Question

I have the below code. What happens if the Upload method takes more than one minute for completion? Does the timer kick off another call to the Upload method immediately after one minute is over or Does it wait until the method finishes executing? I am implementing this as a windows service.

Protected Overrides Sub OnStart(ByVal args() As String)    
  Dim tmr As Timer = New Timer(New TimerCallback(AddressOf Upload), Nothing, 1000, **60000**)

  While Not tmr Is Nothing
  End While
End Sub

Public Sub Upload(ByVal o As Object)
  Dim sr As StreamReader
  ..........
  ..........
End Sub

Answer 1

Though some of your code is confusing (what’s with the While loop?), I can say yes, the Timer will kick off another Thread from the ThreadPool.

You can use Monitor, ReaderWriterLockSlim, or lock to control re-entrancy. You’ll need an object to lock onto and other things tie into your requirements (i.e. the integer specified tells Monitor to try for X milliseconds, where 0 means we want the lock immediately):

VB.NET:

Private ReadOnly LockerObject As Object = New Object()

If (Monitor.TryEnter(LockerObject, 0)) Then
    Try
        'do your processing
    Catch

    Finally
        'always release the lock
        Monitor.Exit(LockerObject)
    End Try
End If

C#:

private readonly object LockerObject = new object();

if (Monitor.TryEnter(LockerObject, 0))
{
    try
    {
        //do your processing
    }
    catch
    {
    }
    finally
    {
        //always release the lock
        Monitor.Exit(LockerObject);
    }
}
else
{
    //do something else here if needs be.
}

EDIT: Oops, this is VB.NET! Let me convert my code…