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Code Inspection: Possible incorrect implementation of Double-Check Locking pattern. Read access to checked field

Consider the following piece of code:

public class Foo { private static volatile Foo instance; private static readonly object padlock = new object(); public static Foo GetValue() { if (instance == null) { lock (padlock) { if (instance == null) { instance = new Foo(); instance.Init(); } } } return instance; } private void Init() { ... } };

If we assume that Init() is a method used to initialize the state of Foo, then the above code may not function as expected due to the memory model not guaranteeing the order of reads and writes. As a result, the call to Init() may actually occur before the variable instance is in a consistent state.

There are two ways to resolve this issue for the code above.

The first, and most obvious, is to simply relocate the contents of Init() to a private constructor.

The second is to perform the initialization in a non-checked variable and then assigned it to the checked one, thus eliminating the ordering problem. Thus, the code in the inner if statement from the above example could be re-written as:

var temp = new Foo(); temp.Init(); instance = temp;

See Also

Last modified: 19 August 2016