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11/18/2011

BlockExpression Class

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

Represents a block that contains a sequence of expressions where variables can be defined.

Inheritance Hierarchy

System.Object
System.Linq.Expressions.Expression
System.Linq.Expressions.BlockExpression

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

Syntax

'Declaration
Public Class BlockExpression _
    Inherits Expression

public class BlockExpression : Expression

The BlockExpression 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.)
	Expressions	Gets the expressions in this block.
	NodeType	Returns the node type of this expression. Extension nodes should return Extension when overriding this method. (Overrides Expression.NodeType.)
	Result	Gets the last expression in this block.
	Type	Gets the static type of the expression that this Expression represents. (Overrides Expression.Type.)
	Variables	Gets the variables defined in this block.

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Methods

	Name	Description
	Accept	Dispatches to the specific visit method for this node type. For example, MethodCallExpression calls the VisitMethodCall. (Overrides Expression.Accept(ExpressionVisitor).)
	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.)
	Update	Creates a new expression that is like this one, but using the supplied children. If all of the children are the same, it will return this 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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Remarks

The Block methods can be used to create a BlockExpression.

Examples

The following code example shows how to create a block expression. The block expression consists of two MethodCallExpression objects and one ConstantExpression object.

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

' The block expression enables you to execute several expressions sequentually.
' When the block expression is executed,
' it returns the value of the last expression in the sequence.
Dim blockExpr As BlockExpression = Expression.Block(
    Expression.Call(
        Nothing,
        GetType(Console).GetMethod("Write", New Type() {GetType(String)}),
        Expression.Constant("Hello ")
       ),
    Expression.Call(
        Nothing,
        GetType(Console).GetMethod("WriteLine", New Type() {GetType(String)}),
        Expression.Constant("World!")
        ),
    Expression.Constant(42)
)

outputBlock.Text &= "The result of executing the expression tree:" & vbCrLf
' The following statement first creates an expression tree,
' then compiles it, and then executes it.           
Dim result = Expression.Lambda(Of Func(Of Integer))(blockExpr).Compile()()

' Print the expressions from the block expression.
outputBlock.Text &= "The expressions from the block expression:" & vbCrLf
For Each expr In blockExpr.Expressions
    outputBlock.Text &= expr.ToString() & vbCrLf
Next

' Print the result of the tree execution.
outputBlock.Text &= "The return value of the block expression:" & vbCrLf
outputBlock.Text &= result & vbCrLf

' This code example produces the following output:
'
' The result of executing the expression tree:
' Hello World!

' The expressions from the block expression:
' Write("Hello ")
' WriteLine("World!")
' 42

' The return value of the block expression:
' 42

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

// The block expression allows for executing several expressions sequentually.
// When the block expression is executed,
// it returns the value of the last expression in the sequence.
BlockExpression blockExpr = Expression.Block(
    Expression.Call(
        null,
        typeof(Console).GetMethod("Write", new Type[] { typeof(String) }),
        Expression.Constant("Hello ")
       ),
    Expression.Call(
        null,
        typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }),
        Expression.Constant("World!")
        ),
    Expression.Constant(42)
);

outputBlock.Text += "The result of executing the expression tree:" + "\n";
// The following statement first creates an expression tree,
// then compiles it, and then executes it.           
var result = Expression.Lambda<Func<int>>(blockExpr).Compile()();

// Print out the expressions from the block expression.
outputBlock.Text += "The expressions from the block expression:" + "\n";
foreach (var expr in blockExpr.Expressions)
   outputBlock.Text += expr.ToString() + "\n";

// Print out the result of the tree execution.
outputBlock.Text += "The return value of the block expression:" + "\n";
outputBlock.Text += result + "\n";

// This code example produces the following output:
//
// The result of executing the expression tree:
// Hello World!

// The expressions from the block expression:
// Write("Hello ")
// WriteLine("World!")
// 42

// The return value of the block expression:
// 42

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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