19

有没有办法用表达式树的对象初始化器创建一个对象的实例？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此 lambda？

更新：

我尝试了自己并编写了以下代码：

    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？

4

2 回答 2

55

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，您可以调用ToString()创建的表达式。在这种情况下，输出如预期：

displayValue => new MyObject() {DisplayValue = displayValue}
于 2012-10-03T09:15:36.873 回答
3

最后我找到了答案：

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，这实际上是我正在寻找的：MemberInit。请参阅@svick 的回答。

于 2012-10-03T06:52:02.120 回答