Visual Basic

' Call the delegate from the subform:
 Private Sub BGWFileRead_DoWork(ByVal sender As Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BGWFileRead.DoWork
     CollectFrames(BGWFileRead, AddressOf MDIMain.ShowProgress)                    <------ here's where the exception is thrown
 End Sub

 Private Sub CollectFrames(ByVal bgw As BackgroundWorker, ByVal Handler As ShowProgressDelegate)
     .. (other stuff) ..
     If Handler IsNot Nothing Then
         Dim ProcessMessage As String = New String(String.Format("Loading file {0} ({1} of {2})", thisFrame.FileToRead.Name, SoFar, Frames.Size))
         Handler.Invoke(CLng(Frames.Size), SoFar, thisFrame.FileToRead.Name, ProcessMessage, ExecutionTime)
     End If
 End Sub

' Main MDI form:
 Public Delegate Sub ShowProgressDelegate(ByVal Files As Long, ByVal TotalFiles As Long, ByVal filename As String, ByVal Message As String, ByVal Time As Integer)

 Public Sub ShowProgress(ByVal Total As Long, ByVal SoFar As Long, ByVal file As String, ByVal Message As String, ByVal Timespan As Integer)
     If Me.InvokeRequired Then
         Dim SPdeleg As New ShowProgressDelegate(AddressOf ShowProgress)
         Me.BeginInvoke(SPdeleg, New Object() {Total, SoFar, file, Message, Timespan})
     Else
         ' actualize StatusStrip
         Dim percent As Integer = CInt((SoFar / Total) * 100)
         Me.StatusProgressBar.Value = percent
         Me.StatusLabel.Text = Message
     End If
 End Sub