Motivation
Higher-order programming allows to manipulate for example methods themselves. This can be useful for many purposes, and is called “delegates” in C#. We explain its basics below, and refer to the sorting lecture for an example of how it can be used.
Definition
a method that has no parameters and does not return a value.
An “Action<T>{=html}” is a method that has a single parameter (of type
T) and does not return a value.
Here are for example three actions:
using System;
public static class ExampleActions
{
public static void Test()
{
Console.WriteLine("Test");
}
public static void Display(int i)
{
Console.WriteLine(i);
}
}
public static class ExampleActions<T>
{
public static void DisplayArray(T[] arrayP)
{
for (int i = 0; i < arrayP.Length; i++)
{
Console.WriteLine(arrayP[i]);
}
}
}Test,DisplayandDisplayArrayall havevoidas their return type,Testdoes not take any argument,Displaytakes anintas an argument,DisplayArraytakes “an array ofT” (that is, a generic type as an argument.
We can call those easily:
ExampleActions.Test();
ExampleActions.Display(3);
ExampleActions<int>.DisplayArray(
new int[] { 20, 30, 40 }
);Manipulating Actions as Variables
We can also store them into variables and then call them:
Action test_variable = ExampleActions.Test;
test_variable();
Action<int> display_variable = ExampleActions.Display;
display_variable(10);
Action<int[]> display_array_variable =
ExampleActions<int>.DisplayArray;
display_array_variable(new int[] { 10, 20, 30 });
// Passing an action as an argument:
CallingAction.Demo(ExampleActions.Test);
CallingAction.DemoT(ExampleActions.Display);
// Done passing an action.
}
}As we can see, ExampleActions.Display is of type Action<int> since
the Display method takes an int as argument.
Action as Parameters
Method can take actions as parameter:
using System;
public static class CallingAction
{
public static void Demo(Action actionP)
{
Console.WriteLine("Now calling action: ");
actionP();
Console.WriteLine("Done calling action.");
}
public static void DemoT(Action<int> actionP)
{
Console.WriteLine(
"Now calling action with arguments ranging from 0 to 9:"
);
for (int i = 0; i < 10; i++)
{
actionP(i);
}
Console.WriteLine("Done calling action.");
}
}and then can be passed an action as an argument:
CallingAction.Demo(ExampleActions.Test);
CallingAction.DemoT(ExampleActions.Display);