Expression 类 (System.Linq.Expressions) | Microsoft Learn

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public ref class Expression abstract

public abstract class Expression

type Expression = class

Public MustInherit Class Expression

Object

Expression

下面的代码示例演示如何创建块表达式。块表达式由两个 MethodCallExpression 对象和一个 ConstantExpression 对象组成。

// Add the following directive to your file:
// using System.Linq.Expressions;
// The block expression allows for executing several expressions sequentually.
// When the block expression is executed,
// it returns the value of the last expression in the sequence.
BlockExpression blockExpr = Expression.Block(
    Expression.Call(
        null,
        typeof(Console).GetMethod("Write", new Type[] { typeof(String) }),
        Expression.Constant("Hello ")
    Expression.Call(
        null,
        typeof(Console).GetMethod("WriteLine", new Type[] { typeof(String) }),
        Expression.Constant("World!")
    Expression.Constant(42)
Console.WriteLine("The result of executing the expression tree:");
// The following statement first creates an expression tree,
// then compiles it, and then executes it.
var result = Expression.Lambda<Func<int>>(blockExpr).Compile()();
// Print out the expressions from the block expression.
Console.WriteLine("The expressions from the block expression:");
foreach (var expr in blockExpr.Expressions)
    Console.WriteLine(expr.ToString());
// Print out the result of the tree execution.
Console.WriteLine("The return value of the block expression:");
Console.WriteLine(result);
// This code example produces the following output:
// The result of executing the expression tree:
// Hello World!
// The expressions from the block expression:
// Write("Hello ")
// WriteLine("World!")
// 42
// The return value of the block expression:
// 42
' Add the following directive to your file:
' Imports System.Linq.Expressions
' The block expression enables you to execute several expressions sequentually.
' When the block expression is executed,
' it returns the value of the last expression in the sequence.
Dim blockExpr As BlockExpression = Expression.Block(
    Expression.Call(
        Nothing,
        GetType(Console).GetMethod("Write", New Type() {GetType(String)}),
        Expression.Constant("Hello ")
    Expression.Call(
        Nothing,
        GetType(Console).GetMethod("WriteLine", New Type() {GetType(String)}),
        Expression.Constant("World!")
    Expression.Constant(42)
Console.WriteLine("The result of executing the expression tree:")
' The following statement first creates an expression tree,
' then compiles it, and then executes it.           
Dim result = Expression.Lambda(Of Func(Of Integer))(blockExpr).Compile()()
' Print the expressions from the block expression.
Console.WriteLine("The expressions from the block expression:")
For Each expr In blockExpr.Expressions
    Console.WriteLine(expr.ToString())
' Print the result of the tree execution.
Console.WriteLine("The return value of the block expression:")
Console.WriteLine(result)
' This code example produces the following output:
' The result of executing the expression tree:
' Hello World!
' The expressions from the block expression:
' Write("Hello ")
' WriteLine("World!")
' The return value of the block expression: