How can a default(CancellationToken) have a corresponding CancellationTokenSource

Question

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).

Answer 1

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.