Why can lambdas convert function calls into Actions?

问题

In this code fragment:

List<String> names = new List<String>();
names.Add("Bruce");
names.Add("Tom");
names.Add("Tim");
names.Add("Richard");

names.ForEach(x => Print(x));

private static string Print(string s)
{
    Console.WriteLine(s);
    return s;
}

Print is not an Action for sure since it is returning string; however x=> Print(x) is, why?

回答1:

The type of the lambda expression x => Print(x) is determined based on its context. Since the compiler knows that the lambda is assigned to Action<string>, the compiler disregards the return type of the Print(s) method as if it were a statement expression.

This is a valid conversion:

Action<string> myAction = y => Print(y);

In other words, both

Print("something");

and

int x = Print("something");

are correct usages of the Print method; they can be used in lambdas in the same way.

回答2:

x => Print(x) is a lambda that is turned into a method here equivalent to:

void MyPrintLambda(string x) { Print(x); }

If the context had called for, say, a Func<string, string>, it would have been this:

string MyPrintLambda(string x) { return Print(x); }

Or if it was a Func<string, object>, it would have been this:

object MyPrintLambda(string x) { return Print(x); }

Because the compiler can turn x => Print(x) into an Action<string> by simply ignoring the return type (i.e. the first example), it can compile.

回答3:

For the same reason that this would be valid:

foreach (string name in names)
{
    Print(name);
}

The Print() method returns a value in that code as well, but no one would expect this to be an error. It's allowed to just throw away the return value.

来源：https://stackoverflow.com/questions/12869426/why-can-lambdas-convert-function-calls-into-actions

标签

.net

c#-4.0