How to get only specific columns in this LINQ expression?

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Hello everybody,

I use a LINQ query from another forum discussion and was able to adapt it, so that it works and even reports activity to the UI (see code below). The result of the query is a BindingList(of FileInfo), which I then use as DataSource of a "FilesBindingSource" in order to feed a DataGridView.

Now I'd like to retrieve / display only a part of the information (FileName + Length + LastWriteTime), and I read that I wanna put another "Select" after the Where-clause... but I've been struggling with the syntax all day long.

Could someone please help me with the correct syntax?

Thank you very much,
Kind regards
Michael

What I have tried:

For Each result As FileInfo In dirInfo.GetFiles(pattern).Select(Function(file) file).Where(Function(s)
       supportedExtensions.Contains(Path.GetExtension(s.Extension).ToLower()))
				queue.Enqueue(result)
				report(dirInfo.FullName)
Next result
Task.WaitAll(dirInfo.GetDirectories().Select(
               Function(d) Task.Factory.StartNew(
			   Sub() InternalCollectFiles(d, pattern, queue))).ToArray())

Posted 4-Feb-19 0:07am

Sonhospa

Updated 4-Feb-19 3:10am

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Graeme_Grant · Accepted Answer · 2019-02-04T03:05:00

Solution 1

Here is the Linq required to do what you want:

VB

Dim results = dirinfo.GetFiles(pattern)
    .Where(Function(x) supportedExtensions.Contains(x.Extension.ToLower()))
    .Select(Function(x) New With {
        .Name = x.Name,
        .Length = x.Length,
        .LastWriteTime = x.LastWriteTime
    })

To help explain the Linq statement, less the selection of output, here is a non-Linq version:

VB

Dim dirinfo = New DirectoryInfo(dirPath)
Dim files = dirinfo.GetFiles(pattern)

For Each fileInfo In files

    If supportedExtensions.Contains(fileInfo.Extension) Then
        queue.Enqueue(fileInfo)
    End If
Next

Posted 4-Feb-19 3:05am

Graeme_Grant

Updated 4-Feb-19 4:32am

v2

Comments

Maciej Los 4-Feb-19 9:15am

5ed!
You've been faster than me, because i had a debate with OP about his other issue.

Sonhospa 4-Feb-19 10:25am

Thank you though, as from the "pupil's" perspective I understand more from your additional explanations. I also appreciate your very good (!) idea to return a specific type.

Graeme_Grant 4-Feb-19 10:32am

Updated my solution with a non-Linq version...

Sonhospa 4-Feb-19 11:02am

Good that you write, now I have a 'Reply' button so that I can thank you as well :-) and apologise for not being used to (and also comfortable with) the changed UI.

Thank you as well for the update / non-Linq version! It's obviously a very cool and brief way to reach the same goal without all the syntax hassle that I unsuccessfully tried to master. And in my learning, the different ways pros go are extremely interesting to me!

Would you think the LINQ version is preferrable for speed or sth else, or is it just a matter of taste?

Graeme_Grant 4-Feb-19 19:39pm

No problems.

The Linq version above is delayed execution, so the code only runs when you iterate over the results. To force it to execute immediately, use .ToList(). Very powerful and flexible when used correctly...

Maciej Los · Accepted Answer · 2019-02-04T03:10:00

It's quite easy. See:

VB

Dim dirinfo As DirectoryInfo = New DirectoryInfo("C:\users\Sonhospa\documents") 'change path to your needs
Dim pattern As String = "*"
Dim supportedExtensions As String() = {".xls", ".doc", ".xlsx", ".docx"}

Dim filelist = dirInfo.GetFiles(pattern) _
    .Where(Function(file) supportedExtensions.Contains(Path.GetExtension(file.Extension).ToLower())) _
    .Select(Function(file) New With _
    { _
        .FileName = file.FullName, _
        .Length = file.Length, _
        .LastWriteTime = file.LastWriteTime _
    }) _
    .ToList()

Note #1: i've moved Where statement up, before Select
Note #2: above code returns List(of AnonymousType(Of String, Int64, DateTime))

If you would like to return a colection of specific type, please see: How to: Return a LINQ Query Result as a Specific Type (Visual Basic) | Microsoft Docs[^]

For example:

VB.NET

Public Class ShortFileInfo

	Public Property FileName As String
	Public Property Length As Long
	Public Property LastWriteTime As DateTime
	
End Class

A query have to be changed this way:

VB.NET

.Select(Function(file) New ShortFileInfo() With _
{ _
    .FileName = file.FullName, _
    .Length = file.Length, _
    .LastWriteTime = file.LastWriteTime _
}) _