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ICollection.SyncRoot Property

When implemented by a class, gets an object that can be used to synchronize access to the ICollection.

[Visual Basic]
ReadOnly Property SyncRoot As Object
[C#]
object SyncRoot {get;}
[C++]
__property Object* get_SyncRoot();
[JScript]
function get SyncRoot() : Object;

Property Value

An object that can be used to synchronize access to the ICollection.

Remarks

For collections whose underlying store is not publicly available, the expected implementation is to return the current instance. Note that the pointer to the current instance might not be sufficient for collections that wrap other collections; those should return the underlying collection's SyncRoot property.

Most collection classes in the System.Collections namespace also implement a Synchronized method, which provides a synchronized wrapper around the underlying collection. However, derived classes can provide their own synchronized version of the collection using the SyncRoot property. The synchronizing code must perform operations on the SyncRoot of the collection, not directly on the collection. This ensures proper operation of collections that are derived from other objects. Specifically, it maintains proper synchronization with other threads that might be simultaneously modifying the collection instance.

In the absence of a Synchronized method on a collection, the expected usage for SyncRoot looks like this:

[C#] 
ICollection MyCollection = ...
lock( MyCollection.SyncRoot ) {
 // Some operation on the collection, which is now thread-safe.
}
[Visual Basic] 
Dim myCollection as New ICollection()
SyncLock myCollection.SyncRoot
 ' Some operation on the collection, which is now thread-safe.
End SyncLock

Enumerating through a collection is intrinsically not a thread-safe procedure. Even when a collection is synchronized, other threads could still modify the collection, which causes the enumerator to throw an exception. To guarantee thread safety during enumeration, you can either lock the collection during the entire enumeration or catch the exceptions resulting from changes made by other threads.

[Visual Basic, C#] The following code example shows how to lock the collection using the SyncRoot during the entire enumeration:

[C#] 
ICollection myCollection = new ICollection();
lock( myCollection.SyncRoot ) {
 foreach ( Object item in myCollection ) {
 // Insert your code here.
 }
}
[Visual Basic] 
Dim myCollection As New ICollection()
Dim item As Object
SyncLock myCollection.SyncRoot
 For Each item In myCollection
 ' Insert your code here.
 Next item
End SyncLock

Requirements

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family, .NET Compact Framework, Common Language Infrastructure (CLI) Standard

See Also

ICollection Interface | ICollection Members | System.Collections Namespace | IsSynchronized

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