C# – the correct way to dispose elements held inside a ThreadLocal

.net-4.0cidisposablethread-local

When you use a ThreadLocal<T> and T implements IDisposable, how are you supposed to dispose of the members being held inside of the ThreadLocal?

According to ILSpy, the Dispose() and Dispose(bool) methods of ThreadLocal are

public void Dispose()
{
    this.Dispose(true);
    GC.SuppressFinalize(this);
}

protected virtual void Dispose(bool disposing)
{
    int currentInstanceIndex = this.m_currentInstanceIndex;
    if (currentInstanceIndex > -1 && Interlocked.CompareExchange(ref this.m_currentInstanceIndex, -1, currentInstanceIndex) == currentInstanceIndex)
    {
        ThreadLocal<T>.s_availableIndices.Push(currentInstanceIndex);
    }
    this.m_holder = null;
}

It does not appear that ThreadLocal attempts to call Dispose on its child members. I can't tell how to reference each thread it internally has allocated so I can take care of it.

I ran a test with the following code, the class is never disposed

static class Sandbox
{
    static void Main()
    {
        ThreadLocal<TestClass> test = new ThreadLocal<TestClass>();
        test.Value = new TestClass();

        test.Dispose();
        Console.Read();
    }
}

class TestClass : IDisposable
{
    public void Dispose()
    {
        Dispose(true);
        GC.SuppressFinalize(this);
    }
    protected void Dispose(bool Disposing)
    {
        Console.Write("I was disposed!");
    }
}

Best Answer

I had a look at the code in ThreadLocal<T> to see what the current Dispose is doing and it appears to be a lot of voodoo. Obviously disposing of thread-related stuff.

But it doesn't dispose of the values if T itself is disposable.

Now, I have a solution - a ThreadLocalDisposables<T> class, but before I give the full definition it's worth thinking about what should happen if you wrote this code:

var tl = new ThreadLocalDisposables<IExpensiveDisposableResource>();
tl.Value = myEdr1;
tl.Value = myEdr2;
tl.Dispose();

Should both myEdr1 & myEdr2 both be disposed? Or just myEdr2? Or should myEdr1 be disposed when myEdr2 was assigned?

It's not clear to me what the semantics should be.

It is clear to me, however, that if I wrote this code:

var tl = new ThreadLocalDisposables<IExpensiveDisposableResource>(
    () => new ExpensiveDisposableResource());
tl.Value.DoSomething();
tl.Dispose();

Then I would expect that the resource created by the factory for each thread should be disposed of.

So I'm not going to allow the direct assignment of the disposable value for ThreadLocalDisposables and only allow the factory constructor.

Here's ThreadLocalDisposables:

public class ThreadLocalDisposables<T> : IDisposable
    where T : IDisposable
{
    private ThreadLocal<T> _threadLocal = null;
    private ConcurrentBag<T> _values = new ConcurrentBag<T>();

    public ThreadLocalDisposables(Func<T> valueFactory)
    {
        _threadLocal = new ThreadLocal<T>(() =>
        {
            var value = valueFactory();
            _values.Add(value);
            return value;
        });
    }

    public void Dispose()
    {
        _threadLocal.Dispose();
        Array.ForEach(_values.ToArray(), t => t.Dispose());
    }

    public override string ToString()
    {
        return _threadLocal.ToString();
    }

    public bool IsValueCreated
    {
        get { return _threadLocal.IsValueCreated; }
    }

    public T Value
    {
        get { return _threadLocal.Value; }
    }
}

Does this help?

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