Common Type System - Type Categories

Type Categories

The common type system supports two general categories of types:

Value types

Value types directly contain their data, and instances of value types are either allocated on the stack or allocated inline in a structure. Value types can be built-in (implemented by the runtime), user-defined, or enumerations.

Reference types

Reference types store a reference to the value's memory address, and are allocated on the heap. Reference types can be self-describing types, pointer types, or interface types. The type of a reference type can be determined from values of self-describing types. Self-describing types are further split into arrays and class types. The class types are user-defined classes, boxed value types, and delegates.

The following example written in Visual Basic .NET shows the difference between reference types and value types:

Imports System Class Class1 Public Value As Integer = 0 End Class 'Class1 Class Test Shared Sub Main Dim val1 As Integer = 0 Dim val2 As Integer = val1 val2 = 123 Dim ref1 As New Class1 Dim ref2 As Class1 = ref1 ref2.Value = 123 Console.WriteLine("Values: {0}, {1}", val1, val2) Console.WriteLine("Refs: {0}, {1}", ref1.Value, ref2.Value) End Sub 'Main End Class 'Test

The output of the above example

Values: 0, 123 Refs: 123, 123