Expression.IfThen(Expression, Expression) 方法

定义

命名空间:

System.Linq.Expressions

程序集:

System.Linq.Expressions.dll

程序集:

System.Core.dll

程序集:

netstandard.dll

创建一个 ConditionalExpression，它表示带 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

要将 Expression 属性设置为与其相等的 Test。

ifTrue

Expression

要将 Expression 属性设置为与其相等的 IfTrue。

返回

ConditionalExpression

一个 ConditionalExpression，其 NodeType 属性等于 Conditional，并且 Test、IfTrue 属性设置为指定值。 IfFalse 属性设置为默认表达式，并且此方法返回的结果 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.