http://stackoverflow.com/questions/4317479/func-vs-action-vs-predicate

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.

 

Func用于有返回值的方法，Action用于没有返回值的方法

 

Func<string>    表示一个有返回值的函数，返回值为string，但是函数没有参数

Func<stirng,string>  表示一个有返回值的函数，返回值为string，函数参数为string

Action<string>  表示一个无返回值的函数，函数参数为string

 

    Action b = Method;
    private static void Method()
    {
    }

    Action<string> a = Method;
    private static void Method(string str)
    {
    }

 

 

    Func<string> a = Method;
    private static string Method()
    {
        return string.Empty;
    }


    Func<string, string> b = Method;
    private static string Method(string str)
    {
        return string.Empty;
    }