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POIs a lock required with a lazy initialization on a deeply immutable type?
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  1. POIs a lock required with a lazy initialization on a deeply immutable type?
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    copied!<p>If I have a deeply immutable type (all members are readonly and if they are reference type members, then they also refer to objects that are deeply immutable).</p> <p>I would like to implement a lazy initialized property on the type, like this:</p> <pre><code>private ReadOnlyCollection&lt;SomeImmutableType&gt; m_PropName = null; public ReadOnlyCollection&lt;SomeImmutableType&gt; PropName { get { if(null == m_PropName) { ReadOnlyCollection&lt;SomeImmutableType&gt; temp = /* do lazy init */; m_PropName = temp; } return m_PropName; } } </code></pre> <p>From what I can tell: </p> <pre><code>m_PropName = temp; </code></pre> <p>...is threadsafe. I'm not worried too much about two threads both racing to initialize at the same time, because it will be rare, both results would be identical from a logical perspective, and I'd rather not use a lock if I don't have to.</p> <p>Will this work? What are the pros and cons?</p> <p><strong>Edit:</strong> Thanks for your answers. I will probably move forward with using a lock. However, I'm surprised nobody brought up the possibility of the compiler realizing that the temp variable is unnecessary, and just assigning straight to m_PropName. If that were the case, then a reading thread could possibly read an object that hasn't finished being constructed. Does the compiler prevent such a situation?</p> <p>(Answers seem to indicate that the runtime won't allow this to happen.)</p> <p><strong>Edit:</strong> So I've decided to go with an Interlocked CompareExchange method inspired by <a href="http://www.bluebytesoftware.com/blog/2007/11/17/ImmutableTypesCanCopyTheWorldSafely.aspx" rel="noreferrer">this article by Joe Duffy</a>.</p> <p>Basically:</p> <pre><code>private ReadOnlyCollection&lt;SomeImmutableType&gt; m_PropName = null; public ReadOnlyCollection&lt;SomeImmutableType&gt; PropName { get { if(null == m_PropName) { ReadOnlyCollection&lt;SomeImmutableType&gt; temp = /* do lazy init */; System.Threading.Interlocked(ref m_PropName, temp, null); } return m_PropName; } } </code></pre> <p>This is supposed to ensure that all threads that call this method on this object instance will get a reference to the same object, so the == operator will work. It is possible to have wasted work, which is fine - it just makes this an optimistic algorithm.</p> <p>As noted in some comments below, this depends on the .NET 2.0 memory model to work. Otherwise, m_PropName should be declared volatile.</p>
 

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