使用对象初始化器创建实例的表达式

Question

有没有什么方法可以使用带有表达式树的对象初始化器来创建对象的实例？我的意思是创建一个表达式树来构建这个 lambda：

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

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

如何使用表达式树创建这个 lambda？

更新：

我自己试了一下，写了以下代码：

    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();
    }

但它会引发以下错误：

无法跳转到未定义的标签''。

大家可以帮帮我吗？

Answer 1

要在表达式中表示对象初始值设定项，您应该使用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);
}

要验证这是否确实符合您的要求，您可以在创建的表达式上调用 ToString()。在这种情况下，输出如预期：

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

Answer 2

终于找到答案了：

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(local, "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), 
        /* I forgot to remove this line:
         * Expression.Return(Expression.Label(expectedType), local, expectedType), 
         * and now it works.
         * */
        returnExpression, 
        returnLabel 
        ); 
    Creator = 
        Expression.Lambda<Func<bool, dynamic>>(block, displayValueParam) 
            .Compile(); 
}

更新：

虽然它工作正常，但@svick 在他的答案中提供了一种更好更短的方法，这正是我一直在寻找的：MemberInit。请参阅@svick 的回答。