Question

我正在编写一些代码，其最终目的是让您使用属性表达式来设置具有类似语法的属性的值，以将变量作为out或ref参数传递。

有些事情：

public static foo(()=>Object.property, value);

将为Object.Property赋值value。

我使用以下代码获取属性的所有对象：

public static object GetOwningObject<T>(this Expression<Func<T>> @this)
    {
        var memberExpression = @this.Body as MemberExpression;
        if (memberExpression != null)
        {
            var fieldExpression = memberExpression.Expression as MemberExpression;
            if (fieldExpression != null)
            {
                var constExpression = fieldExpression.Expression as ConstantExpression;
                var field = fieldExpression.Member as FieldInfo;
                if (constExpression != null) if (field != null) return field.GetValue(constExpression.Value);
            }
        }
        return null;
    }

因此，当在（）=＆gt; Object.Property等属性表达式上使用时，会返回＆＃39; Object＆＃39;的实例。我对使用属性表达式有点新，并且似乎有许多不同的方法来完成事情，但我想扩展到目前为止所做的事情，所以给定一个表达式如（）=＆gt; Foo.Bar .Baz它会给吧，而不是Foo。我总是想要表达式中的最后一个包含对象。

有什么想法吗？提前谢谢。

Answer 1

您需要做的是遍历属性链到最外层的对象。下面的示例是相当自我解释的，并表明扩展方法适用于链接字段和属性：

class Foo
{
    public Bar Bar { get; set; }
}

class Bar
{
    public string Baz { get; set; }
}

class FooWithField
{
    public BarWithField BarField;
}

class BarWithField
{
    public string BazField;
}

public static class LambdaExtensions
{
    public static object GetRootObject<T>(this Expression<Func<T>> expression)
    {
        var propertyAccessExpression = expression.Body as MemberExpression;
        if (propertyAccessExpression == null)
            return null;

        //go up through property/field chain
        while (propertyAccessExpression.Expression is MemberExpression)
            propertyAccessExpression = (MemberExpression)propertyAccessExpression.Expression;

        //the last expression suppose to be a constant expression referring to captured variable ...
        var rootObjectConstantExpression = propertyAccessExpression.Expression as ConstantExpression;
        if (rootObjectConstantExpression == null)
            return null;

        //... which is stored in a field of generated class that holds all captured variables.
        var fieldInfo = propertyAccessExpression.Member as FieldInfo;
        if (fieldInfo != null)
            return fieldInfo.GetValue(rootObjectConstantExpression.Value);

        return null;
    }
}

[TestFixture]
public class Program
{
    [Test]
    public void Should_find_root_element_by_property_chain()
    {
        var foo = new Foo { Bar = new Bar { Baz = "text" } };
        Expression<Func<string>> expression = () => foo.Bar.Baz;
        Assert.That(expression.GetRootObject(), Is.SameAs(foo));
    }

    [Test]
    public void Should_find_root_element_by_field_chain()
    {
        var foo = new FooWithField { BarField = new BarWithField { BazField = "text" } };
        Expression<Func<string>> expression = () => foo.BarField.BazField;
        Assert.That(expression.GetRootObject(), Is.SameAs(foo));
    }
}

Answer 2

如果您的项目是MVC 5项目，并且您有对程序集System.Web.Mvc的引用，则可以使用以下代码：

前一段时间，我写了一个扩展方法来轻松创建多选下拉列表（基于bootstrap 4），它看起来像这样：

public static MvcHtmlString MultiSelectFor<TModel, TProperty>(this HtmlHelper<TModel> htmlHelper, Expression<Func<TModel, TProperty>> expression)
{
/*The challenge I faced here was that the expression you passed could very well be nested, so in order overcome this, I decompiled the dll to see how MVC does it, and I found this piece of code.*/

 string expressionText = System.Web.Mvc.ExpressionHelper.GetExpressionText((LambdaExpression)expression);
 System.Web.Mvc.ModelMetadata metadata = System.Web.Mvc.ModelMetadata.FromStringExpression(expressionText, htmlHelper.ViewData);

}

metadata对象具有一个名为PropertyName的属性和另一个名为Container的属性，该属性是对容器对象实例的引用。

从Property Expression获取属性的拥有对象

2 个答案: