The .NET Framework Standard Library: the System Namespace
- Basic Variable Types
- Attributes
- Utility Objects
- Interfaces
- Delegates
- Exceptions
- Diagram
This chapter is from the book
This chapter is from the book
This chapter is from the book
The System namespace is the root of all namespaces in the .NET Framework, containing all other namespaces as subordinates. It also contains the types that we felt to be the most fundamental and frequently used.
Basic Variable Types
The class Object is the root of the inheritance hierarchy in the .NET Framework. Every class in the .NET Framework ultimately derives from this class. If you define a class without specifying any other inheritance, Object is the implied base class. It provides the most basic methods and properties that all objects need to support, such as returning an identifying string, returning a Type object (think of it as a class descriptor) to use for runtime discovery of the object's contents, and providing a location for a garbage collection finalizer.
The .NET Framework provides two kinds of types, value types and reference types. Instances of value types are allocated on the stack or inline inside an object, which incurs a lower overhead than using the managed heap. Value types are most often used for small, lightweight variables accessed primarily for a single data value, while still allowing them to be treated as objects in the inheritance hierarchy (for example, having methods). All value types must derive from the abstract base class ValueType. Table 1 lists the value types in the System namespace.
Table 1.
|
Name |
Represents |
|---|---|
|
Boolean |
Boolean value (true or false). |
|
Byte |
8-bit unsigned integer. |
|
Char |
UTF-16 code point. |
|
DateTime |
An instant in time, typically expressed as a date and time of day. |
|
Decimal |
Decimal number. |
|
Double |
Double-precision floating-point number. |
|
Enum |
Base class for enumerations. |
|
Int16 |
16-bit signed integer. |
|
Int32 |
32-bit signed integer. |
|
Int64 |
64-bit signed integer. |
|
SByte |
8-bit signed integer. |
|
Single |
Single-precision floating-point number. |
|
TimeSpan |
Time interval. |
|
UInt16 |
16-bit unsigned integer. |
|
UInt32 |
32-bit unsigned integer. |
|
UInt64 |
64-bit unsigned integer. |
All objects that are not value types are by definition reference types. Creating an instance of a reference type allocates the new object from the managed heap and returns a reference to it, hence the name. Most objects are reference types. The class String is a reference type that represents an immutable series of characters. The class CharEnumerator supports iterating over a String and reading its individual characters.
The System namespace also contains the abstract base class Array, which represents a fixed-size, ordered series of objects accessed by index. It contains methods for creating, manipulating, and searching for elements within the array. Programmers will generally not use this class directly. Instead, their programming language will provide an abstraction of it.