How do I return a delegate function or a lambda expression in c#?

Question

I am trying to write a method to return an instance of itself. The pseudo code is

Func<T,Func<T>> MyFunc<T>(T input)
{
    //do some work with input
    return MyFunc;
}

seems simple enough. But I am having problem defining the return type. The return type should be a delegate

 which takes T as parameter, then returns a function 
 which takes T as parameter, then returns a function 
 which takes T as parameter, then returns a function

   ...recursive definition

I am sure there was some subtle thing that I didn't notice. Can someone point it out for me? Thank you.

I was writing a utility method, which is static inside a static class. I also want to have the "fluent" functionality MyFunc(a).MyFunc(b)...but since I am inside a static class. It's not possible to return an instance of the class itself. — Wei Ma, Apr 28 '11 at 04:57

score 15 · Accepted Answer · answered Apr 28 '11 at 05:02

15

You can do it like this:

delegate F<T> F<T>(T obj);

F<T> MyFunc<T>(T obj)
{
    return MyFunc;
}

But it's pretty much useless. The only thing you can really do is something like this, which is weird:

void Main()
{
    MyFunc(1)(2)(3)(4);
}

delegate F<T> F<T>(T obj);

F<T> MyFunc<T>(T obj)
{
    Console.WriteLine(obj);
    return MyFunc;
}

answered Apr 28 '11 at 05:02

porges

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Dunno why I didn't think of that... +1. – Justin Apr 28 '11 at 05:08
1

I have error checking code block for every method I am implementing. And they are all very similar such as if( a == null ) throw new NullArgumentException; so, it is indeed useful to have ErrorCheck(a)(b)(c)(d); This way I can do all the error checking in one line. The code will be much less cluttered. I will try your solution right now. It will be superb if it works :) – Wei Ma Apr 28 '11 at 05:09

score 3 · Answer 2 · answered Apr 28 '11 at 06:35

Another way to do it is to make a combinator. Easy peasy, but you can't do it with generics because of that infinite regress. You've got to declare it directly:

delegate D D(D d);

That is, D is a delegate that takes a D and returns a D.

static D MyCombinator(D d)
{
    return MyCombinator;
}

A few more thoughts on combinators in C# here:

http://blogs.msdn.com/b/ericlippert/archive/2006/06/23/standard-generic-delegate-types-part-two.aspx

score 1 · Answer 3 · answered Apr 28 '11 at 04:57

This sounds a lot like an iterator. If you can refactor your code to be something like:

IEnumerator<P> GetEnumerator<T,P>(T input)
{
  while(<some condition>)
  {
     // do some work with input
     yield return results; 
  }
}

Where T is your input type and P is your result type. Not identical, but it should get the work done.

Edit: If instead you want a fluent interface, the pattern is pretty much set in stone: you create a non-static class and return it from every function (which isn't static) you call. The non-static version of a static class is called a singleton and you can use it for this pattern.

I am actually trying to use this my utility method do some common task to implement the LinQ2Object function such as Where, Select, etc. You code is all good but that's the part for "real" work. My utility method is intend for error checking before running this part. — Wei Ma, Apr 28 '11 at 05:04

Justin · Answer 4 · 2011-04-28T05:06:11.573

1

I'm not sure what you are trying to achieve, but as an intelectual exercise "can I return a method that returns itself?" the following works:

object MyFunc<T>(T input)
{
    Func<T, object> returnValue = MyFunc;
    return returnValue;
}

Like you say, you can't have the method return a Func as this would mean the return type of that delegate would need to be a Func that returns a Func that returns a Func etc...

The only way out of this infinite recusion that I can see is to have it return an object instead, which requires that the caller cast to the correct type.

Edit: Porges answer is better...

edited Apr 28 '11 at 05:06

answered Apr 28 '11 at 04:57

Justin

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As I wrote in the reply for porges. I was writing error checking code that repeat a lot. By doing this, the code is much terser. – Wei Ma Apr 28 '11 at 05:20
@WeiMa An alternative might have been to use `params` - Personally I prefer to make my error handling explicit. Even if its more verbose, it generally makes it clearer to read. In any case this was an interesting question. – Justin Apr 28 '11 at 05:22
Or return dynamic if you're using .NET4 to save having to cast. – Michael Apr 28 '11 at 06:08

How do I return a delegate function or a lambda expression in c#?

4 Answers4