Microsoft KB Archive/316145

= DOC: Confusion in the Threading Design Guideline Help Document =

Article ID: 316145

Article Last Modified on 8/21/2003

-

APPLIES TO


 * Microsoft .NET Framework Software Development Kit 1.0 Service Pack 2

-



This article was previously published under Q316145



SUMMARY
In the Microsoft Developer Network (MSDN) Documentation that installs with Visual Studio .NET, part of the &quot;Threading Design Guidelines&quot; help topic is not very clear and may cause some confusion.



STATUS
This bug was corrected in the Microsoft .NET Framework SDK 1.1.



MORE INFORMATION
The &quot;Threading Design Guidelines&quot; help topic contains the following text:

&quot;Be aware of issues with the lock statement (SyncLock in Visual Basic). It is tempting to use the lock statement to solve all threading problems. However, the System.Threading.Interlocked Class is superior for updates that must be made atomically. It executes a single lock prefix if there is no contention. In a code review, you should watch out for instances like the one shown in the following example.

[Visual Basic] SyncLock Me  myField += 1 End SyncLock [C#] lock(this) {  myField++; } Alternatively, it might be better to use more elaborate code to create rhs outside of the lock, as in the following example. Then, you can use an interlocked compare exchange to update x only if it is still null. This assumes that creation of duplicate rhs values does not cause negative side effects. [Visual Basic] If x Is Nothing Then SyncLock Me     If x Is Nothing Then ' Perform some elaborate code to create rhs. x = rhs End If  End SyncLock End If [C#] if (x == null) {  lock (this) {     if (x == null) {        // Perform some elaborate code to create rhs. x = rhs; }  } } &quot;

To make the topic more clear, replace the previous text with the following:

&quot;Be aware of issues with the lock statement (SyncLock in Visual Basic). It is tempting to use the lock statement to solve all threading problems. However, the System.Threading.Interlocked Class is superior for updates that must be made automatically. This class executes a single lock prefix if there is no contention. For example, in a code review, you should watch out for instances like the one shown in the following example:

[Visual Basic] SyncLock Me  myField += 1 End SyncLock [C#] lock(this) {  myField++; } Replace the previous sample code with the following sample code to improve performance: [Visual Basic] System.Threading.Interlocked.Increment(myField) [C#] System.Threading.Interlocked.Increment(ref myField); Another example is to update an object type variable only if the variable is null. You can use the following code to update the variable and to make the code thread safe: [Visual Basic] If x Is Nothing Then SyncLock Me     If x Is Nothing Then x = y     End If   End SyncLock End If [C#] if (x == null) {  lock (this) {     if (x == null) {         x = y;      } } } For this sample, you can improve the performance if you use the following code to replace the previous code: [Visual Basic] System.Threading.Interlocked.CompareExchange(x, y, Nothing) [C#] System.Threading.Interlocked.CompareExchange(ref x, y, null); &quot;

Keywords: kbbug kbfix kbdocerr KB316145

-

[mailto:TECHNET@MICROSOFT.COM Send feedback to Microsoft]

© Microsoft Corporation. All rights reserved.