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Property Statement

  • Artikkeli

Declares the name of a property, and the property procedures used to store and retrieve the value of the property.

Syntax

[ <attributelist> ] [ Default ] [ accessmodifier ]
[ propertymodifiers ] [ Shared ] [ Shadows ] [ ReadOnly | WriteOnly ] [ Iterator ]
Property name ( [ parameterlist ] ) [ As returntype ] [ Implements implementslist ]
    [ <attributelist> ] [ accessmodifier ] Get
        [ statements ]
    End Get
    [ <attributelist> ] [ accessmodifier ] Set ( ByVal value As returntype [, parameterlist ] )
        [ statements ]
    End Set
End Property
- or -
[ <attributelist> ] [ Default ] [ accessmodifier ]
[ propertymodifiers ] [ Shared ] [ Shadows ] [ ReadOnly | WriteOnly ]
Property name ( [ parameterlist ] ) [ As returntype ] [ Implements implementslist ]

Parts

  • attributelist

    Optional. List of attributes that apply to this property or Get or Set procedure. See Attribute List.

  • Default

    Optional. Specifies that this property is the default property for the class or structure on which it is defined. Default properties must accept parameters and can be set and retrieved without specifying the property name. If you declare the property as Default, you cannot use Private on the property or on either of its property procedures.

  • accessmodifier

    Optional on the Property statement and on at most one of the Get and Set statements. Can be one of the following:

    See Access levels in Visual Basic.

  • propertymodifiers

    Optional. Can be one of the following:

  • Shared

    Optional. See Shared.

  • Shadows

    Optional. See Shadows.

  • ReadOnly

    Optional. See ReadOnly.

  • WriteOnly

    Optional. See WriteOnly.

  • Iterator

    Optional. See Iterator.

  • name

    Required. Name of the property. See Declared Element Names.

  • parameterlist

    Optional. List of local variable names representing the parameters of this property, and possible additional parameters of the Set procedure. See Parameter List.

  • returntype

    Required if Option Strict is On. Data type of the value returned by this property.

  • Implements

    Optional. Indicates that this property implements one or more properties, each one defined in an interface implemented by this property's containing class or structure. See Implements Statement.

  • implementslist

    Required if Implements is supplied. List of properties being implemented.

    implementedproperty [ , implementedproperty ... ]

    Each implementedproperty has the following syntax and parts:

    interface.definedname

    Part Description
    interface Required. Name of an interface implemented by this property's containing class or structure.
    definedname Required. Name by which the property is defined in interface.

  • Get

    Optional. Required if the property is marked ReadOnly. Starts a Get property procedure that is used to return the value of the property. The Get statement is not used with automatically implemented properties.

  • statements

    Optional. Block of statements to run within the Get or Set procedure.

  • End Get

    Terminates the Get property procedure.

  • Set

    Optional. Required if the property is marked WriteOnly. Starts a Set property procedure that is used to store the value of the property. The Set statement is not used with automatically implemented properties.

  • End Set

    Terminates the Set property procedure.

  • End Property

    Terminates the definition of this property.

Remarks

The Property statement introduces the declaration of a property. A property can have a Get procedure (read only), a Set procedure (write only), or both (read-write). You can omit the Get and Set procedure when using an automatically implemented property. For more information, see Automatically implemented properties.

You can use Property only at class level. This means the declaration context for a property must be a class, structure, module, or interface, and cannot be a source file, namespace, procedure, or block. For more information, see Declaration Contexts and Default Access Levels.

By default, properties use public access. You can adjust a property's access level with an access modifier on the Property statement, and you can optionally adjust one of its property procedures to a more restrictive access level.

Visual Basic passes a parameter to the Set procedure during property assignments. If you do not supply a parameter for Set, the integrated development environment (IDE) uses an implicit parameter named value. This parameter holds the value to be assigned to the property. You typically store this value in a private local variable and return it whenever the Get procedure is called.

Rules

  • Mixed Access Levels. If you are defining a read-write property, you can optionally specify a different access level for either the Get or the Set procedure, but not both. If you do this, the procedure access level must be more restrictive than the property's access level. For example, if the property is declared Friend, you can declare the Set procedure Private, but not Public.

    If you are defining a ReadOnly or WriteOnly property, the single property procedure (Get or Set, respectively) represents all of the property. You cannot declare a different access level for such a procedure, because that would set two access levels for the property.

  • Return Type. The Property statement can declare the data type of the value it returns. You can specify any data type or the name of an enumeration, structure, class, or interface.

    If you do not specify returntype, the property returns Object.

  • Implementation. If this property uses the Implements keyword, the containing class or structure must have an Implements statement immediately following its Class or Structure statement. The Implements statement must include each interface specified in implementslist. However, the name by which an interface defines the Property (in definedname) does not have to be the same as the name of this property (in name).

Behavior

  • Returning from a Property Procedure. When the Get or Set procedure returns to the calling code, execution continues with the statement following the statement that invoked it.

    The Exit Property and Return statements cause an immediate exit from a property procedure. Any number of Exit Property and Return statements can appear anywhere in the procedure, and you can mix Exit Property and Return statements.

  • Return Value. To return a value from a Get procedure, you can either assign the value to the property name or include it in a Return statement. The following example assigns the return value to the property name quoteForTheDay and then uses the Exit Property statement to return.

    Private quoteValue As String = "No quote assigned yet."

    ReadOnly Property QuoteForTheDay() As String
    Get
        QuoteForTheDay = quoteValue
        Exit Property
    End Get
End Property

    If you use Exit Property without assigning a value to name, the Get procedure returns the default value for the property's data type.

    The Return statement at the same time assigns the Get procedure return value and exits the procedure. The following example shows this.

    Private quoteValue As String = "No quote assigned yet."

    ReadOnly Property QuoteForTheDay() As String
    Get
        Return quoteValue
    End Get
End Property

Example

The following example declares a property in a class.

Class Class1
    ' Define a local variable to store the property value.
    Private propertyValue As String
    ' Define the property.
    Public Property Prop1() As String
        Get
            ' The Get property procedure is called when the value
            ' of a property is retrieved.
            Return propertyValue
        End Get
        Set(ByVal value As String)
            ' The Set property procedure is called when the value
            ' of a property is modified.  The value to be assigned
            ' is passed in the argument to Set.
            propertyValue = value
        End Set
    End Property
End Class

See also