Статья
11/18/2011

Expression.IfThen Method

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

Creates a ConditionalExpression that represents a conditional block with an if statement.

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

Syntax

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

public static ConditionalExpression IfThen(
    Expression test,
    Expression ifTrue
)

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.

Return Value

Type: System.Linq.Expressions.ConditionalExpression
A ConditionalExpression that has the NodeType property equal to Conditional and the Test, IfTrue, properties set to the specified values. The IfFalse property is set to default expression and 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 ifThenExpr As Expression = Expression.IfThen(
     Expression.Constant(test),
     Expression.Call(
         Nothing,
         GetType(Console).GetMethod("WriteLine", New Type() {GetType(String)}),
         Expression.Constant("The condition is true.")
     )
)

' The following statement first creates an expression tree,
' then compiles it, and then runs it.
Expression.Lambda(Of Action)(ifThenExpr).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 ifThenExpr = Expression.IfThen(
    Expression.Constant(test),
    Expression.Call(
        null,
        typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }),
        Expression.Constant("The condition is true.")
       )
);

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