C＃Lambda表达式-将委托结果添加到通用列表

Question

Question: I have just wrote my first code using c# lambda expressions. It works, but I am not sure if this is the best way to do it. Any recommendations on a better way to do the lambda expression? It seems odd to have numerous lines of code in the expression like I do below.

Background: I have a generic list of delegates. Each delegate function returns an enum value indicating what happened in the function. Upon evaluation of the delegate, I need to add the enum to a List if it was not a specific enum value.

Disclaimer: Code here is very generic, the real code actually does stuff in the delegates to determine the return value!

---

class Class1
{
    public enum WhatHappened
    {
        ThingA,
        ThingB,
        Nothing
    }

    private delegate WhatHappened del();

    public static List<WhatHappened> DoStuff()
    {
        List<del> CheckValues = new List<del>();

        List<WhatHappened> returnValue = new List<WhatHappened> { };

        CheckValues.Add(delegate { return method1(); });
        CheckValues.Add(delegate { return method2(); });

        CheckValues.ForEach(x =>
        {
            WhatHappened wh = x();
            if (wh != WhatHappened.Nothing)
                returnValue.Add(wh);
        });

        return returnValue;

    }

    private static WhatHappened method1()
    {
        return WhatHappened.Nothing;
    }

    private static WhatHappened method2()
    {
        return WhatHappened.ThingA;
    }

}

Note: I originally had the lambda like adding all the items (see below), then removing the ones I didn't want (WhatHappened.Nothing).

CheckValues.ForEach(x => returnValue.Add(x()));

Answer 1

Okay, a few suggestions:

• Don't call your delegate del . In this case, I'd use Func<WhatHappened> - but if you do want to declare your own delegate type, give it a more descriptive name, and obey the .NET naming conventions.

• Instead of using anonymous methods to add to CheckValues , you can just use:

   CheckValues.Add(method1); CheckValues.Add(method2); 

  The compiler will convert the method groups into delegates.

• I'd recommend not using Pascal case for a local variable name to start with.

• Your collection initializer for returnValues isn't really doing anything for you - just call the List<T> constructor as normal, or use my code below which doesn't require a local variable to start with.
• If your list really only has two delegates in it, I'd just call them separately. It's a lot simpler.

• Otherwise you can indeed use LINQ as Jared suggests, but I'd do it slightly differently:

   return CheckValues.Select(x => x()) .Where(wh => wh != WhatHappened.Nothing) .ToList(); 

EDIT: As suggested, here's the full example. It's not quite the same as Denis's though... I've made a couple of changes :)

public static List<WhatHappened> DoStuff()
{
    var functions = new List<Func<WhatHappened>> { Method1, Method2 };

    return functions.Select(function => function())
                    .Where(result => result != WhatHappened.Nothing)
                    .ToList();
}

(I'm assuming that method1 and method2 have been renamed to fit the naming convention. Of course in real life I'm sure they'd have more useful names anyway...)

Answer 2

You can go lambda all the way by chaining Select (map) and Where (filter) instead of multiple FOR loops and IF statements

// get results from the list of functions
var results = CheckValues.Select(x => x());

// filter out only the relevant ones.
var returnValues = results.Where(x => x != WhatHappened.Nothing);

Basically, you should think more declaratively instead of imperatively when work ing with lambdas. It'll help you write more elegant code.

Answer 3

I would simply use Linq, but that's just me:

public static List<WhatHappened> DoStuff()
{
    List<del> CheckValues = new List<del>();

    List<WhatHappened> returnValue = new List<WhatHappened>();

    CheckValues.Add(method1);
    CheckValues.Add(method2);

    return CheckValues
               .Select(dlg => dlg())
               .Where( res => res != WhatHappened.Nothing)
               .ToList();
}

Note that you can also use Func instead of declaring a Delegate type if you want, but that's less terse in that case. Also, I'd return an IEnumerable<WhatHappened> instead of a List, but it's all about the context.

Answer 4

It's a bit more idiomatic to write the following instead of using the delegate keyword. It doesn't change the underlying functionality though.

CheckValues.Add( () => method1() );

Also, I find it more readable to rewrite the ForEach as the following

CheckValues = CheckValues.
  Select(x => x()).
  Where(wh => wh != WhatHappened.Nothing ). 
  ToList();

Answer 5

In my opinion, based on the example, it looks fine. You could refactor even more by replacing:

CheckValues.Add(delegate { return method1(); });
CheckValues.Add(delegate { return method2(); });

with:

CheckValues.Add(() => WhatHappened.Nothing);
CheckValues.Add(() => WhatHappened.ThingA);

Answer 6

Here's a LINQ-free solution:

return CheckValues
    .ConvertAll<WhatHappened>(x => x())
    .FindAll(y => y != WhatHappened.Nothing);

caveat

This is not the most performant solution, as it would iterate twice.

Answer 7

I can't fathom the purpose of the code.. however here goes.
Used delegate chaining Update: and picked up some Enumerable goodness from Jon n Jared's posts

private delegate WhatHappened WhatHappenedDelegate();

public static List<WhatHappened> DoStuff()
{
    WhatHappenedDelegate delegateChain = null;
    delegateChain += method1;
    delegateChain += method2;

    return delegateChain.GetInvocationList() 
            .Select(x => (WhatHappened) x.DynamicInvoke())
            .Where( wh => (wh != WhatHappened.Nothing))
            .ToList<WhatHappened>();
}