Статья
11/18/2011

Expression.IfThenElse Method

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Creates a ConditionalExpression that represents a conditional block with if and else statements.

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

Syntax

'Declaration
Public Shared Function IfThenElse ( _
    test As Expression, _
    ifTrue As Expression, _
    ifFalse As Expression _
) As ConditionalExpression

public static ConditionalExpression IfThenElse(
    Expression test,
    Expression ifTrue,
    Expression ifFalse
)

Parameters

test
Type: System.Linq.Expressions.Expression
An Expression to set the Test property equal to.

ifTrue
Type: System.Linq.Expressions.Expression
An Expression to set the IfTrue property equal to.

ifFalse
Type: System.Linq.Expressions.Expression
An Expression to set the IfFalse property equal to.

Return Value

Type: System.Linq.Expressions.ConditionalExpression
A ConditionalExpression that has the NodeType property equal to Conditional and the Test, IfTrue, and IfFalse properties set to the specified values. The type of the resulting ConditionalExpression returned by this method is Void.

Examples

The following code example shows how to create an expression that represents a conditional block.

' Add the following directive to the file:
' Imports System.Linq.Expressions
Dim test As Boolean = True

' This expression represents the conditional block.
Dim ifThenElseExpr As Expression = Expression.IfThenElse(
     Expression.Constant(test),
     Expression.Call(
         Nothing,
         GetType(Console).GetMethod("WriteLine", New Type() {GetType(String)}),
         Expression.Constant("The condition is true.")
     ),
     Expression.Call(
         Nothing,
         GetType(Console).GetMethod("WriteLine", New Type() {GetType(String)}),
         Expression.Constant("The condition is false.")
     )
)

' The following statement first creates an expression tree,
' then compiles it, and then runs it.
Expression.Lambda(Of Action)(ifThenElseExpr).Compile()()

' This code example produces the following output:
'
' The condition is true.

// Add the following directive to the file:
// using System.Linq.Expressions;  
bool test = true;

// This expression represents the conditional block.
Expression ifThenElseExpr = Expression.IfThenElse(
    Expression.Constant(test),
    Expression.Call(
        null,
        typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }),
        Expression.Constant("The condition is true.")
    ),
    Expression.Call(
        null,
        typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }),
        Expression.Constant("The condition is false.")
    )
);

// The following statement first creates an expression tree,
// then compiles it, and then runs it.
Expression.Lambda<Action>(ifThenElseExpr).Compile()();

// This code example produces the following output:
//
// The condition is true.

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.

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