在 MVC 3 vb.net 中使用 EF 选择单行
我正在尝试使用以下代码行从 EF 数据库模型中获取单个值。
Dim _classRoom As classrm = db.classrms.Select(Function(b) b.Course_ID = _CurrCourse.course_ref)
其中 classrm 是实体的名称，而 db 被声明为新实体。我想做的是根据匹配的 Course_ID 从实体中选择一行，在该模型中是一个字符串。这样我以后就可以使用变量 _classRoom 从同一行中获取其他项目。但是我收到以下错误：
Unable to cast object of type 'System.Data.Objects.ObjectQuery`1[System.Boolean]' to type 'Trial_Online.classrm'.
有人有任何想法吗？？？我必须与几个不同的实体执行类似的任务，但如果我指向正确的方向，我可以从那里进行管理......
I am trying to get back a single from from a EF database model using the below line of code..
Dim _classRoom As classrm = db.classrms.Select(Function(b) b.Course_ID = _CurrCourse.course_ref)
Where classrm is the name of the entity and db is declared as a new entity. What I am trying to do is select a row from the entity based on matching Course_ID which in this model is a string. So that I can later use the variable _classRoom to get other items out of the same row.. However I am getting the following error:
Unable to cast object of type 'System.Data.Objects.ObjectQuery`1[System.Boolean]' to type 'Trial_Online.classrm'.
Anyone got any ideas??? I have to preform similar tasks with several different Entities but if I get pointed in the right direction I can manage from there...
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使用
Where而不是Select来过滤结果，并在末尾添加First()或Single()：First()和Single()的区别在于Single()如果数组中的元素超过 1 个，则会抛出异常结果。如果结果序列为空，两者都会抛出异常。如果没有结果，您还可以使用
FirstOrDefault()和SingleOrDefault()返回Nothing。Use
Whereinstead ofSelectto filter the results, and addFirst()orSingle()to the end:The difference between
First()andSingle()is thatSingle()will throw an exception if there is more than 1 element in the result. Both will throw an exception if the result sequence is empty.You can also use
FirstOrDefault()andSingleOrDefault()to returnNothingif there are no results.不太熟悉 VB.NET 中的查询语法，但在 C# 中，我很确定您必须将
First()或Single()添加到查询末尾它将结果投影到对象实例中。否则我认为你会得到一个结果集（可能是一个 IQueryable 对象）。Not too familiar with query syntax in VB.NET, but in C# I'm pretty sure you have to add
First()orSingle()to the end of the query for it to project the result into an object instance. Otherwise I think you get a resultset (probably anIQueryableobject).