ICollection.SyncRoot Property (System.Collections)

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 property 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 as follows:

Dim myCollection As ICollection = someCollection
SyncLock myCollection.SyncRoot
    ' Some operation on the collection, which is now thread safe.
End SyncLock

ICollection myCollection = someCollection;
lock(myCollection.SyncRoot)
{
    // Some operation on the collection, which is now thread safe.
}

ICollection^ myCollection = someCollection;
bool lockTaken = false;
try
{
    Monitor::Enter(myCollection->SyncRoot, lockTaken);
    // Some operation on the collection, which is now thread safe.
}
finally
{
    if (lockTaken)
    {
        Monitor::Exit(myCollection->SyncRoot);
    }
}

Enumerating through a collection is intrinsically not a thread-safe procedure. Even when a collection is synchronized, other threads can 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.

The following code example shows how to lock the collection using the SyncRoot property during the entire enumeration.

Dim myCollection As ICollection = someCollection
SyncLock myCollection.SyncRoot
    For Each item In myCollection
        ' Insert your code here.
    Next item
End SyncLock

ICollection myCollection = someCollection;
lock(myCollection.SyncRoot)
{
    foreach (object item in myCollection)
    {
        // Insert your code here.
    }
}

ICollection^ myCollection = someCollection;
bool lockTaken = false;
try
{
    Monitor::Enter(myCollection->SyncRoot, lockTaken);
    for each (Object^ item in myCollection);
    {
        // Insert your code here.
    }
}
finally
{
    if (lockTaken)
    {
        Monitor::Exit(myCollection->SyncRoot);
    }
}

.NET Framework

.NET Framework Client Profile

Portable Class Library

Windows 8 Consumer Preview, Windows Server 8 Beta, Windows 7, Windows Server 2008 SP2, Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Reference