Query Generic List

Question

I have to log FTP activity using VendorID. Some vendors have multiple FTP user names, but I only need to log one (the latest).

For example:

• VendorID 1 (two user names)
• Mike

• Pete

• VendorID 4 (two user names)

• Jack
• Luis

So I created a class to store the FTP activity:

Public Class VendorClass
  Public VendorID   As Integer
  Public FTPUserName As String  
  Public File   As String
  Public Date As DateTime
End Class

• I load the file into a list of VendorClass

Question:

How query the list and group by UserID, and return the latest date and time.

Original List

VendorID    FTP UserName    File                Date
1           Mike            BalanceSheet.csv    6/17/2015 10:20
1           Pete            BalanceSheet.csv    6/17/2015 10:40
2           Ron             BalanceSheet.csv    6/17/2015 10:20
3           Julia           BalanceSheet.csv    6/17/2015 14:20:02 
4           Jack            BalanceSheet.csv    6/17/2015 11:20
4           Luis            BalanceSheet.csv    6/17/2015 09:20

Grouped result:

VendorID    FTP UserName    File                Date
1           Pete            BalanceSheet.csv    6/17/2015 10:40
2           Ron             BalanceSheet.csv    6/17/2015 10:20
3           Julia           BalanceSheet.csv    6/17/2015 14:20:02
4           Jack            BalanceSheet.csv    6/17/2015 11:20

My function is only able to select 1 vendor at the time:

Private Function FlattenDuplicateEntries(ActivityList As List(Of VendorClass)) As List(Of VendorClass)
    FlattenDuplicateEntries = New List(Of VendorClass)

    For Each vendor In FTPUserNameList
        Dim val As New VendorClass
        val = ActivityList.Find(Function(value As VendorClass) value.VendorID = vendor.VendorID)
    Next    
End Function

Answer 1

Private Function FlattenDuplicateEntries(ActivityList As List(Of VendorClass)) As List(Of VendorClass) 
    Dim filtered As New Dictionary(Of Integer, VendorClass)()
    For Each vendor In ActivityList
        If Not filtered.ContainsKey(vendor.VendorID) Then
            filtered.Add(vendor.VendorID, vendor)
        Else If filtered(vendor.VendorID).Date < vendor.Date Then
            filtered(vendor.VendorID) = vendor
        End If
    Next 
    Return filtered.Values.ToList()   
End Function