Distinct() linq query

我正在尝试使用distinct()方法筛选linq查询，但我一直在获取所有数据记录(包括重复(。我尝试了以下变体，但似乎都失败了。

           int total = Data.Count();
           // Data = Data.GroupBy(member => member.Tag).Select(x => x.OrderBy(y => y.Name).First());
           // Data = Data.OrderByDescending(c => c.UploadDate);
            Data = Data.Distinct().OrderBy(value => value.Tag);
            var data = Data.ToList();

如何通过显示按唯一tags字段名筛选的所有数据字段名来筛选查询？我的tag字段名也包含NULL数据。

以下是我的整个方法，供进一步参考：

        [Authorize]
    [HttpPost]
   private HttpResponseMessage method(HttpContext request, Query query)
    {
        if (User.IsInRole("admin") || User.IsInRole("art"))
        {
            IQueryable<database_B> Data = null;
            if (!string.IsNullOrEmpty(query.name))
            {
                var ids = query.name.Split(',');
               // var dataMatchingTags = db.database_B.Where(c => ids.Any(id => c.Name.Contains(id)));
                if (Data == null)
                    Data = dataMatchingTags;
                else
                    Data = Data.Union(dataMatchingTags);
            }
            if (Data == null) // If no tags or name is being queried, apply filters to the whole set of products
                Data = db.database_B;
            if (query.endDate != null)
            {
                Data = Data.Where(c => c.UploadDate <= query.endDate);
            }
            if (query.startDate != null)
            {
                Data = Data.Where(c => c.UploadDate >= query.startDate);
            }
            int total = Data.Count();
           // Data = Data.GroupBy(member => member.Tag).Select(x => x.OrderBy(y => y.Name).First());
           // Data = Data.OrderByDescending(c => c.UploadDate);
            Data = Data.Distinct().OrderBy(value => value.Tag);
            var data = Data.ToList();
            if (!data.Any())
            {
                var message = string.Format("No data found");
                return Request.CreateErrorResponse(HttpStatusCode.NotFound, message);
            }
           // return Request.CreateResponse(HttpStatusCode.OK, data);
            return Request.CreateResponse(HttpStatusCode.OK, new { total, data });
        }

感谢您的进一步帮助。