VB.net对象数组引发异常

Question

I am getting an exception when running the following code.

Public Function getSongs() As Song()
   ' Dim dir As New DirectoryInfo(Application.ExecutablePath)
     Dim dir As New DirectoryInfo(directory)
     Dim songsInDir() As Song = Nothing
     Dim i As Integer = 0
     For Each file As FileInfo In dir.GetFiles()
        'only read ".mp3" files
        If file.Extension = ".mp3" Then
            songsInDir(i) = New Song(file.Name)
            i = +i
        End If
    Next
    Return songsInDir
End Function

I get an error on line:

songsInDir(i) = New Song(file.Name)

I get an uncaught exception that says:

"Object reference not set to an instance of an object."

The song object has a:

Public Sub new(By Val filename as String)

... sub that sets a variable and retrieves file info (this code works)

Any help would be appreciated!

Answer 1

Try using a list:

Public Function getSongs() As Song()
  Dim dir As New DirectoryInfo(directory)
  Dim songsInDir() As New List(of Song)
  For Each file As FileInfo In dir.GetFiles()
    'only read ".mp3" files
    If file.Extension = ".mp3" Then
      songsInDir.Add(New Song(file.Name)
    End If
  Next
  Return songsInDir.ToArray()
End Function

Answer 2

Your problem is that arrays need a size when they're initialized and setting it to Nothing gives you exactly that. Give the array a size and don't set it to Nothing. Also, there's a much cleaner way to do this.

Public Function getSongs() As Song()
    Dim songFiles As String() = Directory.GetFiles(directory, "*.mp3")
    Dim songsInDir(songFiles.Length) As Song
    Dim i As Integer = 0
    For Each file As String In songFiles
        songsInDir(i) = New Song(Path.GetFileName(file))
        i = +i
    Next
    Return songsInDir
End Function 

Answer 3

You should specify the array size

Dim i as Integer = dir.GetFiles().count or dir.FilesCount()

Dim songsInDir(i) As Song = Nothing

or you can use dynamic array

put this line inside your for loop

ReDim Preserve songsInDir(i)