Question

Is this code correct or is there any possibility of some random threading deadlocks etc?

Is it a good idea to use static properties and locking together? Or is static property thread-safe anyway?

Private Shared _CompiledRegExes As List(Of Regex) Private Shared Regexes() As String = {'test1.Regex', 'test2.Regex'} Private Shared RegExSetupLock As New Object  Private Shared ReadOnly Property CompiledRegExes() As List(Of Regex)     Get         If _CompiledRegExes Is Nothing Then         SyncLock RegExSetupLock              If _CompiledRegExes Is Nothing Then                 _CompiledRegExes = New List(Of Regex)(Regexes.Length - 1)                  For Each exp As String In Parser.Regexes                     _CompiledRegExes.Add(New Regex(exp, RegexOptions.Compiled Or RegexOptions.CultureInvariant Or RegexOptions.IgnoreCase))                 Next              End If          End SyncLock      End If      Return _CompiledRegExes  End Get End Property 

If it’s not obvious code is compiling regexes and storing in a List(Of Regex) so they can run faster. And it’s shared so every instance of class can get benefit out of it.

Answer 1

Your code isn’t thread-safe because you’re using double-checked locking without making the field volatile. Unfortunately VB.NET doesn’t have a volatile modifier, so you can’t apply the normal fix. Just acquire the lock every time instead, or use static initialization to initialize _CompiledRegExes when the type is initialized.

See my singleton page for a general discussion of this with regards to singletons – I know this isn’t quite a singleton, but it’s close. The page gives C# code, but it should be fairly easy to understand.

Additionally, I generally recommend making lock variables read-only. You really don’t want to be changing the value 🙂

In general terms:

• No, static properties aren’t thread-safe automatically
• Yes, it’s okay to lock on a static variable (but initialize it explicitly, like you are doing)
• Don’t try to write lock-free or low-lock code unless you really know what you’re doing. I regard myself as reasonably knowledgeable about threads, and I still don’t try using double-checked locking etc.
• Type initialization is thread-safe (with a few caveats if you have complex initializers which end up referencing each other) so that’s a good time to do initialization like this – then you really don’t need a lock.

EDIT: You don’t need to make the type a singleton. Just write function to initialize the list and return it, then use that function in the initializer for the variable:

' This has to be declared *before* _CompiledRegExes ' ' as the initializer will execute in textual order ' ' Alternatively, just create the array inside BuildRegExes ' ' and don't have it as a field at all. Unless you need the array ' ' elsewhere, that would be a better idea. ' Private Shared ReadOnly Regexes() As String = {'test1.Regex', 'test2.Regex'}  Private Shared ReadOnly _CompiledRegExes As List(Of Regex) = BuildRegExes()  Private Shared ReadOnly Property CompiledRegExes() As List(Of Regex)     Get         Return _CompiledRegExes     End Get End Property  Private Shared Function BuildRegExes() As List(Of Regex)     Dim list = New List(Of Regex)(Regexes.Length - 1)      For Each exp As String In Regexes         _CompiledRegExes.Add(New Regex(exp, RegexOptions.Compiled Or RegexOptions.CultureInvariant Or RegexOptions.IgnoreCase))     Next     Return list End Function