教下