Dim SumJanuar As HaushaltskasseDataSet.Eintraege.Where Jahr= "2019" And _
    Monat="Januar".Sum
        tbx_Summe_Januar.Text = SumJanuar

Public Class frmDemo
    Dim dt As New DataTable
 
    Private Sub frmDemo_Load(sender As Object, 
                             e As EventArgs) Handles MyBase.Load
 
        'Einige Testdaten
        dt.Columns.Add("Jahr", GetType(String))
        dt.Columns.Add("Monat", GetType(String))
        dt.Columns.Add("Einnahmen", GetType(Decimal))
 
        Dim rndm As New Random(1234)
        For i As Integer = 1 To 1000
            dt.Rows.Add(rndm.Next(2016, 2020).ToString,
                       rndm.Next(1, 13).ToString,
                       CDec(rndm.Next(50, 150)))
        Next i
 
        Dim jahr, monat, rowcount As Integer
        Dim summe As Decimal
 
        For i As Integer = 0 To dt.Rows.Count - 1
            Dim row As DataRow = dt.Rows(i)
            'Konvertieren
            If Integer.TryParse(row.Field(Of String)("Jahr"), jahr) AndAlso
               Integer.TryParse(row.Field(Of String)("Monat"), monat) Then
                'Auswahlbedingungen prüfen 
                If jahr = 2019 AndAlso monat = 1 Then
                    summe += row.Field(Of Decimal)("Einnahmen")
                    rowcount += 1
                End If
            End If
        Next i
 
    End Sub
End Class

 Dim Januar_Einnahmen = (From r In HaushaltskasseDataSet.Eintraege Where _
   r.Monat = "Januar" And r.Jahr = cbx_Jahr.Text Select r.Betrag_Einnahmen).Sum( _
   )
        tbx_Januar_Einnahmen.Text = CStr(Januar_Einnahmen)