用对象初始化器创建一个实例的表达式 [英] Expression to create an instance with object initializer

问题描述

有没有办法使用表达式树创建对象初始化器的对象实例？我的意思是创建一个表达式树来构建这个lambda：

  //我的类
 public class MyObject {
 public bool DisplayValue {get;组; 
} 
 
 //我的lambda：
 var lambda =（Func< bool，MyObject>）
（displayValue => new MyObject {DisplayValue = displayValue} ）; 
  

如何使用表达式树创建此lambda？

更新：

我尝试了自己写下以下代码：

  public static Func< bool，dynamic>造物主; 
 
 static void BuildLambda（）{
 var expectedType = typeof（MyObject）; 
 var displayValueParam = Expression.Parameter（typeof（bool），displayValue）; 
 var ctor = Expression.New（expectedType）; 
 var local = Expression.Parameter（expectedType，obj）; 
 var displayValueProperty = Expression.Property（ctor，DisplayValue）; 
 
 var returnTarget = Expression.Label（expectedType）; 
 var returnExpression = Expression.Return（returnTarget，local，expectedType）; 
 var returnLabel = Expression.Label（returnTarget，Expression.Default（expectedType））; 
 
 var block = Expression.Block（
 new [] {local}，
 Expression.Assign（local，ctor），
 Expression.Assign（displayValueProperty，displayValueParam ）
 Expression.Return（Expression.Label（expectedType），local，expectedType），
 returnExpression，
 returnLabel 
）; 
 Creator = 
 Expression.Lambda  .Compile（）; 
} 
  

但它会引发以下错误：

无法跳转到未定义的标签''。

可以帮助我吗？

解决方案

要在Expression中表示对象初始值设置，您应该使用 Expression.MemberInit（） ：

 表达式< Func< bool，MyObject>> BuildLambda（）{
 var createdType = typeof（MyObject）; 
 var displayValueParam = Expression.Parameter（typeof（bool），displayValue）; 
 var ctor = Expression.New（createdType）; 
 var displayValueProperty = createdType.GetProperty（DisplayValue）; 
 var displayValueAssignment = Expression.Bind（
 displayValueProperty，displayValueParam）; 
 var memberInit = Expression.MemberInit（ctor，displayValueAssignment）; 
 
 return 
 Expression.Lambda } 
  

为了验证这实际上是你想要的，你可以调用 ToString（）在创建的表达式上。在这种情况下，输出是预期的：

  displayValue => new MyObject（）{DisplayValue = displayValue} 
  

Is there any way to create an instance of an object with object initializer with an Expression Tree? I mean create an Expression Tree to build this lambda:

// my class
public class MyObject {
    public bool DisplayValue { get; set; }
}

// my lambda:
var lambda = (Func<bool, MyObject>)
             (displayValue => new MyObject { DisplayValue = displayValue });

How can I create this lambda with an Expression Tree?

UPDATE:

I tryed myself and write following code:

    public static Func<bool, dynamic> Creator;

    static void BuildLambda() {
        var expectedType = typeof(MyObject);
        var displayValueParam = Expression.Parameter(typeof(bool), "displayValue");
        var ctor = Expression.New(expectedType);
        var local = Expression.Parameter(expectedType, "obj");
        var displayValueProperty = Expression.Property(ctor, "DisplayValue");

        var returnTarget = Expression.Label(expectedType);
        var returnExpression = Expression.Return(returnTarget,local, expectedType);
        var returnLabel = Expression.Label(returnTarget, Expression.Default(expectedType));

        var block = Expression.Block(
            new[] { local },
            Expression.Assign(local, ctor),
            Expression.Assign(displayValueProperty, displayValueParam),
            Expression.Return(Expression.Label(expectedType), local, expectedType),
            returnExpression,
            returnLabel
            );
        Creator =
            Expression.Lambda<Func<bool, dynamic>>(block, displayValueParam)
                .Compile();
    }

But it throws the following error:

Cannot jump to undefined label ''.

Can everybody help me please?

解决方案

To represent object initializers in an Expression, you should use Expression.MemberInit():

Expression<Func<bool, MyObject>> BuildLambda() { 
    var createdType = typeof(MyObject);
    var displayValueParam = Expression.Parameter(typeof(bool), "displayValue"); 
    var ctor = Expression.New(createdType);
    var displayValueProperty = createdType.GetProperty("DisplayValue");
    var displayValueAssignment = Expression.Bind(
        displayValueProperty, displayValueParam);
    var memberInit = Expression.MemberInit(ctor, displayValueAssignment);

    return
        Expression.Lambda<Func<bool, MyObject>>(memberInit, displayValueParam);
}

To verify this actually does what you want, you can call ToString() on the created expression. In this case, the output is as expected:

displayValue => new MyObject() {DisplayValue = displayValue}

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