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Value Types and Reference Types

  • บทความ

There are two kinds of types in Visual Basic: reference types and value types. Variables of reference types store references to their data (objects), while variables of value types directly contain their data. With reference types, two variables can reference the same object; therefore, operations on one variable can affect the object referenced by the other variable. With value types, each variable has its own copy of the data, and it is not possible for operations on one variable to affect the other (except in the case of the ByRef modifier on parameters).

Value Types

A data type is a value type if it holds the data within its own memory allocation. Value types include the following:

  • All numeric data types

  • Boolean, Char, and Date

  • All structures, even if their members are reference types

  • Enumerations, since their underlying type is always SByte, Short, Integer, Long, Byte, UShort, UInteger, or ULong

Every structure is a value type, even if it contains reference type members. For this reason, value types such as Char and Integer are implemented by .NET Framework structures.

You can declare a value type by using the reserved keyword, for example, Decimal. You can also use the New keyword to initialize a value type. This is especially useful if the type has a constructor that takes parameters. An example of this is the Decimal(Int32, Int32, Int32, Boolean, Byte) constructor, which builds a new Decimal value from the supplied parts.

Reference Types

A reference type stores a reference to its data. Reference types include the following:

  • String

  • All arrays, even if their elements are value types

  • Class types, such as Form

  • Delegates

A class is a reference type. Note that every array is a reference type, even if its members are value types.

Since every reference type represents an underlying .NET Framework class, you must use the New Operator keyword when you initialize it. The following statement initializes an array.

Dim totals() As Single = New Single(8) {}

Elements That Are Not Types

The following programming elements do not qualify as types, because you cannot specify any of them as a data type for a declared element:

  • Namespaces

  • Modules

  • Events

  • Properties and procedures

  • Variables, constants, and fields

Working with the Object Data Type

You can assign either a reference type or a value type to a variable of the Object data type. An Object variable always holds a reference to the data, never the data itself. However, if you assign a value type to an Object variable, it behaves as if it holds its own data. For more information, see Object Data Type.

You can find out whether an Object variable is acting as a reference type or a value type by passing it to the IsReference method in the Information class of the Microsoft.VisualBasic namespace. Information.IsReference returns True if the content of the Object variable represents a reference type.

See also