Is .NET's StringBuilder thread-safe

Question

The regular "Thread Safety" section of the MSDN documentation for StringBuilder states that:

...any instance members are not guaranteed to be thread safe...

but this statement feels like it has been copied and pasted for almost every class in the Framework:

http://msdn.microsoft.com/en-us/library/system.text.stringbuilder.aspx

However, these blog posts by Gavin Pugh mention thread-safe behaviours of StringBuilder:

http://www.gavpugh.com/2010/03/23/xnac-stringbuilder-to-string-with-no-garbage/

http://www.gavpugh.com/2010/04/01/xnac-avoiding-garbage-when-working-with-stringbuilder/

Additionally, the source of StringBuilder revealed by Reflector, and the accompanying comments in the SSCLI source, also suggest many implementation considerations to ensure thread-safety:

http://labs.developerfusion.co.uk/SourceViewer/browse.aspx?assembly=SSCLI&namespace=System.Text&type=StringBuilder

Does anyone have any more insight into whether a StringBuilder instance is safe to share among multiple concurrent threads?

Answer 1

Absolutely not; here's a simple example lifted from 4.0 via reflector:

[SecuritySafeCritical]
public StringBuilder Append(char value)
{
    if (this.m_ChunkLength < this.m_ChunkChars.Length)
    {
        this.m_ChunkChars[this.m_ChunkLength++] = value;
    }
    else
    {
        this.Append(value, 1);
    }
    return this;
}

The attribute just handles callers, not thread-safety; this is absolutely not thread-safe.

Update: looking at the source he references, this is clearly not the current .NET 4.0 code-base (comparing a few methods). Perhaps he is talking about a particular .NET version, or maybe XNA - but it is not the case in general. The 4.0 StringBuilder does not have a m_currentThread field, which Gavin's source material uses; there's a hint (an unused constant ThreadIDField) that it used to exist, but... no longer.

---

If you want a direct disproof - run this on 4.0; it will most likely give the wrong length (I've seen a few in the 4k region, a few in the 2k region - it should be exactly 5000), but some other Append methods (Append(char) for example) tend more likely to throw exceptions, depending on timing:

var gate = new ManualResetEvent(false);
var allDone = new AutoResetEvent(false);
int counter = 0;
var sb = new StringBuilder();
ThreadStart work = delegate
{
    // open gate when all 5 threads are running
    if (Interlocked.Increment(ref counter) == 5) gate.Set();
    else gate.WaitOne();

    for (int i = 0; i < 1000; i++) sb.Append("a");

    if (Interlocked.Decrement(ref counter) == 0) allDone.Set();
};
for(int i = 0 ; i < 5 ; i++)
{
    new Thread(work).Start();
}
allDone.WaitOne();
Console.WriteLine(sb.Length);