C#:Creating Multicast delegate with boolean return

问题:

Hai Techies,

in C#, how can we define the multicast delegate which accepts a DateTime object and return a boolean.

Thanks

回答1:

public delegate bool Foo(DateTime timestamp);

This is how to declare a delegate with the signature you describe. All delegates are potentially multicast, they simply require initialization. Such as:

public bool IsGreaterThanNow(DateTime timestamp)
{
    return DateTime.Now < timestamp;
}

public bool IsLessThanNow(DateTime timestamp)
{
    return DateTime.Now > timestamp;
}

Foo f1 = IsGreaterThanNow;
Foo f2 = IsLessThanNow;
Foo fAll = f1 + f2;

Calling fAll, in this case would call both IsGreaterThanNow() and IsLessThanNow().

What this doesn't do is give you access to each return value. All you get is the last value returned. If you want to retrieve each and every value, you'll have to handle the multicasting manually like so:

List<bool> returnValues = new List<bool>();
foreach(Foo f in fAll.GetInvocationList())
{
    returnValues.Add(f(timestamp));
}

回答2:

Any delegate can be a multicast delegate

delegate bool myDel(DateTime s);
myDel s = someFunc;
s += someOtherFunc;

msdn

A useful property of delegate objects is that they can be assigned to one delegate instance to be multicast using the + operator. A composed delegate calls the two delegates it was composed from. Only delegates of the same type can be composed.

EDIT: A delagate has a method GetInvocationList which returns a list with the attached methods.

Here is a reference about Delegate invocation

foreach(myDel d in s.GetInvocationList())
{
   d();
}

回答3:

class Test
{
    public delegate bool Sample(DateTime dt);
    static void Main()
    {
        Sample j = A;
        j += B;
        j(DateTime.Now);

    }
    static bool A(DateTime d)
    {
        Console.WriteLine(d);
        return true;
    }
    static bool B(DateTime d)
    {
        Console.WriteLine(d);
        return true;
    }
}

回答4:

I was stumbling upon the same problem. I searched and found this in msdn.

http://msdn.microsoft.com/en-us/library/2e08f6yc(v=VS.100).aspx

There are two methods on delegates

• BeginInvoke
• EndInvoke

The link describes these in detail, with code samples.

We can hook into these methods to handle the return values of the delegates.

回答5:

In your case , Instead of creating a delegate yourself ,

it is better to use pre-defined delegates in C# such as Func and Predicate :

public delegate TResult Func<in T1, in T2, out TResult>(T1 arg1, T2 arg2);

and

public delegate bool Predicate<in T>(T obj);