使用平均值时LINQ查询中的错误 [英] Error in linq query when using average

问题描述

我的数据库中有两个表picture和pictureratings.

I have two tables, picture and pictureratings in my database.

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; } 
}

将为每个评级创建一个新的pictureratings行.我想按图片ID将pictureratings表分组，然后计算喜欢人数.我想在totalrating属性的picture表中显示喜欢的人.

For every rating a new pictureratings row will be created. I want to group pictureratings table by the picture id and then count the likes. I want to show those likes in picture table in totalrating property.

所以按照我现在的样子，我可以编写以下代码

So as per my now I am able to write the following code

 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Web API返回查询总数.我正在显示picture属性，例如iduser，picTitle，picFilename，pictime，nuditylevel和fakeslevel.

I am using web api to return query total. I am showing picture properties like iduser, picTitle, picFilename, pictime, nuditylevel and fakeslevel.

目前，一切正常，但是当我添加totalrating = pl.Average(c => c.likenumber)时，我得到一个异常提示

As per now every thing runs smooth, but when I add totalrating = pl.Average(c => c.likenumber) , I get an exception saying

强制转换为值类型"Double"，因为实例化值为null.结果类型的通用参数或查询必须使用可为空的类型.

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.

如何清除错误?

推荐答案

也许问题在于pl没有记录，请尝试

Maybe the problem is that pl has no records, try

替换

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

使用

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

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