Expression.IfThen(Expression, Expression) 方法

定义

命名空间:
System.Linq.Expressions
程序集:
System.Core.dll, System.Linq.Expressions.dll
程序集:
System.Core.dll
程序集:
System.Linq.Expressions.dll
程序集:
netstandard.dll

创建一个 ConditionalExpression 表示带有 if 语句的条件块。

public:
 static System::Linq::Expressions::ConditionalExpression ^ IfThen(System::Linq::Expressions::Expression ^ test, System::Linq::Expressions::Expression ^ ifTrue);
public static System.Linq.Expressions.ConditionalExpression IfThen(System.Linq.Expressions.Expression test, System.Linq.Expressions.Expression ifTrue);
static member IfThen : System.Linq.Expressions.Expression * System.Linq.Expressions.Expression -> System.Linq.Expressions.ConditionalExpression
Public Shared Function IfThen (test As Expression, ifTrue As Expression) As ConditionalExpression

参数

test
Expression

要设置的属性Expression等于的一个Test。

ifTrue
Expression

要设置的属性Expression等于的一个IfTrue。

返回

ConditionalExpression

一个属性等于和 <a0/> 属性设置为指定值。 此属性 IfFalse 设置为默认表达式，此方法返回的结果 ConditionalExpression 的类型为 Void。

示例

下面的代码示例演示如何创建表示条件块的表达式。

// 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.

' 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.

适用于