Articolo
11/18/2011

DefaultExpression Class

Microsoft Silverlight will reach end of support after October 2021. Learn more.

Represents the default value of a type or an empty expression.

Inheritance Hierarchy

System.Object
System.Linq.Expressions.Expression
System.Linq.Expressions.DefaultExpression

Namespace: System.Linq.Expressions
Assembly: System.Core (in System.Core.dll)

Syntax

'Declaration
Public NotInheritable Class DefaultExpression _
    Inherits Expression

public sealed class DefaultExpression : Expression

The DefaultExpression type exposes the following members.

Properties

	Name	Description
	CanReduce	Indicates that the node can be reduced to a simpler node. If this returns true, Reduce() can be called to produce the reduced form. (Inherited from Expression.)
	NodeType	Returns the node type of this expression. Extension nodes should return Extension when overriding this method. (Overrides Expression.NodeType.)
	Type	Gets the static type of the expression that this Expression represents. (Overrides Expression.Type.)

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Methods

	Name	Description
	Accept	Dispatches to the specific visit method for this node type. For example, MethodCallExpression calls the VisitMethodCall. (Inherited from Expression.)
	Equals(Object)	Determines whether the specified Object is equal to the current Object. (Inherited from Object.)
	Finalize	Allows an object to try to free resources and perform other cleanup operations before the Object is reclaimed by garbage collection. (Inherited from Object.)
	GetHashCode	Serves as a hash function for a particular type. (Inherited from Object.)
	GetType	Gets the Type of the current instance. (Inherited from Object.)
	MemberwiseClone	Creates a shallow copy of the current Object. (Inherited from Object.)
	Reduce	Reduces this node to a simpler expression. If CanReduce returns true, this should return a valid expression. This method can return another node which itself must be reduced. (Inherited from Expression.)
	ReduceAndCheck	Reduces this node to a simpler expression. If CanReduce returns true, this should return a valid expression. This method can return another node which itself must be reduced. (Inherited from Expression.)
	ReduceExtensions	Reduces the expression to a known node type (that is not an Extension node) or just returns the expression if it is already a known type. (Inherited from Expression.)
	ToString	Returns a textual representation of the Expression. (Inherited from Expression.)
	VisitChildren	Reduces the node and then calls the visitor delegate on the reduced expression. The method throws an exception if the node is not reducible. (Inherited from Expression.)

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Examples

The following code example shows how to create an expression that represents a default value for a given type by using the Default method.

' Add the following directive to your file:
' Imports System.Linq.Expressions  

' This expression represents the default value of a type
' (0 for integer, null for a string, and so on).
Dim defaultExpr As Expression = Expression.Default(
                                        GetType(Byte)
                                    )

' Print the expression.
outputBlock.Text &= defaultExpr.ToString() & vbCrLf

' The following statement first creates an expression tree,
' then compiles it, and then executes it.
outputBlock.Text &=
    Expression.Lambda(Of Func(Of Byte))(defaultExpr).Compile()() & vbCrLf

' This code example produces the following output:
'
' default(Byte)
' 0

// Add the following directive to your file:
// using System.Linq.Expressions;  

// This expression represents the default value of a type
// (0 for integer, null for a string, etc.)
Expression defaultExpr = Expression.Default(
                            typeof(byte)
                        );

// Print out the expression.
outputBlock.Text += defaultExpr.ToString() + "\n";

// The following statement first creates an expression tree,
// then compiles it, and then executes it.
outputBlock.Text +=
    Expression.Lambda<Func<byte>>(defaultExpr).Compile()() + "\n";

// This code example produces the following output:
//
// default(Byte)
// 0

Version Information

Silverlight

Supported in: 5, 4

Platforms

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.

Thread Safety

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

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