Getting only records not in second table LINQ - vb.net

Question

i need help figuring out how to do this, i have a DataSet that contains records by Date and currencyCode, what i want is to compare two datasets and know if one (left table) have records that don´t exists on the second one (right table), i have the following code for joining the table based on date and code:
vLINQ = (From ContentExchangeRates In myDataset.Tables(0).AsEnumerable() _
Join PartnerExchangeRates In PartnerDataSet.Tables(0).AsEnumerable() _
On ContentExchangeRates.Field(Of Date)("EffectiveDate") Equals PartnerExchangeRates.Field(Of Date)("RateDate") _
And ContentExchangeRates.Field(Of String)("CurrencyCode") Equals PartnerExchangeRates.Field(Of Date)("CurrencyCode") _
Select New With { .PartnerRateDate = PartnerExchangeRates.Field(Of Date)("RateDate"), _ etc}).ToList
Using Jeff answer the code looks like this:
vLINQ = (From ContentExchangeRates In myDataSet.Tables(0).AsEnumerable() _
Group Join PartnerExchangeRates In PartnerDataSet.Tables(0).AsEnumerable() _
On PartnerExchangeRates.Field(Of Date)("RateDate") Equals ContentExchangeRates.Field(Of Date)("EffectiveDate") _
And PartnerExchangeRates.Field(Of String)("CurrencyCode") Equals ContentExchangeRates.Field(Of String)("ConvertCurrencyCode") _
Into result = Group From m In result.DefaultIfEmpty _
Select New With _
{ _
.PartnerRateDate = m.Field(Of Date)("RateDate") _
}).ToList
But i get the following error: Value cannot be null.
Parameter name: row
I want to be able to make a left outer join, that will only retrieve records that do not exists on PartnerExchangeRates table, but i don´t know what to do any more :(
Any help is appreciated!

Answer 1

To perform a left outer join, you need to do a group join and go through the results of the group join and use DefaultIfEmpty. It will translate to a left outer join.
Dim query =
From cer In myDataset.Tables(0).AsEnumerable
Group Join per In PartnerDataSet.Tables(0).AsEnumerable
On New With
{
.Date = cer.Field(Of Date)("EffectiveDate"),
.Currency = cer.Field(Of String)("CurrencyCode")
}
Equals New With
{
.Date = per.Field(Of Date)("RateDate"),
.Currency = per.Field(Of String)("CurrencyCode")
}
Into pers = Group
From per in pers.DefaultIfEmpty
Where per Is Nothing
Select PartnerRateDate = per.Field(Of Date)("RateDate"), ...

Answer 2

You can use the Enumerable.Except(IEnumerable, IEnumerable, IEqualityComparer) method:
Option Infer On
Module Module1
Class RowComparer : Implements IEqualityComparer(Of DataRow)
Public Function Equals1(x As DataRow, y As DataRow) As Boolean Implements IEqualityComparer(Of DataRow).Equals
' use .Equals for the DateTime and Is for the String
Return x(0).Equals(y(0)) AndAlso x(1) Is y(1)
End Function
Public Function GetHashCode1(obj As DataRow) As Integer Implements IEqualityComparer(Of DataRow).GetHashCode
Return obj(0).GetHashCode Xor obj(1).GetHashCode
End Function
End Class
Sub ShowDt(dt As DataTable)
For Each r As DataRow In dt.Rows
Console.WriteLine(CDate(r(0)).ToString("yyyy-MM-dd") & " " & CStr(r(1)))
Next
Console.WriteLine("-------------")
End Sub
Sub Main()
Dim dt(1) As DataTable
For i = 0 To 1
dt(i) = New DataTable
dt(i).Columns.Add(New DataColumn With {.ColumnName = "EffectiveDate", .DataType = Type.GetType("System.DateTime")})
dt(i).Columns.Add(New DataColumn With {.ColumnName = "CurrencyCode", .DataType = Type.GetType("System.String")})
Next
' test data
For i = 0 To 1
For j = 1 To 10
Dim nr = dt(i).NewRow
nr("EffectiveDate") = New DateTime(2015, 1, j)
nr("CurrencyCode") = "USD"
dt(i).Rows.Add(nr)
Next
Next
' remove a couple of rows from the second table
dt(1).Rows.RemoveAt(9)
dt(1).Rows.RemoveAt(2)
ShowDt(dt(0))
ShowDt(dt(1))
' find the rows in dt(0) which are not in dt(1)...
Dim missings = (dt(0).AsEnumerable.Except(dt(1).AsEnumerable, New RowComparer())).CopyToDataTable()
ShowDt(missings)
Console.ReadLine()
End Sub
End Module
But if it happens that your data is amenable to being stored in lists, your code could be cleaner and more readable:
Option Infer On
Module Module1
Public Class DateAndCurrency
Public Property EffectiveDate As DateTime
Public Property CurrencyCode As String
Public Overrides Function ToString() As String
Return Me.EffectiveDate.ToString("yyyy-MM-dd") & " " & Me.CurrencyCode
End Function
End Class
Public Class DateAndCurrencyComparer : Implements IEqualityComparer(Of DateAndCurrency)
Public Function Equals1(x As DateAndCurrency, y As DateAndCurrency) As Boolean Implements IEqualityComparer(Of DateAndCurrency).Equals
Return x.CurrencyCode = y.CurrencyCode AndAlso x.EffectiveDate = y.EffectiveDate
End Function
Public Function GetHashCode1(obj As DateAndCurrency) As Integer Implements IEqualityComparer(Of DateAndCurrency).GetHashCode
Return obj.CurrencyCode.GetHashCode Xor obj.EffectiveDate.GetHashCode
End Function
End Class
Sub Main()
' test data
Dim daccs As New List(Of List(Of DateAndCurrency))
For i = 0 To 1
Dim dacc As New List(Of DateAndCurrency)
For j = 1 To 10
dacc.Add(New DateAndCurrency With {.EffectiveDate = New DateTime(2015, 1, j), .CurrencyCode = "USD"})
Next
daccs.Add(dacc)
Next
' remove a couple of rows from the second list
daccs(1).RemoveAt(9)
daccs(1).RemoveAt(2)
For i = 0 To 1
daccs(i).ForEach(Sub(x) Console.WriteLine(x.ToString()))
Console.WriteLine("-------------")
Next
' find the items in daccs(0) which are not in daccs(1)...
Dim missings = daccs(0).Except(daccs(1), New DateAndCurrencyComparer())
missings.ToList().ForEach(Sub(x) Console.WriteLine(x.ToString()))
Console.WriteLine("-------------")
Console.ReadLine()
End Sub
End Module