我正在研究一些代码,其最终目的是让您使用属性表达式来设置属性的值,其语法类似于将变量作为 out 或 ref 参数传递。
大致如下:
public static foo(()=>Object.property, value);
并且 Object.Property 将被分配值的值。
我使用以下代码来获取属性的 owining 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,当在像 ()=>Object.Property 这样的属性表达式上使用时,这将返回"对象"的实例。 我对使用属性表达式有点陌生,似乎有许多不同的方法来完成事情,但我想扩展我目前所拥有的,以便给定一个表达式,例如 ()=>Foo.Bar.Baz,它将给出 Bar,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。
你要做的是遍历属性链到最外部的对象。下面的示例不言自明,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));
}
}
如果您的项目是 MVC 5 项目,并且您有对程序集的引用System.Web.Mvc
则可以使用以下方法:
前段时间,我编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。