Is default(CancellationToken) equivalent to Cancel

问题:

Looking at the implementation of CancellationToken.None, it is simply returning default(CancellationToken). However, I see no reference in CancellationToken's documentation that the two are equivalent.

I'd like to offer an API like this but not until I'm sure it'll always work:

Task DoSomething(CancellationToken token = default(CancellationToken))

Is it defined behavior that default(CancellationToken) is the same as CancellationToken.None, or is this just an implementation detail?

回答1:

After filing an issue with corefx, the documentation remarks have been updated to make this a guaranteed feature:

You can also use the C# default(CancellationToken) statement to create an empty cancellation token.

回答2:

They are the same. Check source code

public static CancellationToken None
{
    get { return default(CancellationToken); }
}

From https://referencesource.microsoft.com/#mscorlib/system/threading/CancellationToken.cs,36b17ded8b1a228c

回答3:

CancellationToken.None simply returns new CancellationToken:

public static CancellationToken None
{    
    get
    {
        return new CancellationToken();
    }
}

Thus CancellationToken is a struct, then default(CancellationToken) will return same value. C# Spec 5.2:

For a variable of a value-type, the default value is the same as the value computed by the value-type’s default constructor

UPDATE: This behavior is not defined on MSDN, so you can rely only on current implementation.

回答4:

default is defined for every type. It is null for reference types. It is an "empty" instance for structs, i.e. one with all properties initialized to their respective default values.