Question

Okay, I am trying to write a LINQ query with a Join in it, using VB on .NET 4.0.

I get the red squiggle under my line of code and when I mouse over it, it shows:

“Cannot not transform LINQ”

Note the double-negative.

Anyone have a clue what this error means? Grammatically it would mean there is NO error, but I don’t know why there should even be an error message then. This “error” also kills auto-complete on the line in question, as if there actually IS an error. So I’m confused.

Here are my variable declarations:

Dim dateTimes As Date() = GetDates()
Dim readings = (
    From dr As DataRow In dbReadings.Rows 
    Where dr("SENSORID") = sensorid 
    Select New Device.Reading With {
        .Dated = CDate(dr("DATEDT")), 
        .Value = CSng(dr("VALUE")), 
        .Exception = CBool(dr("EXCEPTION"))
    })

Here is my query which I’m being told cannot be queried:

Dim joined = From dt As Date In dateTimes 
             Join r As Device.Reading In readings On r.Dated = dt 
             Into DateTimeReadings From r In DateTimeReadings.DefaultIfEmpty() 
             Select r.Value

I’m trying to get a list of readings for all the date-times in dateTimes, including blank entries for dates for which there is no reading.

Answer 1

Try using Cast(Of T)() since you can’t use linq against data row collection

    Dim joined = From dt As Date In dateTimes 
Join dr As DataRow In sreadings.rows.Cast(Of DataRow)() 
             On CDate(dr("DATEDT")) = dt

http://msdn.microsoft.com/en-us/library/bb341406.aspx#Y0

Try this (sorry, it’s in C#)

var res1 = dates.Join(sreadings.rows.Cast<DataRow>()
                , c => c, d => d.Field<DateTime>("DATEDT"), (date, row) => new { Date = date, Row = row });