Delegates: Predicate vs. Action vs. Func

Question

Can someone provide a good explanation (hopefully with examples) of these 3 most important delegates:

• Predicate
• Action
• Func

Solution 1

• Predicate: essentially Func<T, bool>; asks the question "does the specified argument satisfy the condition represented by the delegate?" Used in things like List.FindAll.

• Action: Perform an action given the arguments. Very general purpose. Not used much in LINQ as it implies side-effects, basically.

• Func: Used extensively in LINQ, usually to transform the argument, e.g. by projecting a complex structure to one property.

Other important delegates:

• EventHandler/EventHandler<T>: Used all over WinForms

• Comparison<T>: Like IComparer<T> but in delegate form.

Solution 2

Action, Func and Predicate all belong to the delegate family.

Action : Action can take n input parameters but it returns void.

Func : Func can take n input parameters but it will always return the result of the provided type. Func<T1,T2,T3,TResult>, here T1,T2,T3 are input parameters and TResult is the output of it.

Predicate : Predicate is also a form of Func but it will always return bool. In simple words it is wrapper of Func<T,bool>.

Solution 3

In addition to Jon's answer, there is also

• Converter<TInput, TOutput>: It's essentially Func<TInput, TOutput>, but with semantics. Used by List.ConvertAll and Array.ConvertAll, but personally haven't seen it anywhere else.

Solution 4

A simple example about the arguments and what retutn each type

This Func take two int arguments and return an int.Func always has return type

 Func<int, int, int> sum = (a, b) => a + b;
 Console.WriteLine(sum(3, 5));//Print 8

In this case func doesn't have arguments but return a string

Func<string> print = () => "Hello world";
Console.WriteLine(print());//Print Hello world

This Action take two int arguments and return void

Action<int, int> displayInput = (x, y) => Console.WriteLine("First number is :" + x + " , Second number is "+ y);
displayInput(4, 6); //Print First number is :4 , Second number is :6

This Predicate take one argument and always return bool.Generally Predicates always return bool.

Predicate<int> isPositive = (x) => x > 0;
Console.WriteLine(isPositive(5));//Print True

Solution 5

MethodInvoker is one which WinForms developers may use; it accepts no arguments and returns no results. It predates Action, and is still often used when invoking onto the UI thread since BeginInvoke() et al accept an untyped Delegate; although Action will do just as well.

myForm.BeginInvoke((MethodInvoker)delegate
{
  MessageBox.Show("Hello, world...");
});

I'd also be aware of ThreadStart and ParameterizedThreadStart; again most people will substitute an Action these days.

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Comments

[G-Wiz]

There's also System.Converter<TInput, TOutput>, though it's rarely used.

[Michael Stum]

The Converter is a nice delegate when a lot of Converting of Model into Business classes is needed, i.e. stum.de/2009/12/23/…

[Andy]

EventHandler/EventHandler<T> appear all over outside of WinForms too.

[Jon Skeet]

@Andy: Somewhat... But less so in WPF for example. I agree that there's nothing WinForms-specific to it.