c# - 使用平均值时 linq 查询出错

Question

我的数据库中有两个表，picture 和 pictureratings。

public partial class picture
{
    public int idpicture { get; set; }
    public int iduser { get; set; }
    public string picTitle { get; set; }
    public string picFilename { get; set; }
    public System.DateTime pictime { get; set; }
    public int nuditylevel { get; set; }
    public int fakeslevel { get; set; }

    // This property will hold the total accumulated/summed 
    // up rating of a picture
    public int totalrating { get; set; }    
}

public partial class pictureratings 
{
    public int idrating { get; set; }
    public int idpictures { get; set; }
    public int iduser { get; set; }
    public System.DateTime iddatetime { get; set; }
    public int iduserrateddby { get; set; }
    public int rating { get; set; } 
}

对于每个评级，都会创建一个新的 pictureratings 行。我想按图片 ID 对 pictureratings 表进行分组，然后计算点赞数。我想在 totalrating 属性的 picture 表中显示那些喜欢的内容。

根据我现在的情况，我可以编写以下代码

 var combo = from p in db.picturedetails
                    join l in db.picturelikes on p.idpictures equals l.idpictures into pl
                    select new LikesandPictureDetails
                    {
                             IdUserPic = p.iduser,
                           IdPicturess =p.idpictures,
                           Likes = p.likes,
                           NudityLevel = p.nuditylevel,
                           PicTitle = p.picTitle,
                           PicTime = p.pictime,
                           picFilename=p.picFilename,
                           Count=pl.Count(),
                           totalrating = pl.Average(c => c.likenumber) // This line is causing error
                    };

我正在使用网络 API 返回查询总数。我正在显示 picture 属性，例如 iduser、picTitle、picFilename、pictime、nuditylevel 和 fakeslevel。

现在一切都运行顺利，但是当我添加 totalrating = pl.Average(c => c.likenumber) 时，我得到一个异常提示

The cast to value type 'Double' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.

如何消除错误？

Answer 1

可能是pl没有记录的问题，试试

替换

totalrating = pl.Average(c => c.likenumber) // This line is causing error

与

totalrating = pl.DefaulIfEmpty(0).Average(c => c.likenumber)