Static Members

Static vs. instance members

We’ve used the static keyword regularly throughout this course, but haven’t really talked about what that means.

Consider the following class:

class Person
{
	public int Age { get; set; }
	public string Name { get; set; }
}

To use this class, we have to create its instance:

var person1 = new Person();
person1.Age = 41;
person1.Name = "John Doe";

var person2 = new Person();
person2.Age = 38;
person2.Age = "Jim Terry";

We now have two instances of the Person class, and each instance has its own copy of the Age and Name properties.

In contrast to instance members that are bound to concrete instances of the class, static members are bound to the class definition itself:

class Person
{
	public static long Population { get; private set; } = 7000000000;

	public int Age { get; set; }
	public string Name { get; set; }

	public Person()
	{
		Population += 1;
	}
}

Because the Population property isn’t bound to a concrete instance, it can be used as a shared variable between class instances.

In terms of computer memory, static members are located in the heap and are accessible by all instances of the class where the static member is defined.

Memory for a static member is allocated when the program starts, even if no instance of this class was created.

In our example, we increase the Population by one whenever a new instance of the Person class is constructed.

Access static members

Because we don’t need a class instance to use static members, we access them through the class name: