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

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。 但是，如果我将Option Strict ON设为打开，则无法在Linq查询中使用此功能：

    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

它将产生以下错误：

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'..........

“ Where”子句是System.Linq.Queryable的成员

公共共享函数where（Of TSource）（ByVal来源为System.Linq.IQueryable（Of TSource），ByVal谓词为System.Linq.Expressions.Expression（Of System.Func（Of TSource，Boolean）））作为System.Linq。 IQueryable（TSource的）

并且“ DefaultIfEmpty”是System.Linq.Queryable的成员

公共共享函数DefaultIfEmpty（Of TSource）（ByVal source as System.Linq.IQueryable（Of TSource））作为System.Linq.IQueryable（Of TSource）

如果我将Option Strict设置为OFF ， 那没有问题 。

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

Answer 1

看起来CountryId是可为空的值类型，因此

c.CountryId = st.CountryId

将为可为空的布尔值，而不是常规布尔值。

试试这个

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，看来您无论如何都在寻找团体加入：

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, "")}