如何在VB.NET项目中使用System.Linq扩展方法，并使用Option Strict ON

Question

我正在使用.NET 3.5在我的DataLayer类中我引用了System.Core，System.Data.Linq，System.Data.DataSetExtensions 。

Dim query = From st In db.Students _ 
     From c In db.Countries.Where(Function(c) c.Id = st.CountryId).DefaultIfEmpty _ 
     From r In db.Rooms.Where(Function(r) r.Id = st.RoomId).DefaultIfEmpty _ 
     From b In db.Buildings.Where(Function(b) b.Id = r.BuildingId).DefaultIfEmpty _ 
     From es In db.Essays.Where(Function(es) es.StudentId = st.Id).DefaultIfEmpty _ 
     Select st.Id, st.FullName, c.CountryName, r.RoomNumber, b.BuildingName, es.Eassay 

这将产生以下错误：但 如果我有选项严格ON我cantnot使用Linq查询此功能

Overload resolution failed because no accessible 'Where' can be called with these arguments: 
    Extension method 'Public Function Where(predicate As System.Linq.Expressions.Expression(Of System.Func(Of Country, Integer, Boolean))) As System.Linq.IQueryable(Of Country)' 

defined in 'System.Linq.Queryable': Nested function does not have the same signature as delegate 'System.Func(Of Country, Integer, Boolean)'. 

Extension method 'Public Function Where(predicate As System.Linq.Expressions.Expression(Of System.Func(Of Country, Boolean))) As System.Linq.IQueryable(Of Country)' defined in 'System.Linq.Queryable': Option Strict On disallows implicit conversions from 'Boolean?' to 'Boolean'. 
    Extension method 'Public Function Where(predicate As System.Func(Of Country, Integer, Boolean)) As System.Collections.Generic.IEnumerable(Of Country)' defined in 'System.Linq.Enumerable': Nested function does not have the same signature as delegate 'System.Func(Of Country, Integer, Boolean)'. 
    Extension method 'Public Function Where(predicate As System.Func(Of Country, Boolean)) As System.Collections.Generic.IEnumerable(Of Country)' defined in 'System.Linq.Enumerable': Option Strict On disallows implicit conversions from 'Boolean?' to 'Boolean'.......... 

“去哪儿”的条款是系统的成员。 Linq.Queryable

Public Shared Function Where(Of TSource)(ByVal source As System.Linq.IQueryable(Of TSource), ByVal predicate As System.Linq.Expressions.Expression(Of System.Func(Of TSource, Boolean))) As System.Linq.IQueryable(Of TSource)

而 “DefaultIfEmpty” 是System.Linq.Queryable

会员

Public Shared Function DefaultIfEmpty(Of TSource)(ByVal source As System.Linq.IQueryable(Of TSource)) As System.Linq.IQueryable(Of TSource)

如果我设置选项严格关，没有任何问题。

如何在使用Option Strict ON的VB.NET项目中使用这些System.Linq扩展方法？谢谢

Answer 1

貌似CountryId是一个空值类型，因此

c.CountryId = st.CountryId 

将是一个可空布尔值，而不是常规的布尔值。

尝试这样的事情

From st In db.Students _ 
From c In db.Countries.Where(Function(c) If(c.CountryId = st.CountryId, False)) _ 
Select st.FirstName, c.CountryName 

顺便说一句，这似乎是你寻找一个组加入总之：

From s In db.Students 
Group Join c In db.Countries On s.CountryID Equals c.CountryID Into Group 
From g In Group.DefaultIfEmpty 
Select New With {.Name = s.Name, .CountryName = If(g IsNot Nothing, g.CountryName, "")}