Как получить имена всех файлов с данным расширением в данной папке

    Private Function GetFiles(ByVal Path As String, ByVal Filter As String)
 
        Dim d As New DirectoryInfo(Path)
        Dim files As List(Of FileInfo) = New List(Of FileInfo)
        files.AddRange(d.GetFiles(Filter))
        Return files
 
    End Function
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim Opendlg As New OpenFileDialog
        Dim dir, ext As String
 
        If opendlg.ShowDialog = Windows.Forms.DialogResult.OK Then
            dir = Path.GetDirectoryName(opendlg.FileName)
            ext = Path.GetExtension(opendlg.FileName)
        Else
            'что-то
        End If
 
 
        Dim files As List(Of FileInfo) = GetFiles(dir, "*" & ext)
 
        For Each a In files
            msgbox(a.name)
        Next
    End Sub

Imports System.IO
 
Public Class Form1
    Private Function GetFiles(ByVal Path As String, ByVal Filter As String)
 
        Dim d As New DirectoryInfo(Path)
        Dim files As List(Of FileInfo) = New List(Of FileInfo)
        files.AddRange(d.GetFiles(Filter))
        Return files
 
    End Function
    Public files As List(Of FileInfo)
    Public dir, ext As String
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim Opendlg As New OpenFileDialog
 
        If Opendlg.ShowDialog = Windows.Forms.DialogResult.OK Then
            dir = Path.GetDirectoryName(Opendlg.FileName)
            ext = Path.GetExtension(Opendlg.FileName)
            PictureBox1.Load(Opendlg.FileName)
        Else
            Exit Sub
        End If
 
        files = GetFiles(dir, "*" & ext)
 
    End Sub
    
    Public imgcount As Integer = 0
    ' >>>
    Private Sub Button3_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button3.Click
 
        If imgcount < files.Count - 1 Then
            imgcount += 1
            PictureBox1.Load(files(imgcount).FullName)
        End If
    End Sub
    ' <<<
    Private Sub Button2_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button2.Click
        If imgcount <= files.Count - 1 And imgcount > 0 Then
            imgcount -= 1
            PictureBox1.Load(files(imgcount).FullName)
        End If
    End Sub
End Class

Dim converter As ImageSourceConverter = new ImageSourceConverter()
Dim imageSource As ImageSource = DirectCast(converter.ConvertFromString(path), ImageSource)
IMG.Source = imageSource

 Public count As Integer = 0
 
 
    Private Sub Next_Click(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles след.Click
 
        Try
            Dim files As List(Of IO.FileInfo) = GetFiles(dir, "*" & ext)
            If count <= files.Count - 1 And count > 0 Then
                count -= 1
                Image1.Source = New ImageSourceConverter().ConvertFrom(files(count).FullName)
                Image1.Height = Image1.Source.Height
                Image1.Width = Image1.Source.Width
            End If
        Catch ex As Exception
        
        End Try
    End Sub
 
    Private Sub Previous_Click(sender As System.Object, e As System.Windows.RoutedEventArgs) Handles Previous.Click
        Try
            Dim files As List(Of IO.FileInfo) = GetFiles(dir, "*" & ext)
            If count < files.Count - 1 Then
                count += 1
                Image1.Source = New ImageSourceConverter().ConvertFrom(files(count).FullName)
                Image1.Height = Image1.Source.Height
                Image1.Width = Image1.Source.Width
            End If
        Catch ex As Exception
     
        End Try
    End Sub

              files = GetFiles(dir, "*" & "jpg")
                files = GetFiles(dir, "*" & "jpeg")
                files = GetFiles(dir, "*" & "bmp")
                files = GetFiles(dir, "*" & "tiff")
                files = GetFiles(dir, "*" & "png") заполняем массив
                For Each a In files
                    If a.FullName = openfile.FileName Then count = files.IndexOf(a)- проверка на соответствие
                Next
                im = count -получение индекса текущего элемента

   Try
 
          
            If im < files.Count - 1 Then
                im += 1
                Image1.Source = New ImageSourceConverter().ConvertFrom(files(im).FullName) 
                Image1.Height = Image1.Source.Height
                Image1.Width = Image1.Source.Width
                Slider1.Value = 11
            End If
        Catch ex As Exception
            Next_Click(sender, e)
        End Try

  Try
 
          
            If im <= files.Count - 1 And im > 0 Then
                im -= 1
                Image1.Source = New ImageSourceConverter().ConvertFrom(files(im).FullName) 
                Image1.Height = Image1.Source.Height
                Image1.Width = Image1.Source.Width
                Slider1.Value = 11
            End If
        Catch ex As Exception
            Previous_Click(sender, e)
        End Try