Thread not getting value in named slot

[Guest]

I am launching a new thread from my application's main process (using VB.net
2003), and I can't get the child to receive the parameter I'm attempting to
send it in a named data slot.

The code for launching the thread:

Dim NewThread As New Thread(AddressOf LaunchCommThread)
NewThread.AllocateNamedDataSlot("Offset")
NewThread.IsBackground = True
NewThread.Name = SIMclass.SIM(1).strCtrlDesignator
NewThread.SetData(NewThread.GetNamedDataSlot("Offset"), CType(1,
Object))
NewThread.Start()


The beginning of the LaunchCommThread function:

'get thread data and controller # to use
Dim Ctrlr As Int16 'used to ID current (from data
slot)
Ctrlr =
Thread.CurrentThread.GetData(Thread.CurrentThread.GetNamedDataSlot("Offset"))


'declare per-thread variables for TCP communication
Dim address As IPAddress =
IPAddress.Parse(SIMclass.SIM(Ctrlr).strCtrlIP)



The results from the main thread:
?newThread.GetData(newThread.GetNamedDataSlot("Offset"))
Nothing
?newthread.Name
"M1C1"

The results from the child:
?thread.CurrentThread.Name
"M1C1"
?thread.CurrentThread.GetData(thread.CurrentThread.GetNamedDataSlot("Offset"))
Nothing

Strangely, this syntax works from the main thread (does anyone know why?):
?newThread.GetData(Thread.CurrentThread.GetNamedDataSlot("Offset"))
1 {Integer}
[Integer]: 1 {Integer}

Q:
1) Am I misunderstanding the purpose of named data slots?
2) Am I misusing named data slots by using one to pass a parameter?
3) Why does this code not work? I am able to place the value into the
current thread's slot but that isn't where I am directing it?

TIA
Fred B

 

[Larry Lard]

I am launching a new thread from my application's main process (using VB.net
2003), and I can't get the child to receive the parameter I'm attempting to
send it in a named data slot.

The code for launching the thread:

Dim NewThread As New Thread(AddressOf LaunchCommThread)
NewThread.AllocateNamedDataSlot("Offset")
NewThread.IsBackground = True
NewThread.Name = SIMclass.SIM(1).strCtrlDesignator
NewThread.SetData(NewThread.GetNamedDataSlot("Offset"), CType(1,
Object))
NewThread.Start()


The beginning of the LaunchCommThread function:

'get thread data and controller # to use
Dim Ctrlr As Int16 'used to ID current (from data
slot)
Ctrlr =
Thread.CurrentThread.GetData(Thread.CurrentThread.GetNamedDataSlot("Offset"))


'declare per-thread variables for TCP communication
Dim address As IPAddress =
IPAddress.Parse(SIMclass.SIM(Ctrlr).strCtrlIP)



The results from the main thread:
?newThread.GetData(newThread.GetNamedDataSlot("Offset"))
Nothing
?newthread.Name
"M1C1"

The results from the child:
?thread.CurrentThread.Name
"M1C1"
?thread.CurrentThread.GetData(thread.CurrentThread.GetNamedDataSlot("Offset"))
Nothing

Strangely, this syntax works from the main thread (does anyone know why?):
?newThread.GetData(Thread.CurrentThread.GetNamedDataSlot("Offset"))
1 {Integer}
[Integer]: 1 {Integer}

Unfortunately you are being misled by one of the loosenesses of VB
syntax. The various *NamedDataSlot functions are actually *Shared*
functions in Thread, and should be invoked with this syntax:

Thread.WhateverNamedDataSlot

The thread they apply to is always the *current* thread - as the docs
for AllocateNamedDataSlot say,

"No other thread (not even a child thread) can get that data."

The fact that VB allows access to Shared members through an instance
variable is (IMO) a mistake, and in VB2005 such code produces a warning.

So when you say

NewThread.AllocateNamedDataSlot("Offset")

you are actually allocating a named data slot on the *current* thread.
Bearing this in mind, everything that subsequently happens makes sense.

Q:
1) Am I misunderstanding the purpose of named data slots?

A bit.

2) Am I misusing named data slots by using one to pass a parameter?
Yes.

3) Why does this code not work? I am able to place the value into the
current thread's slot but that isn't where I am directing it?

The easiest way to pass information to new threads is to create a helper
class:

Public Class CommLauncher
Public Ctrlr As Int16
'wrap in a property in a real app, posible readonly

Public Sub LaunchComm()
'declare per-thread variables for TCP communication
Dim address As IPAddress = _
IPAddress.Parse(SIMclass.SIM(Ctrlr).strCtrlIP)

' etc
End Sub
End Class

Then to launch, we just

Dim cl As New CommLauncher
cl.Ctrlr = 1
Dim t As New Thread(AddressOf cl.LaunchComm)
t.IsBackground = True
t.Name = SIMclass.SIM(1).strCtrlDesignator
t.Start()


For more about threading, I recommend Jon Skeet's C# threading pages at
<http://www.yoda.arachsys.com/csharp/threads/> or Joseph Albahari's C#
threading pages at <http://www.albahari.com/threading/>. Most threading
info tends to be in C#...

One final note: I see that both the main thread and the worker thread
are accessing SIMclass - if you aren't already, make sure that access to
thread-shared resources is correctly synchronized.