Expression.IfThenElse(Expression, Expression, Expression) 方法

定義

命名空間:

System.Linq.Expressions

組件:

System.Linq.Expressions.dll

來源:

ConditionalExpression.cs

建立 ConditionalExpression，代表具有 if 和 else 陳述式的條件區塊。

public:
 static System::Linq::Expressions::ConditionalExpression ^ IfThenElse(System::Linq::Expressions::Expression ^ test, System::Linq::Expressions::Expression ^ ifTrue, System::Linq::Expressions::Expression ^ ifFalse);

public static System.Linq.Expressions.ConditionalExpression IfThenElse (System.Linq.Expressions.Expression test, System.Linq.Expressions.Expression ifTrue, System.Linq.Expressions.Expression ifFalse);

static member IfThenElse : System.Linq.Expressions.Expression * System.Linq.Expressions.Expression * System.Linq.Expressions.Expression -> System.Linq.Expressions.ConditionalExpression

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

參數

test

Expression

要將 Expression 屬性設定為與之相等的 Test。

ifTrue

Expression

要將 Expression 屬性設定為與之相等的 IfTrue。

ifFalse

Expression

要將 Expression 屬性設定為與之相等的 IfFalse。

傳回

ConditionalExpression

ConditionalExpression，其 NodeType 屬性等於 Conditional，且 Test、IfTrue 和 IfFalse 屬性設定為指定的值。 這個方法所傳回之結果 ConditionalExpression 的類型為 Void。

範例

下列程式代碼範例示範如何建立代表條件式區塊的表達式。

// 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.

' 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.