delegate, template and alias

[Heromyth]

== Quote from Philippe Sigaud (philippe.sigaud at gmail.com)'s article
> On Sun, Dec 18, 2011 at 14:13, Heromyth <bitworld at qq.com> wrote:
> > I have a delegate as a parameter in a function which is a template function.
> > And I want to use alias for the delegate parameter.
> > Is there a better way for this?
> I suppose you do *not*want the commented line?
> You can extract a template parameter with an is() expression. But the
> extracted type is only accessible inside a static if.
> Solution: expose it through an alias:
> template AsynchronousActionParam(T)
> {
>     static if (is(T t == void delegate(U), U))
> 	alias U AsynchronousActionParam ;
>     else
> 	static assert(false, "Bad AsynchronousActionParam call: " ~  T.stringof);
> }
> class TestC
> {
>     int b = 3;
>     void test(F)(F func ) if (is(AsynchronousActionParam!F))
>     {
>         static if (is(AsynchronousActionParam!F == string))
> 	    func("It's me");
>         else
>             writeln("test called with void delegate(" ~
> AsynchronousActionParam!F ~ ").");
>     }
>     this(int x)
>     {
>         b = x;
>     }
> }
> void main()
> {
>     auto c = new TestC(3);
>     void foo(string s) { writeln("foo: ", s);}
>     c.test(&foo);
> }

Thanks greatly. Your code makes me understanding D's template much more.
The template in D is so amazing.

>             writeln("test called with void delegate(" ~
> AsynchronousActionParam!F ~ ").");

writeln("test called with void delegate(" ~
AsynchronousActionParam!F.stringof ~ ").");


> I suppose you do *not*want the commented line?

I want to convert a delegate type define in C# to D's, as such:
    public delegate void AsynchronousAction<T>(T argument, AsyncContinuation asyncContinuation);

    public static void ForEachItemSequentially<T>(IEnumerable<T> items, AsyncContinuation asyncContinuation, AsynchronousAction<T> action)
    {
        ......
    }