Question

I’m new to threading and I came accross a custom thread pool implementation example in a blog. I’m pasting just the necessary parts of the code:

Public Class ThreadPool

    Private CountLock As New Object
    Private _Count As Integer

    Public ReadOnly Property ThreadCount() As Integer
      Get
         SyncLock CountLock
           Return _Count
         End SyncLock
      End Get
    End Property

    Public Sub Open()
      Interlocked.Increment(_Count)
    End Sub

    Public Sub Close()
      Interlocked.Decrement(_Count)
      ....
    End Sub

EndClass

My question is, why do we need a lock to implement the readonly ThreadCount property?

Answer 1

The lock will force a memory barrier, so that a stale value from the CPU cache isn’t read if the last value written was written by a different CPU. The same could be done with Thread.VolatileRead() without locking.