Question

I’m using .NET 3.5 In my DataLayer class I have references of System.Core,System.Data.Linq, System.Data.DataSetExtensions. But
I cantnot use this feature in Linq query if I have Option Strict ON:

    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

It will yield the following error:

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'..........

The “Where” clause is Member of System.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)

And “DefaultIfEmpty” is Member of System.Linq.Queryable

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

If I set Option Strict OFF, there’s no problem.

How to use these System.Linq extension methods in VB.NET project with Option Strict ON ? Thank you

Answer 1

Looks like CountryId is a nullable value type, hence

c.CountryId = st.CountryId

will be a nullable boolean instead of a regular boolean.

Try something like this

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

BTW, it’s seems your looking for a group join anyhow:

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, "")}