Monitor.Enter Method (Object)


The .NET API Reference documentation has a new home. Visit the .NET API Browser on to see the new experience.

Acquires an exclusive lock on the specified object.

Namespace:   System.Threading
Assembly:  mscorlib (in mscorlib.dll)

static member Enter : 
        obj:Object -> unit


Type: System.Object

The object on which to acquire the monitor lock.

Exception Condition

The obj parameter is null.

Use Enter to acquire the Monitor on the object passed as the parameter. If another thread has executed an Enter on the object but has not yet executed the corresponding Exit, the current thread will block until the other thread releases the object. It is legal for the same thread to invoke Enter more than once without it blocking; however, an equal number of Exit calls must be invoked before other threads waiting on the object will unblock.

Use Monitor to lock objects (that is, reference types), not value types. When you pass a value type variable to Enter, it is boxed as an object. If you pass the same variable to Enter again, it is boxed as a separate object, and the thread does not block. In this case, the code that Monitor is supposedly protecting is not protected. Furthermore, when you pass the variable to Exit, still another separate object is created. Because the object passed to Exit is different from the object passed to Enter, Monitor throws SynchronizationLockException. For more information, see the conceptual topic Monitors.

Interrupt can interrupt threads that are waiting to enter a Monitor on an object. A ThreadInterruptedException will be thrown.

Use a C# tryfinally block (TryFinally in Visual Basic) to ensure that you release the monitor, or use the C# lock statement (SyncLock statement in Visual Basic), which wraps the Enter and Exit methods in a tryfinally block.

The following example demonstrates how to use the Enter method.

No code example is currently available or this language may not be supported.

Universal Windows Platform
Available since 8
.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
Windows Phone
Available since 8.1
Return to top