LockedWait.qhelp

<!DOCTYPE qhelp PUBLIC
  "-//Semmle//qhelp//EN"
  "qhelp.dtd">
<qhelp>
<overview>
<p>Holding a lock during a call to <code>System.Threading.Monitor.Wait()</code> can lead to
performance problems or deadlock because the lock can prevent other threads from running. 
This can be caused by nesting the call to <code>System.Threading.Monitor.Wait()</code> in
two <code>lock</code> statements, or by waiting on a different object to the one
which is locked.</p>

<p>Synchronized methods (with the attribute <code>[MethodImpl(MethodImplOptions.Synchronized)]</code>)
can also cause problems, because they are equivalent to <code>lock(this)</code> or 
<code>lock(typeof(Class))</code>.</p>

</overview>
<recommendation>
<p>Ensure that the call to <code>System.Threading.Monitor.Wait()</code> occurs within a single
<code>lock</code> statement and ensure that the argument to <code>System.Threading.Monitor.Wait()</code>
matches the variable in the <code>lock</code> statement.</p>

</recommendation>
<example>
<p>The following example locks two variables, <code>countLock</code> and <code>textLock</code>,
then calls <code>System.Threading.Monitor.Wait()</code>. However for the duration of <code>Wait()</code>,
the variable <code>countLock</code> is locked, which deadlocks the program.</p>

<sample src="LockedWait.cs" />

<p>The problem can be fixed by moving the <code>lock</code> statement so that <code>countLock</code>
is not locked for the duration of the wait.</p>

<sample src="LockedWaitFix.cs" />

</example>
<references>

   <li>MSDN, C# Reference: <a href="http://msdn.microsoft.com/en-gb/library/c5kehkcz.aspx">lock Statement</a>.</li>


</references>
</qhelp>