Func vs. Action vs. Predicate Delegates

  • 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.

 

The difference between Func and Action is simply whether you want the delegate to return a value (use Func) or not (use Action).

Func is probably most commonly used in LINQ – for example in projections:

 list.Select(x => x.SomeProperty)

or filtering:

 list.Where(x => x.SomeValue == someOtherValue)

or key selection:

 list.Join(otherList, x => x.FirstKey, y => y.SecondKey, ...)

Action is more commonly used for things like List<T>.ForEach: execute the given action for each item in the list. I use this less often than Func, although I do sometimes use the parameterless version for things like Control.BeginInvoke and Dispatcher.BeginInvoke.

Predicate is just a special cased Func<T, bool> really, introduced before all of the Func and most of the Action delegates came along. I suspect that if we’d already had Func and Action in their various guises, Predicate wouldn’t have been introduced… although it does impart a certain meaning to the use of the delegate, whereas Func and Action are used for widely disparate purposes.

Predicate is mostly used in List<T> for methods like FindAll and RemoveAll.

 

source: stackoverflow

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