what is this code: Func < T, string > that I don't understand

Question

I have some code (not writted by me) that I don't understand:

private Func<ControllerContext, string> ThemeSelector { get; set; }

So this is not a type, this is Func < T, string > and I don't know how this kind of thing is named but I am unable to find some explanations on Google. Does someone can gives me some explanations of giving me a link to explain?

Thanks.

This might be a duplicate of this : http://stackoverflow.com/questions/7901609/theme-selector-idea-for-a-blog-engine-written-with-asp-net-mvc — radu florescu, Feb 04 '12 at 08:35
@Floradu88: I try to find by myself but I don't know how it is named so it is difficult to search... On msdn it is not clear for me. — Bronzato, Feb 04 '12 at 08:36

Oybek · Answer 1 · 2012-02-04T08:47:37.837

This is a delegate type, i.e. this is a property and it can be set. And it can be called as method.

for example

/* Action<String> is a delegate, get it as an object*/
public void Foo (String[] arr, Action<String> instruction) {
    foreach (var element in arr) {
        //use it as a method
        instruction(element);
    }
}

EDIT

The same applies to Func

// Func<Int32, Boolean> means that this is a function which accepts one argument as an integer and returns boolean.
public void Foo(Int32 data, Int32 otherData, Func<Int32, Boolean> instruction) {
    var result1 = instruction(data);
    var result2 = instruction(data2);
    Console.WriteLine(result1==result2);
}

EDIT

Even more comprehensive example.

//This methods accepts the third parameter as a function with 2 input arguments of type
// Int32 and return type String
public static void DoSomething(Int32 data1, Int32 data2, Func<Int32, Int32, String> whatToDo) {
    var result = whatToDo(data1, data2);
    Console.WriteLine(result);
}

public static Main(String[] args) {
    // here we are calling the DoSomething and
    // passing the Concat method as if it was an object.
    DoSomething(1, 2, Concat);
    // The same with multiply concat.
    DoSomething(7, 2, MultiplyConcat);
}
public static String Concat(Int32 arg1, Int32 arg2) {
    return arg1.ToString() + " " + arg2.ToString():
}
public static String MultiplyConcat(Int32 arg1, Int32 arg2) {
    return (arg1 * arg2).ToString();
}

score 3 · Accepted Answer · answered Feb 04 '12 at 08:47

Func<ControllerContext, string> is a specific type of the generic Func<T,K>. You need to learn about generics first before you can fully understand that.

Therefore ThemeSelector is just a property of that type, which has setter and getter.

The Func type is a delegate type, which represents a function that takes one parameter of type T and returns an instance of type K.

This means that you can assign any such function to the property. E.g.:

private string MyFunction(ControllerContext context) {
    return "Hello world!";
}

...

ThemeSelector = MyFunction;

Console.WriteLine(ThemeSelector(null));

will print "Hello world!" in the console.

+1 for code that almost gives an example that matches the question — Murph, Feb 04 '12 at 09:20

score 1 · Answer 3 · answered Feb 04 '12 at 08:50

It is a type ! Actually it a delegate type. It represents a method that can be used without explictely declaring a delegate.

The type is a generic type. The one in your example is of type Func<T, TResult>, which represents a method that have one parameter of type T and returns a value of type TResult. Specifically to your example, it represents a method that have a ControllerContext parameter and returns a string value:

string ThemeSelectorHandler(ControllerContext context) {
    returns context.ToString();
}

what is this code: Func < T, string > that I don't understand

4 Answers4