With real examples and their use, can someone please help me understand:
Func<T, ..>
delegate?Action<T>
delegate?Predicate<T>
delegate?Func - When you want a delegate for a function that may or may not take parameters and returns a value. The most common example would be Select from LINQ:
var result = someCollection.Select( x => new { x.Name, x.Address });
Action - When you want a delegate for a function that may or may not take parameters and does not return a value. I use these often for anonymous event handlers:
button1.Click += (sender, e) => { /* Do Some Work */ }
Predicate - When you want a specialized version of a Func that evaluates a value against a set of criteria and returns a boolean result (true for a match, false otherwise). Again, these are used in LINQ quite frequently for things like Where:
var filteredResults =
someCollection.Where(x => x.someCriteriaHolder == someCriteria);
I just double checked and it turns out that LINQ doesn't use Predicates. Not sure why they made that decision...but theoretically it is still a situation where a Predicate would fit.
Action is a delegate (pointer) to a method, that takes zero, one or more input parameters, but does not return anything.
Func is a delegate (pointer) to a method, that takes zero, one or more input parameters, and returns a value (or reference).
Predicate is a special kind of Func often used for comparisons.
Though widely used with Linq, Action and Func are concepts logically independent of Linq. C++ already contained the basic concept in form of typed function pointers.
Here is a small example for Action and Func without using Linq:
class Program
{
static void Main(string[] args)
{
Action<int> myAction = new Action<int>(DoSomething);
myAction(123); // Prints out "123"
// can be also called as myAction.Invoke(123);
Func<int, double> myFunc = new Func<int, double>(CalculateSomething);
Console.WriteLine(myFunc(5)); // Prints out "2.5"
}
static void DoSomething(int i)
{
Console.WriteLine(i);
}
static double CalculateSomething(int i)
{
return (double)i/2;
}
}
Source: Stackoverflow.com