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How can I use IBindStatusCallBack interface in VB.NET?

Hi!

I wrote a program which download a specified file from the Internet to a local file.
I used the URLDownloadToFile function (urlmon.dll). I would like to show a progressbar while the file is downloading. I have found Edanmo's type library (olelib.tlb) which contains the IBindStatusCallBack interface and made a class which implements it.
The last parameter of URLDownloadToFile is an integer but according to the description of URLDownLoadToFile it is a pointer referred to the caller's IBindStatusCallBack interface.
Private Declare Function URLDownloadToFile Lib "urlmon" Alias "URLDownloadToFileA" (ByVal pCaller As Integer, _
ByVal szURL As String, _
ByVal szFileName As String, _
ByVal dwReversed As Integer, _
ByVal lpfnCB As Integer) As Integer

How can I pass this parameter to the URLDownLoad function? Is it possible to solve this problem in VB.NET at all?
If there is a solution in C#, it would be helpful for me.

Thanks

Bob Learned

Do you know what signature is for the callback function from the IBindStatusCallBack interface? You need to define a delegate function and pass the address of the delegate:

Public Delegate Sub StatusCallBackDelegate(<signature goes here>)

Private Declare Function URLDownloadToFile Lib "urlmon" Alias "URLDownloadToFileA" (ByVal pCaller As Integer, _
ByVal szURL As String, _
ByVal szFileName As String, _
ByVal dwReversed As Integer, _
ByVal lpfnCB As StatusCallBackDelegate) As Integer

Bob

kacsab

ASKER

Thank you for your reply, Bob!

As you suggested I used delegate function, but my program doesn't work. A System.NullReferenceException occurs.
Could you tell me where is the error?
Here is the code:

Public Class clsBSC
Implements olelib.IBindStatusCallback
...
Public Sub OnProgress(ByVal ulProgress As Integer, _
ByVal ulProgressMax As Integer, _
ByVal ulStatusCode As olelib.BINDSTATUS, _
ByVal szStatusText As Integer) Implements olelib.IBindStatusCallback.OnProgress

RaiseEvent ProgressEvent(ulProgress, ulProgressMax, ulStatusCode, szStatusText)
End Sub

Public Event ProgressEvent(ByVal ulProgress As Integer, _
ByVal ulProgressMax As Integer, _
ByVal ulStatusCode As olelib.BINDSTATUS, _
ByVal szStatusText As Integer)

Public Declare Function URLDownloadToFile Lib "urlmon" Alias "URLDownloadToFileA" (ByVal pCaller As Integer, _
ByVal szURL As String, _
ByVal szFileName As String, _
ByVal dwReversed As Integer, _
ByVal lpfnCB As OnProgressDelegate) As Integer

Public Delegate Sub OnProgressDelegate(ByVal ulProgress As Integer, _
ByVal ulProgressMax As Integer, _
ByVal ulStatusCode As olelib.BINDSTATUS, _
ByVal szStatusText As Integer)

End Class

I have a form with a button, a progressbar and two textboxes on it:

Public Class Form1
Inherits System.Windows.Forms.Form
...
Private WithEvents BSC As clsBSC
Private mOnProgressDelegate As clsBSC.OnProgressDelegate

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
BSC = New clsBSC
mOnProgressDelegate = New clsBSC.OnProgressDelegate(AddressOf BSC.OnProgress)
BSC.URLDownloadToFile(0, TextBox1.Text, TextBox2.Text, 0, mOnProgressDelegate) '<====The exception occurs at this row.
End Sub

Private Sub BSC_ProgressEvent(ByVal ulProgress As Integer, _
ByVal ulProgressMax As Integer, _
ByVal ulStatusCode As olelib.BINDSTATUS, _
ByVal szStatusText As Integer) Handles BSC.ProgressEvent
ProgressBar1.Maximum = ulProgressMax
ProgressBar1.Value = ulProgress
End Sub

End Class

Thank you in anticipation

ASKER CERTIFIED SOLUTION

Bob Learned

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