When I create a default CancellationToken
I can see in the debugger that the CancellationToken
has a CancellationTokenSource
associated with it which is stored in the private m_source
field:
I am wondering how can that be as for structs the default
keyword "will return each member of the struct initialized to zero or null depending on whether they are value or reference types" and CancellationTokenSource
is a reference type.
CancellationToken
does have 2 constructors that set this field however they are irrelevant as default(CancellationToken)
doesn't call constructors and new CancellationToken()
(which has the exact same behavior) doesn't call a constructor becuase structs can't have parameterless constructors (yet).
You can also use the C# default(CancellationToken) statement to create an empty cancellation token. Two empty cancellation tokens are always equal.
CancellationTokenSource is quite a heavyweight object and its not normally cancelled; however it can't be pooled or reused because its registrations cannot be cleared.
A CancellationToken enables cooperative cancellation between threads, thread pool work items, or Task objects. You create a cancellation token by instantiating a CancellationTokenSource object, which manages cancellation tokens retrieved from its CancellationTokenSource.
default(CancellationToken)
does create a CancellationToken
where m_source
is null
. You can see that by getting the value of that private field using reflection:
Console.WriteLine(typeof (CancellationToken).
GetField("m_source", BindingFlags.NonPublic | BindingFlags.Instance).
GetValue(default(CancellationToken)) ?? "null");
Output:
null
You can also see that by pining only the relevant field in the debugger:
So, what happens?
The debugger, in order to display the contents of the CancellationToken
, accesses its properties one by one. When the inner CancellationTokenSource
is null
the WaitHandle
property creates and sets a default CancellationTokenSource
before delegating to its WaitHandle
property:
public WaitHandle WaitHandle
{
get
{
if (m_source == null)
{
m_source = CancellationTokenSource.InternalGetStaticSource(false);
}
return m_source.WaitHandle;
}
}
In conclusion, default(CancellationToken)
and new CancellationToken
create an empty struct where m_source
is null
but by looking at the struct in the debugger you are filling that field with a default CancellationTokenSource
instance that can't be cancelled.
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