我想防止这种情况的发生，但是要以不同的方式，而不是将DateCreated的类型设置为可为null的DateTime。
完整的异常如下所示:
当我在数据库上执行以下查询时，我可以看到那里没有空记录，因此我相信我应该避免将DateCreate属性设置为可空的DateTime，因为这对我来说意义不大(即使我知道它可以解决)问题)。我有什么理解不对的地方吗？
查询中的以下行引起了该问题:
DateCreated = subJoined.dateUploaded
这是完整的LINQ查询:
using (var db = new ABEntities())
{
var features = (from textObject in db.textObjects
join container in db.containers.DefaultIfEmpty() on textObject.textObjectPK equals
container.textObjectPK into tObjsContainerJoined
from subContainerJoin in tObjsContainerJoined.DefaultIfEmpty()
join tObjsMedia in db.media on subContainerJoin.mediaID equals
tObjsMedia.mediaID into tObjsMediaJoined
from subJoined in tObjsMediaJoined.DefaultIfEmpty()
from textContainer in tObjsContainerJoined
where
textObject.version == Constants.Versions.LATEST &&
textObject.textObjectTypeID == Constants.News.FEATURES &&
textObject.deployDate <= DateTime.Now
select new TextObject
{
Id = textObject.textObjectID,
Title = textObject.title,
ContainerId = textContainer.containerID,
Description = textContainer.container1,
DateCreated = textObject.deployDate,
Media = new Media
{
Title = subJoined.title,
MediaFormat = subJoined.extension,
MediaTypeID = subJoined.mediaTypeID,
MediaFile = subJoined.fileName,
Credit = subJoined.credit,
MembersOnly = subJoined.membersOnly,
LastModified = subJoined.lastModified,
DateCreated = subJoined.dateUploaded
},
TypeId = textObject.textObjectTypeID
}).OrderByDescending(t => t.DateCreated).ToList();
return features;
}
这是媒体类的定义:
[Serializable]
public class Media
{
public int Id { get; set; }
public string MediaFile { get; set; }
public string Title { get; set; }
public string Credit { get; set; }
public int? MediaTypeID { get; set; }
public string MediaFormat { get; set; }
public bool? isYoutube { get; set; }
public string YoutubeID { get; set; }
public int Width { get; set; }
public int Height { get; set; }
public int Views { get; set; }
public string Description { get; set; }
public int SiloID { get; set; }
public DateTime DateCreated { get; set; }
public bool IsVideo { get; set; }
public int SegmentId { get; set; }
public string Extension { get; set; }
public bool? ShowOnHomepage { get; set; }
public bool? MembersOnly { get; set; }
public DateTime? LastModified { get; set; }
}
正在生成的查询-由SQL Profiler捕获:
SELECT
[Project1].[textObjectPK] AS [textObjectPK],
[Project1].[textObjectID] AS [textObjectID],
[Project1].[title] AS [title],
[Project1].[containerID] AS [containerID],
[Project1].[container] AS [container],
[Project1].[deployDate] AS [deployDate],
[Project1].[title1] AS [title1],
[Project1].[extension] AS [extension],
[Project1].[mediaTypeID] AS [mediaTypeID],
[Project1].[fileName] AS [fileName],
[Project1].[credit] AS [credit],
[Project1].[membersOnly] AS [membersOnly],
[Project1].[C1] AS [C1],
[Project1].[dateUploaded] AS [dateUploaded],
[Project1].[textObjectTypeID] AS [textObjectTypeID]
FROM ( SELECT
[Extent1].[textObjectPK] AS [textObjectPK],
[Extent1].[textObjectID] AS [textObjectID],
[Extent1].[textObjectTypeID] AS [textObjectTypeID],
[Extent1].[title] AS [title],
[Extent1].[deployDate] AS [deployDate],
CAST( [Extent3].[lastModified] AS datetime2) AS [C1],
[Extent3].[mediaTypeID] AS [mediaTypeID],
[Extent3].[fileName] AS [fileName],
[Extent3].[title] AS [title1],
[Extent3].[extension] AS [extension],
[Extent3].[credit] AS [credit],
[Extent3].[dateUploaded] AS [dateUploaded],
[Extent3].[membersOnly] AS [membersOnly],
[Join4].[containerID] AS [containerID],
[Join4].[container] AS [container]
FROM [dbo].[textObjects] AS [Extent1]
LEFT OUTER JOIN (SELECT [Extent2].[textObjectPK] AS [textObjectPK], [Extent2].[mediaID] AS [mediaID]
FROM ( SELECT 1 AS X ) AS [SingleRowTable1]
INNER JOIN [dbo].[containers] AS [Extent2] ON 1 = 1 ) AS [Join1] ON [Extent1].[textObjectPK] = [Join1].[textObjectPK]
LEFT OUTER JOIN [dbo].[media] AS [Extent3] ON [Join1].[mediaID] = [Extent3].[mediaID]
INNER JOIN (SELECT [Extent4].[containerID] AS [containerID], [Extent4].[textObjectPK] AS [textObjectPK], [Extent4].[container] AS [container]
FROM ( SELECT 1 AS X ) AS [SingleRowTable2]
INNER JOIN [dbo].[containers] AS [Extent4] ON 1 = 1 ) AS [Join4] ON [Extent1].[textObjectPK] = [Join4].[textObjectPK]
WHERE (1 = [Extent1].[version]) AND (2 = [Extent1].[textObjectTypeID]) AND ([Extent1].[deployDate] <= (SysDateTime()))
) AS [Project1]
ORDER BY [Project1].[deployDate] DESC
最佳答案
您可以将查询中的值转换为可为空的类型，然后检查是否为Nullable，而不是将数据类型设置为null。
DateCreated = (DateTime?)subJoined.dateUploaded ?? DateTime.Now
您的查询将如下所示:
using (var db = new ABEntities())
{
var features = (from textObject in db.textObjects
join container in db.containers.DefaultIfEmpty() on textObject.textObjectPK equals container.textObjectPK into tObjsContainerJoined
from subContainerJoin in tObjsContainerJoined.DefaultIfEmpty()
join tObjsMedia in db.media on subContainerJoin.mediaID equals tObjsMedia.mediaID into tObjsMediaJoined
from subJoined in tObjsMediaJoined.DefaultIfEmpty()
from textContainer in tObjsContainerJoined
where textObject.version == Constants.Versions.LATEST &&
textObject.textObjectTypeID == Constants.News.FEATURES && textObject.deployDate <= DateTime.Now
select new TextObject
{
Id = textObject.textObjectID,
Title = textObject.title,
ContainerId = textContainer.containerID,
Description = textContainer.container1,
DateCreated = textObject.deployDate,
Media = new Media
{
Title = subJoined.title,
MediaFormat = subJoined.extension,
MediaTypeID = subJoined.mediaTypeID,
MediaFile = subJoined.fileName,
Credit = subJoined.credit,
MembersOnly = subJoined.membersOnly,
LastModified = subJoined.lastModified,
DateCreated = (DateTime?)subJoined.dateUploaded ?? DateTime.Now
},
TypeId = textObject.textObjectTypeID
}).OrderByDescending(t => t.DateCreated).ToList();
return features;
}
为什么在该特定行上显示错误？
免责声明:这只是一个猜测(如果我错了，请纠正我...)
可能是因为您进行了左联接，并且EF期望某些值可以为null，并且作为预防措施会引发错误。它只会在
DateTime数据类型上引发错误，因为它知道在其他属性为null(字符串和可为null的类型)时该怎么办。关于c# - 阻止 “' System.DateTime'失败，因为实例化值为null”，我们在Stack Overflow上找到一个类似的问题：https://stackoverflow.com/questions/29223975/