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Synchronizing data for multithreading

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When multiple threads can make calls to the properties and methods of a single object, it is critical that those calls be synchronized. Otherwise one thread might interrupt what another thread is doing, and the object could be left in an invalid state. A class whose members are protected from such interruptions is called thread-safe.

.NET provides several strategies to synchronize access to instance and static members:

  • Synchronized code regions. You can use the Monitor class or compiler support for this class to synchronize only the code block that needs it, improving performance.

  • Manual synchronization. You can use the synchronization objects provided by the .NET class library. See Overview of Synchronization Primitives, which includes a discussion of the Monitor class.

  • Synchronized contexts. For .NET Framework and Xamarin applications only, you can use the SynchronizationAttribute to enable simple, automatic synchronization for ContextBoundObject objects.

  • Collection classes in the System.Collections.Concurrent namespace. These classes provide built-in synchronized add and remove operations. For more information, see Thread-Safe Collections.

The common language runtime provides a thread model in which classes fall into a number of categories that can be synchronized in a variety of different ways depending on the requirements. The following table shows what synchronization support is provided for fields and methods with a given synchronization category.

Category Global fields Static fields Static methods Instance fields Instance methods Specific code blocks
No Synchronization No No No No No No
Synchronized Context No No No Yes Yes No
Synchronized Code Regions No No Only if marked No Only if marked Only if marked
Manual Synchronization Manual Manual Manual Manual Manual Manual

No synchronization

This is the default for objects. Any thread can access any method or field at any time. Only one thread at a time should access these objects.

Manual synchronization

The .NET class library provides a number of classes for synchronizing threads. See Overview of Synchronization Primitives.

Synchronized code regions

You can use the Monitor class or a compiler keyword to synchronize blocks of code, instance methods, and static methods. There is no support for synchronized static fields.

Both Visual Basic and C# support the marking of blocks of code with a particular language keyword, the lock statement in C# or the SyncLock statement in Visual Basic. When the code is executed by a thread, an attempt is made to acquire the lock. If the lock has already been acquired by another thread, the thread blocks until the lock becomes available. When the thread exits the synchronized block of code, the lock is released, no matter how the thread exits the block.

Note

The lock and SyncLock statements are implemented using Monitor.Enter and Monitor.Exit, so other methods of Monitor can be used in conjunction with them within the synchronized region.

You can also decorate a method with a MethodImplAttribute with a value of MethodImplOptions.Synchronized, which has the same effect as using Monitor or one of the compiler keywords to lock the entire body of the method.

Thread.Interrupt can be used to break a thread out of blocking operations such as waiting for access to a synchronized region of code. Thread.Interrupt is also used to break threads out of operations like Thread.Sleep.

Important

Do not lock the type — that is, typeof(MyType) in C#, GetType(MyType) in Visual Basic, or MyType::typeid in C++ — in order to protect static methods (Shared methods in Visual Basic). Use a private static object instead. Similarly, do not use this in C# (Me in Visual Basic) to lock instance methods. Use a private object instead. A class or instance can be locked by code other than your own, potentially causing deadlocks or performance problems.

Compiler support

Both Visual Basic and C# support a language keyword that uses Monitor.Enter and Monitor.Exit to lock the object. Visual Basic supports the SyncLock statement; C# supports the lock statement.

In both cases, if an exception is thrown in the code block, the lock acquired by the lock or SyncLock is released automatically. The C# and Visual Basic compilers emit a try/finally block with Monitor.Enter at the beginning of the try, and Monitor.Exit in the finally block. If an exception is thrown inside the lock or SyncLock block, the finally handler runs to allow you to do any clean-up work.

Synchronized Context

In .NET Framework and Xamarin applications only, you can use the SynchronizationAttribute on any ContextBoundObject to synchronize all instance methods and fields. All objects in the same context domain share the same lock. Multiple threads are allowed to access the methods and fields, but only a single thread is allowed at any one time.

See also