find files then parse the results before displaying the results

I'm working on an app for our customer service division. The user will enter a print number, and a mfg date. the app will then do a get files returning results of all files found with that drawing number (multiple revisions). the files are named in this format drawingnumber_rev_pages_6-digit date.pdf. Once I have the list of that drawing number, I then take a right(string) of 10 characters to strip the 6-digit date off, and do a Date.ParseExact to compare to the user input mfg date. I was grabbing anything prior to that date, and showing them in a listbox. the criteria has now changed, and they want me to only show the file that would pertain to that build date. The problem is, I need the first rev prior to that date. and I don't fully understand the (function) portion of my getfiles statement. So I don't know what to search for to even lookup google results. as I search for getfiles, function isn't mentioned, if I look up orderby... function isn't mentioned, is it linq I need to search under? how would you propose I approach this? original version example new version, I can filter result and find everything before date ... but what I want is the highest revision before the mfg. date. return a single result. current version

thank you all- here's my sample code.

Try
    Dim results = Directory.GetFiles(fp, f).
        OrderByDescending(Function(x) New FileInfo(x).FullName)

    For Each result In results
        Dim dp As String = ""
        Dim d As Date
        dp = Strings.Right(result, 10)
        dp = Strings.Left(dp, 6)
        d = Date.ParseExact(dp, "MMddyy", New Globalization.CultureInfo("en-us"))
        Debug.Print(dp)
        Debug.Print(d)
        Dim udt As String = ""
        Dim ud As Date 'user date
        udt = Trim(Replace(txtMfgDate.Text, "/", ""))
        If udt.ToString.Length = 0 Then lstFound.Items.Add(result)

        ud = Date.ParseExact(udt, "MMddyy", New Globalization.CultureInfo("en-us"))
        If d.Date <= ud.Date Then
            lstFound.Items.Add(result)
        End If

        Debug.Print(d)
        Debug.Print(ud)
        If result <> Nothing Then

        End If
    Next
Catch x As Exception
    MessageBox.Show("Error finding file: " & f)
End Try

Solution

LINQ can do everything you need with the right conditions. I also tried to clean up some of the code in other ways. I switched the LINQ expression to query comprehension to so I could use Let.

Try
    Dim enusCulture = New Globalization.CultureInfo("en-us") ' consider CultureInfo.CurrentCulture instead?
    Dim udt As String = Trim(Replace(txtMfgDate.Text, "/", ""))
    Dim ud As Date

    If udt.Length > 0 Then ud = Date.ParseExact(udt, "MMddyy", enusCulture) ' user mfg date
    ' 0071911_a_1_072115.pdf

    Dim results = From filename In Directory.GetFiles(fp, f)
        Let filedate = Date.ParseExact(filename.Substring(filename.Length - 10, 6), "MMddyy", enusCulture)
        Where udt.Length = 0 OrElse filedate.Date < ud.Date
        Order By filedate Descending

    If udt.Length> 0 Then
        results = results.Take(1)
    End If

    lstFound.Items.AddRange(results)
Catch x As Exception
    MessageBox.Show("Error finding file: " & f)
End Try

The Function(x) syntax is a lambda expression, and it is part of LINQ method syntax for the OrderByDescending function, if you want to research that.