Question

I expect the following code to deadlock when Clear tries to lock on the same object that Build has already locked:

void Main()
{
    (new SiteMap()).Build();
}

class SiteMap
{
    private readonly object _lock = new object();

    public void Build()
    {
        lock (_lock)
        {
            Clear();

            Console.WriteLine("Build");
        }
    }

    public void Clear()
    {
        lock (_lock)
        {
            Console.WriteLine("Clear");
        }
    }
}

Output:

Clear

Build

Edit 1

Thank you all for your answers.

If I add a call to Build inside the lock of Clear (keeping the rest of the code the same):

public void Clear()
{
    lock (_lock)
    {
        Build();

        Console.WriteLine("Clear");
    }
}

A deadlock does occur (or at least that’s what I think, LINQ Pad crashes).

According to your answers, this shouldn’t happen, because it’s still the same thread.

Thanks!

Answer 1

In C#, a thread holding a lock can enter the same lock without blocking.

The lock statement, as well as the Monitor class on which it is built, is reentrant in .NET.

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Edit in response to your edit:

When you add the call to Build inside clear, the code doesn’t deadlock – it is calling itself recursively. It’s not blocking, but rather running forever (until, eventually, you hit a StackOverflowException), because Build calls Clear which calls Build again which calls Clear, etc….