/// <summary>
/// This method implements a low-level idiom of calling a [typically initialization] method
/// only once in a multithreaded environment; all other threads must wait
/// for the initialization method to finish (much like what happens in STL for call_once).
///
/// It was initially put here in order to be used in the ILC-generated method
/// Mcg.StartupCodeTrigger.Initialize which uses it to make sure that InternalInitialize
/// gets called only once and all concurrent threads wait until the initialization finishes.
///
/// Please keep that in mind - by the time StartupCodeTrigger.Initialize calls CallOnce,
/// most BCL hasn't yet been initialized as this is what StartupCodeTrigger.InternalInitialize
/// does among other tasks.
///
/// Please refer to EagerStaticConstructorOrder enumeration to see what early
/// framework initialization occurs in StartupCodeTrigger.Initialize for the shared library.
/// </summary>
/// <param name="callOnceAction">Delegate to call once</param>
/// <param name="callOnceGuard">Helper variable used to synchronize the initialization among threads</param>
public static void CallOnce(Action callOnceAction, ref int callOnceGuard)
{
if (callOnceGuard != CallOnceState.HasRun)
{
if (Interlocked.CompareExchange(ref callOnceGuard, CallOnceState.Running, CallOnceState.NotRun) == CallOnceState.NotRun)
{
// We have won the race for calling the callOnceAction
callOnceAction();
Volatile.Write(ref callOnceGuard, CallOnceState.HasRun);
}
else
{
// We have lost the race so just wait until another thread finishes the initialization
while (Volatile.Read(ref callOnceGuard) != CallOnceState.HasRun)
{
SpinWait.Yield();
}
}
}
}