Static and instance fields

When a field declaration includes a static modifier, the fields introduced by the declaration are static fields. When no static modifier is present, the fields introduced by the declaration are instance fields. Static fields and instance fields are two of the several kinds of variables (§5) supported by C#, and at times they are referred to as static variables and instance variables, respectively.

A static field is not part of a specific instance; instead, it is shared amongst all instances of a closed type (§4.4.2). No matter how many instances of a closed class type are created, there is only ever one copy of a static field for the associated application domain.

For example:

class C<V>
{
static int count = 0;

public C() {
count++;
}

public static int Count {
get { return count; }
}
}

class Application
{
static void Main() {
C<int> x1 = new C<int>();
Console.WriteLine(C<int>.Count); // Prints 1

C<double> x2 = new C<double>();
Console.WriteLine(C<int>.Count); // Prints 1

C<int> x3 = new C<int>();
Console.WriteLine(C<int>.Count); // Prints 2
}
}

An instance field belongs to an instance. Specifically, every instance of a class contains a separate set of all the instance fields of that class.

When a field is referenced in a member-access (§7.6.4) of the form E.M, if M is a static field, E must denote a type containing M, and if M is an instance field, E must denote an instance of a type containing M.

The differences between static and instance members are discussed further in §10.3.7.