防止“'System.DateTime' 失败，因为物化值为空”
Preventing "'System.DateTime' failed because the materialized value is null"
我想以不同的方式防止这种情况发生，而不是将 DateCreated 的类型设置为可为 null 的 DateTime。
完整的异常是这样的：
The cast to value type 'System.DateTime' failed because the
materialized value is null. Either the result type's generic parameter
or the query must use a nullable type.
当我 运行 在数据库上查询时，我可以看到那里没有空记录，所以我相信我应该避免将 DateCreate 属性 设置为可为空的 DateTime，因为它不会对我来说很有意义（即使我知道它会解决问题）。是不是我理解有误？
查询中的以下行导致了问题：
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;
}
媒体class定义如下：
[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 - 被 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
除了将数据类型设置为 Nullable，您还可以将查询中的值转换为可为 null 的类型，并检查这是否为 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;
}
为什么它会在特定行给出错误？
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 并且它会抛出错误作为预防措施。它只在 DateTime 数据类型上抛出错误，因为它知道当其他属性为 null（字符串和可空类型）时要做什么。
我想以不同的方式防止这种情况发生，而不是将 DateCreated 的类型设置为可为 null 的 DateTime。
完整的异常是这样的：
The cast to value type 'System.DateTime' failed because the materialized value is null. Either the result type's generic parameter or the query must use a nullable type.
当我 运行 在数据库上查询时，我可以看到那里没有空记录，所以我相信我应该避免将 DateCreate 属性 设置为可为空的 DateTime，因为它不会对我来说很有意义（即使我知道它会解决问题）。是不是我理解有误？
查询中的以下行导致了问题：
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;
}
媒体class定义如下：
[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 - 被 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
除了将数据类型设置为 Nullable，您还可以将查询中的值转换为可为 null 的类型，并检查这是否为 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;
}
为什么它会在特定行给出错误？
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 并且它会抛出错误作为预防措施。它只在 DateTime 数据类型上抛出错误，因为它知道当其他属性为 null（字符串和可空类型）时要做什么。