Lazy<T>: "The function evaluation requires all threads to run"

Question

I have a static class with some static properties. I initialized all of them in a static constructor, but then realized that it is wasteful and I should lazy-load each property when needed. So I switched to using the System.Lazy<T> type to do all the dirty work, and told it to not to use any of its thread safety features since in my case execution was always single threaded.

I ended up with the following class:

public static class Queues
{
    private static readonly Lazy<Queue> g_Parser = new Lazy<Queue>(() => new Queue(Config.ParserQueueName), false);
    private static readonly Lazy<Queue> g_Distributor = new Lazy<Queue>(() => new Queue(Config.DistributorQueueName), false);
    private static readonly Lazy<Queue> g_ConsumerAdapter = new Lazy<Queue>(() => new Queue(Config.ConsumerAdaptorQueueName), false);

    public static Queue Parser { get { return g_Parser.Value; } }
    public static Queue Distributor { get { return g_Distributor.Value; } }
    public static Queue ConsumerAdapter { get { return g_ConsumerAdapter.Value; } }
}

When debugging, I noticed a message I've never seen:

The function evaluation requires all threads to run

Before using Lazy<T>, the values were displayed directly. Now, I need to click on the round button with the threads icon to evaluate the lazy value. This happens only on my properties that are retrieving the .Value of Lazy<T>. When expanding the debugger visualizer node of the actual Lazy<T> object, the Value property simply displays null, without any message.

What does that message mean and why is it displayed in my case?

Answer 1

I've found an MSDN page titled "How to: Refresh Watch Values" explaining it:

When you evaluate an expression in the debugger, one of two refresh icons might appear in the Value column. One refresh icon is a circle that contains two arrows, which circle in opposite directions. The other is a circle that contains two wavy lines that resemble threads.

...

If the two threads appear, the expression was not evaluated because of a potential cross-thread dependency. A cross-thread dependency means that evaluating the code requires other threads in your application to run temporarily. When you are in break mode, all threads in your application are typically stopped. Allowing other threads to run temporarily can have unexpected effects on the state of your program and causes the debugger to ignore events such as breakpoints.

I'd still like a better explanation if anyone can give it. Questions that this doesn't answer include: What kind of evaluation requires all threads to run? How does the debugger identify such a case? What exactly happens when you click the thread refresh icon?

EDIT: I think I've stumbled across the answer when examining Lazy<T> under ILSpy (for a completely different reason). The getter of the Value property has a call to a Debugger.NotifyOfCrossThreadDependency(). MSDN has this to say:

[...] performing a function evaluation typically requires freezing all threads except for the thread that is performing the evaluation. If the function evaluation requires execution on more than one thread, as might occur in remoting scenarios, the evaluation will block. The NotifyOfCrossThreadDependency notification informs the debugger that it has to release a thread or abort the function evaluation.

So basically, to prevent the annoying case where you try to evaluate some expression and Visual Studio just hangs for 30 seconds and then informs you that "a function evaluation has timed out", the code has a chance to inform the debugger that it must unfreeze other threads for the evaluation to succeed or otherwise the evaluation will block forever.

Since running other threads may disrupt your debugging session, as usually when you evaluate an expression all other threads are kept frozen, the debugger doesn't automatically proceeed and warns you before letting you jump down the rabbit hole.