Can a C# lambda expression ever return void?

问题

I have the following method, and I want to know if there is anything that can go in place default(void) below because there is a compiler error that says that void is not valid here:

private void applyDefaultsIfNecessary(ApplicationConfiguration configuration)
{
    var defaults = new Dictionary<Predicate<ApplicationConfiguration>, Action<ApplicationConfiguration>>()
    {
       // { rule, action } - if rule is true, execute action 
       { (c) => c.ConnectionString == null , (c) => c.ConnectionString = "foo" },
       { (c) => c.OutputExcelFilePath == null, (c) => c.ConnectionString = "bar" },
       { (c) => c.OutputDirectory == null, (c) => c.OutputDirectory = "baz" }

    };

    //Nothing to select, but we want to loop throough the dict and invoke action, if rule is true.
    //It is a pity there is no extension method called DoForEach on collections.
    defaults.Select((item) => item.Key.Invoke(configuration) ? item.Value.Invoke(configuration) : default(void)  );
}

I realize that I can use the if-else statement instead of the ternary operator (or that I could call a dummy method to return void). Also, the Select extension method does not like lambdas that return void. It seems to say that the type cannot be inferred, but of course if I specify the type like this, either:

defaults.Select<ApplicationConfiguration, void>((item) => { if (item.Key.Invoke(configuration)) item.Value.Invoke(configuration); } );

I was curious from a language design standpoint, why we don't have expressions that can return void or the data type for variables that is void.

回答1:

I refer you to section 7.1 of the specification, which states:

[An expression may be classified as] "nothing". This occurs when the expression is an invocation of a method with a return type of void. An expression classified as nothing is only valid in the context of a statement expression.

[Emphasis added].

That is to say that the only time you may use an expression which is a void-returning method invocation is when the expression makes up an entire statement. Like this:

M();

回答2:

This is, in effect, a violation of functional programming rules. This has the same flaw Eric Lippert described about List.ForEach: You're philosphically trying to cause side effects on your collection.

Enumerable.Select is intended to return a new collection - filtering the input. It is not intended to execute code.

That being said, you can work around this by doing:

defaults.Where(item => item.Key.Invoke(configuration)).ToList().ForEach( item => item.Value.Invoke(configuration));

It's just not as clear as doing:

var matches = defaults.Where(item => item.Key.Invoke(configuration));
foreach(var match in matches)
    match.Value.Invoke(configuration);

回答3:

Firstly, you should really avoid putting side-effects in standard linq query operators, and secondly this won't actually work since you aren't enumerating the Select query anywhere. If you want to use linq you could do this:

foreach(var item in defaults.Where(i => i.Key.Invoke(configuration)))
{
   item.Value.Invoke(configuration);   
}

Regarding your question, I'm pretty sure there are no possible values of void and you can't return it explicity. In functional languages such as F#, void is replaced with 'unit' i.e. a type with only one possible value - if you wanted you could create your own unit type and return that. In this case you could do something like this:

defaults.Select(item => {
    if(item.Key.Invoke(configuration))
    {
        item.Value.Invoke(configuration);
    }
    return Unit.Value;
}).ToList();

But I really can't recommend doing this.

回答4:

From a language standpoint, void means "does not exist", which begs the question: what value would there be in declaring a variable that does not exist?

The problem here is not a language limitation but the fact that you're using a construct (the ternary operator) that demands two rvalues -- where you only have one.

If I may be blunt, I'd say you're avoiding if/else in favor of pointless brevity. Any tricks you come up with to replace default(void) will only serve to confuse other developers, or you, in the future, long after you've stopped bothering with this sort of thing.

回答5:

default doesn't work with void; but it works with a type. The Action class produces no result, but the Func<> object always has to return a result. Whatever item.Value.Invoke() returns just return the default of that, as in:

default(object)

or if it's a specific type:

default(SomeType)

Like that.

来源：https://stackoverflow.com/questions/2029433/can-a-c-sharp-lambda-expression-ever-return-void

标签

.net

syntax

lambda

void