Generic parameter delegate?

Question

I'm a bit fuzzy on the new Action/Func/Variance/CoVariance stuff, which is probably what I need.

What I want is to be able to pass a delegate as a parameter to a method, that takes a string and returns a bool. The problem is that I can't use a typed delegate or interface since it will be used in different libraries which doesn't share libraries, and both will be invoked by a third.

So in general I want the delegate to be inferred by it's input and returning type.

So it'll be like:

delegate bool IncludeItemDelegate(string item);

ClassA.FetchItems(int someParameter,includeItemDelegate);
ClassB.FetchItems(int someParameter,string someOtherParam,includeItemDelegate);

Where A and B doesnt share any libraries, can it be done?

Answer 1

How about Func<string,bool> ?

Answer 2

Predicate is built-in and also signals intent:

ClassA.FetchItems(int someParameter, Predicate<string> filter);

Answer 3

What you want is a Func:

aLibrary.ClassName.MethodName(new Func<string, bool>(myCallback));

Though your reasoning "I can't use a typed delegate or interface since it will be used in different libraries" is not 100% as you can just declare your typed delegate in the namespace of each library. FYI: Action is used when your callback method is void.

Update: Incidentally you neednt bother declaring your delegate in your caller as C# 3.0 (and above) will do it for you (the callback method's signature just needs to the same as expected by the method you are passing it from):

aLibrary.ClassName.MethodName(myCallbackMethod);

Answer 4

It's also possible to pass the Predicate as a lambda

class A
{
    static public IEnumerable<string> FetchItems(int max, Predicate<string> filter)
    {
        var l = new List<string>() {"test", "fest", "pest", "häst"};
        return l.FindAll(filter).Take(max);
    }
}

like this

var res = A.FetchItems(2, s => s.EndsWith("est"));