SelectedValue 失效

/// <summary>
        /// 绑定代表下拉选项
        /// </summary>
        public void BindOwnerDataII(string unitId, string bindid)
        {

            BaseCore obj = new BaseCore();
            DataTable dt = obj.GetOwnerByBusiness(unitId);
            this.ddlOwner.DataSource = dt;
            this.ddlOwner.DataTextField = "fullname";
            this.ddlOwner.DataValueField = "systemuserid";
            this.ddlOwner.DataBind();
            this.ddlOwner.SelectedValue = bindid; 
            this.ddlOwner.Items.Insert(0, new ListItem("全部", "00000000-0000-0000-0000-000000000000"));

}

我的问题是 应该是由于 字符的大小写引起的, 因为 this.ddlOwner.SelectedValue = bindid;  bindid 为大写的GUID,但是 dt中的元素都是小写的,这里是什么原因我不知道,因为

GetOwnerByBusiness(unitId);从数据库中取到的GUID全部为大写的.   

改为使用下面的语句就看出问题来了,如果是大写的GUID会报错,所以把bindid 转换为小写了,这样就正常了.

  this.ddlOwner.Items.FindByValue(bindid.ToLower()).Selected = true;