Func delegates

Syntax

  • public delegate TResult Func<in T, out TResult>(T arg)
  • public delegate TResult Func<in T1, in T2, out TResult>(T1 arg1, T2 arg2)
  • public delegate TResult Func<in T1, in T2, in T3, out TResult>(T1 arg1, T2 arg2, T3 arg3)
  • public delegate TResult Func<in T1, in T2, in T3, in T4, out TResult>(T1 arg1, T2 arg2, T3 arg3, T4 arg4)

Parameters

ParameterDetails
arg or arg1the (first) parameter of the method
arg2the second parameter of the method
arg3the third parameter of the method
arg4the fourth parameter of the method
T or T1the type of the (first) parameter of the method
T2the type of the second parameter of the method
T3the type of the third parameter of the method
T4the type of the fourth parameter of the method
TResultthe return type of the method

Covariant & Contravariant Type Parameters

Func also supports Covariant & Contravariant

// Simple hierarchy of classes.
public class Person { }
public class Employee : Person { }

class Program
{
    static Employee FindByTitle(String title)
    {
        // This is a stub for a method that returns
        // an employee that has the specified title.
        return new Employee();
    }

    static void Test()
    {
        // Create an instance of the delegate without using variance.
        Func<String, Employee> findEmployee = FindByTitle;

        // The delegate expects a method to return Person,
        // but you can assign it a method that returns Employee.
        Func<String, Person> findPerson = FindByTitle;

        // You can also assign a delegate 
        // that returns a more derived type 
        // to a delegate that returns a less derived type.
        findPerson = findEmployee;

    }
}

Lambda & anonymous methods

An anonymous method can be assigned wherever a delegate is expected:

Func<int, int> square = delegate (int x) { return x * x; }

Lambda expressions can be used to express the same thing:

Func<int, int> square = x => x * x;

In either case, we can now invoke the method stored inside square like this:

var sq = square.Invoke(2);

Or as a shorthand:

var sq = square(2);

Notice that for the assignment to be type-safe, the parameter types and return type of the anonymous method must match those of the delegate type:

Func<int, int> sum = delegate (int x, int y) { return x + y; } // error
Func<int, int> sum = (x, y) => x + y; // error

With multiple variables

static int Sum(int a, int b)
{
    return a + b;
}

static int Multiplication(int a, int b)
{
    return a * b;
}

static void Main(string[] args)
{
    Func<int, int, int> method = Sum;
    // method points to the Sum method
    // that retuns 1 int variable and takes 2 int variables  
    int sum = method(1, 1);

    method = Multiplication;
    // now method points to the Multiplication method

    int multiplication = method(1, 1);
}

Without parameters

This example shows how to create a delegate that encapsulates the method that returns the current time

static DateTime UTCNow()
{
    return DateTime.UtcNow;
}

static DateTime LocalNow()
{
    return DateTime.Now;
}

static void Main(string[] args)
{
    Func<DateTime> method = UTCNow;
    // method points to the UTCNow method
    // that retuns current UTC time  
    DateTime utcNow = method();

    method = LocalNow;
    // now method points to the LocalNow method
    // that returns local time

    DateTime localNow = method();
}