Func

The C# Func type handles many arguments. It is a parameterized type. We specify the number and kinds of parameters, and the type of the return value.

The Func type provides a way to store anonymous methods in a generalized and simple way. We can even use it in a data structure like an array to call a method by an index.

First program

We use the lambda expression syntax with Func type instances to store anonymous methods. We see type parameters: these are specified in the angle brackets.

using System;

// Part 1: create a Func that has 1 parameter and 1 return value.
// ... Parameter is an integer, result value is a string.
Func<int, string> func1 = (x) => string.Format("string = {0}", x);

// Part 2: Func instance with two parameters and one result.
// ... Receives bool and int, returns string.
Func<bool, int, string> func2 = (b, x) => string.Format("string = {0} and {1}", b, x);

// Part 3: Func instance that has no parameters and one result value.
Func<double> func3 = () => Math.PI / 2;

// Part 4: call the Invoke instance method on the anonymous functions.
Console.WriteLine(func1.Invoke(5));
Console.WriteLine(func2.Invoke(true, 10));
Console.WriteLine(func3.Invoke());
string = 5
string = True and 10
1.5707963267949

Call directly

We can call a Func directly, without specifying the Invoke() function. We just use the Func like a normal method and pass it the required arguments.

using System;

class Program
{
    static void Main()
    {
        Func<int, int> addOneHundred = a => a + 100;
        // We can call the Func directly without Invoke.
        Console.WriteLine(addOneHundred(10));
    }
}
110

Performance, Func

Should you change all your C# code to use Func invocations? This might decrease performance—using classic constructs like switch may be faster.

using System;
using System.Diagnostics;

class Program
{
    const int _max = 10000000;
    static void Main()
    {
        // A Func lookup table.
        var lookup = new Func<int, int>[]
        {
            a => a + 10,
            a => a + 20,
            a => a + 30
        };

        // Version 1: use Func table.
        var s1 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            if (Version1(lookup, i) == -1)
            {
                return;
            }
        }
        s1.Stop();
        // Version 2: use switch.
        var s2 = Stopwatch.StartNew();
        for (int i = 0; i < _max; i++)
        {
            if (Version2(i) == -1)
            {
                return;
            }
        }
        s2.Stop();
        Console.WriteLine(((double)(s1.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
        Console.WriteLine(((double)(s2.Elapsed.TotalMilliseconds * 1000000) / _max).ToString("0.00 ns"));
    }

    static int Version1(Func<int, int>[] lookup, int i)
    {
        // Use lookup table.
        return lookup[i % 3](i);
    }

    static int Version2(int i)
    {
        // Use switch.
        switch (i % 3)
        {
            default:
            case 0:
                return i + 10;
            case 1:
                return i + 20;
            case 2:
                return i + 30;
        }
    }
}
2.82 ns    Func lookup table
5.36 ns    Switch

You cannot call the Invoke method with an invalid number of parameters. The Func type stores the required parameter types and uses those to generate the Invoke method signature.

The Func parameterized type is often used with C# lambda expression syntax. It is possible to store anonymous functions inside Func pointers and then Invoke() them.