Default parameter for CancellationToken

问题:

I have some async code that I would like to add a CancellationToken to. However, there are many implementations where this is not needed so I would like to have a default parameter - perhaps CancellationToken.None. However,

Task<x> DoStuff(...., CancellationToken ct = null)

yields

A value of type '' cannot be used as a default parameter because there are no standard conversions to type 'System.Threading.CancellationToken'

and

Task<x> DoStuff(...., CancellationToken ct = CancellationToken.None)

Default parameter value for 'ct' must be a compile-time constant

Is there any way to have a default value for CancellationToken?

回答1:

It turns out that the following works:

Task<x> DoStuff(...., CancellationToken ct = default(CancellationToken))

which, according to the documentation, is interpreted the same as CancellationToken.None:

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

回答2:

Is there any way to have a default value for CancellationToken?

Unfortunately, this is not possible, as CancellationToken.None is not a compile time constant, which is a requirement for default values in optional arguments.

You can provide the same effect, however, by making an overloaded method instead of trying to use default parameters:

Task<x> DoStuff(...., CancellationToken ct)
{
    //...
}

Task<x> DoStuff(....)
{
    return DoStuff(...., CancellationToken.None);
}

回答3:

Here are several solutions, in descending order of general goodness:

1. Using default(CancellationToken) as default value:

Task DoAsync(CancellationToken ct = default(CancellationToken)) { … }

Semantically, CancellationToken.None would be the ideal candidate for the default, but cannot be used as such because it isn't a compile-time constant. default(CancellationToken) is the next best thing because it is a compile-time constant and officially documented to be equivalent to CancellationToken.None.

2. Providing a method overload without a CancellationToken parameter:

Or, if you prefer method overloads over optional parameters (see this and this question on that topic):

Task DoAsync(CancellationToken ct) { … } // actual method always requires a token
Task DoAsync() => DoAsync(CancellationToken.None); // overload producing a default token

For interface methods, the same can be achieved using extension methods:

interface IFoo
{
    Task DoAsync(CancellationToken ct);
}

static class Foo
{
    public static Task DoAsync(this IFoo foo) => foo.DoAsync(CancellationToken.None);
}

This results in a slimmer interface and spares implementers from explicitly writing the forwarding method overload.

3. Making the parameter nullable and using null as default value:

Task DoAsync(…, CancellationToken? ct = null)
{
    … ct ?? CancellationToken.None …
}

I like this solution least because nullable types come with a small runtime overhead, and references to the cancellation token become more verbose because of the null coalescing operator ??.

回答4:

Another option is to use a Nullable<CancellationToken> parameter, default it to null, and deal with it inside the method:

Task<x> DoStuff(...., CancellationToken? ct = null) {
    var token = ct ?? CancellationToken.None;
    ...
}

回答5:

Tofutim's answer is one way, but from the comments I see that people have issues with it.

In that case, I did suggest that one can have a method as follows:

Task<x> DoStuff(...., CancellationToken ct)
{
} 

and overload it as:

Task<x> DoStuff(....)
{
    return DoStuff(...., CancellationToken.None);
}

This compiles, because the value of CancellationToken.None is not required at compile time.