Lazy<'T> Constructor (Func<'T>, LazyThreadSafetyMode)
Initializes a new instance of the Lazy<'T> class that uses the specified initialization function and thread-safety mode.
Assembly: mscorlib (in mscorlib.dll)
The following example demonstrates the use of this constructor to create a lazy initializer that enables multiple threads to race to create an object lazily. Multiple threads might succeed in creating instances, but all threads use the instance that was created first. In addition, the example demonstrates that exceptions are never cached when you specify PublicationOnly, even if initialization is performed by a function instead of by the default constructor of the lazily created type.
For an example that demonstrates how to use this constructor in single-threaded scenarios (specifying LazyThreadSafetyMode.None for mode), see the Lazy<'T>(Boolean) constructor. For an example that demonstrates how to use this constructor to provide locking instead of race conditions in multithreaded scenarios (specifying LazyThreadSafetyMode.ExecutionAndPublication for mode), see the Lazy<'T>() constructor.
The example defines a LargeObject class that will be initialized lazily by any of several threads. The four key sections of code illustrate the creation of the initializer, the actual initialization, the initialization function, and the constructor and finalizer of the LargeObject class. At the beginning of the Main method, the example creates the Lazy<'T> object that performs lazy initialization of the LargeObject:
The lazy initializer uses a function to perform the initialization. In this case, a function is required because there is no default constructor for the LargeObject class.
The example creates and starts three threads that block on a ManualResetEvent object, so that the example can release the threads all at once. In the ThreadProc method that's used by all three threads, calling the Value property creates the LargeObject instance:
In the third key section of code, the lazy initialization function is called to create the LargeObject instance. The function throws an exception the first time it's called:
With any other LazyThreadSafetyMode setting, an unhandled exception in the initialization function would be cached. However, PublicationOnly suppresses exception caching. The output from the example demonstrates that a subsequent attempt to initialize the object succeeds.
The exception message usually appears after messages indicating that other threads have successfully initialized the object. This is because of the delay introduced by throwing and catching the exception.
Because the constructor for the Lazy<'T> instance specified LazyThreadSafetyMode.PublicationOnly, all three threads are allowed to create LargeObject instances. The example demonstrates this by displaying console messages in the constructor and in the finalizer of the LargeObject class:
The Lazy<'T> object ensures that only one instance is used by all threads (except the thread where the initialization function throws an exception). The output from the example shows this.
For simplicity, this example uses a global instance of Lazy<'T>, and all the methods are static (Shared in Visual Basic). These are not requirements for the use of lazy initialization.
Available since 8
Available since 4.0
Portable Class Library
Supported in: portable .NET platforms
Available since 4.0
Windows Phone Silverlight
Available since 8.0
Available since 8.1